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1399 lines
75 KiB
1399 lines
75 KiB
# Copyright (c) 2013-2014 Sandstorm Development Group, Inc. and contributors
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# Licensed under the MIT License:
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#
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# Permission is hereby granted, free of charge, to any person obtaining a copy
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# of this software and associated documentation files (the "Software"), to deal
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# in the Software without restriction, including without limitation the rights
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# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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# copies of the Software, and to permit persons to whom the Software is
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# furnished to do so, subject to the following conditions:
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#
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# The above copyright notice and this permission notice shall be included in
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# all copies or substantial portions of the Software.
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#
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# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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# THE SOFTWARE.
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@0xb312981b2552a250;
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# Recall that Cap'n Proto RPC allows messages to contain references to remote objects that
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# implement interfaces. These references are called "capabilities", because they both designate
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# the remote object to use and confer permission to use it.
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#
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# Recall also that Cap'n Proto RPC has the feature that when a method call itself returns a
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# capability, the caller can begin calling methods on that capability _before the first call has
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# returned_. The caller essentially sends a message saying "Hey server, as soon as you finish
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# that previous call, do this with the result!". Cap'n Proto's RPC protocol makes this possible.
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#
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# The protocol is significantly more complicated than most RPC protocols. However, this is
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# implementation complexity that underlies an easy-to-grasp higher-level model of object oriented
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# programming. That is, just like TCP is a surprisingly complicated protocol that implements a
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# conceptually-simple byte stream abstraction, Cap'n Proto is a surprisingly complicated protocol
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# that implements a conceptually-simple object abstraction.
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#
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# Cap'n Proto RPC is based heavily on CapTP, the object-capability protocol used by the E
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# programming language:
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# http://www.erights.org/elib/distrib/captp/index.html
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#
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# Cap'n Proto RPC takes place between "vats". A vat hosts some set of objects and talks to other
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# vats through direct bilateral connections. Typically, there is a 1:1 correspondence between vats
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# and processes (in the unix sense of the word), although this is not strictly always true (one
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# process could run multiple vats, or a distributed virtual vat might live across many processes).
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#
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# Cap'n Proto does not distinguish between "clients" and "servers" -- this is up to the application.
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# Either end of any connection can potentially hold capabilities pointing to the other end, and
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# can call methods on those capabilities. In the doc comments below, we use the words "sender"
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# and "receiver". These refer to the sender and receiver of an instance of the struct or field
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# being documented. Sometimes we refer to a "third-party" that is neither the sender nor the
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# receiver. Documentation is generally written from the point of view of the sender.
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#
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# It is generally up to the vat network implementation to securely verify that connections are made
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# to the intended vat as well as to encrypt transmitted data for privacy and integrity. See the
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# `VatNetwork` example interface near the end of this file.
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#
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# When a new connection is formed, the only interesting things that can be done are to send a
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# `Bootstrap` (level 0) or `Accept` (level 3) message.
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#
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# Unless otherwise specified, messages must be delivered to the receiving application in the same
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# order in which they were initiated by the sending application. The goal is to support "E-Order",
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# which states that two calls made on the same reference must be delivered in the order which they
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# were made:
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# http://erights.org/elib/concurrency/partial-order.html
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#
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# Since the full protocol is complicated, we define multiple levels of support that an
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# implementation may target. For many applications, level 1 support will be sufficient.
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# Comments in this file indicate which level requires the corresponding feature to be
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# implemented.
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#
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# * **Level 0:** The implementation does not support object references. Only the bootstrap interface
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# can be called. At this level, the implementation does not support object-oriented protocols and
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# is similar in complexity to JSON-RPC or Protobuf services. This level should be considered only
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# a temporary stepping-stone toward level 1 as the lack of object references drastically changes
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# how protocols are designed. Applications _should not_ attempt to design their protocols around
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# the limitations of level 0 implementations.
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#
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# * **Level 1:** The implementation supports simple bilateral interaction with object references
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# and promise pipelining, but interactions between three or more parties are supported only via
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# proxying of objects. E.g. if Alice (in Vat A) wants to send Bob (in Vat B) a capability
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# pointing to Carol (in Vat C), Alice must create a proxy of Carol within Vat A and send Bob a
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# reference to that; Bob cannot form a direct connection to Carol. Level 1 implementations do
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# not support checking if two capabilities received from different vats actually point to the
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# same object ("join"), although they should be able to do this check on capabilities received
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# from the same vat.
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#
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# * **Level 2:** The implementation supports saving persistent capabilities -- i.e. capabilities
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# that remain valid even after disconnect, and can be restored on a future connection. When a
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# capability is saved, the requester receives a `SturdyRef`, which is a token that can be used
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# to restore the capability later.
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#
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# * **Level 3:** The implementation supports three-way interactions. That is, if Alice (in Vat A)
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# sends Bob (in Vat B) a capability pointing to Carol (in Vat C), then Vat B will automatically
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# form a direct connection to Vat C rather than have requests be proxied through Vat A.
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#
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# * **Level 4:** The entire protocol is implemented, including joins (checking if two capabilities
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# are equivalent).
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#
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# Note that an implementation must also support specific networks (transports), as described in
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# the "Network-specific Parameters" section below. An implementation might have different levels
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# depending on the network used.
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#
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# New implementations of Cap'n Proto should start out targeting the simplistic two-party network
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# type as defined in `rpc-twoparty.capnp`. With this network type, level 3 is irrelevant and
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# levels 2 and 4 are much easier than usual to implement. When such an implementation is paired
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# with a container proxy, the contained app effectively gets to make full use of the proxy's
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# network at level 4. And since Cap'n Proto IPC is extremely fast, it may never make sense to
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# bother implementing any other vat network protocol -- just use the correct container type and get
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# it for free.
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using Cxx = import "/capnp/c++.capnp";
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$Cxx.namespace("capnp::rpc");
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# ========================================================================================
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# The Four Tables
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#
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# Cap'n Proto RPC connections are stateful (although an application built on Cap'n Proto could
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# export a stateless interface). As in CapTP, for each open connection, a vat maintains four state
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# tables: questions, answers, imports, and exports. See the diagram at:
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# http://www.erights.org/elib/distrib/captp/4tables.html
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#
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# The question table corresponds to the other end's answer table, and the imports table corresponds
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# to the other end's exports table.
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#
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# The entries in each table are identified by ID numbers (defined below as 32-bit integers). These
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# numbers are always specific to the connection; a newly-established connection starts with no
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# valid IDs. Since low-numbered IDs will pack better, it is suggested that IDs be assigned like
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# Unix file descriptors -- prefer the lowest-number ID that is currently available.
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#
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# IDs in the questions/answers tables are chosen by the questioner and generally represent method
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# calls that are in progress.
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#
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# IDs in the imports/exports tables are chosen by the exporter and generally represent objects on
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# which methods may be called. Exports may be "settled", meaning the exported object is an actual
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# object living in the exporter's vat, or they may be "promises", meaning the exported object is
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# the as-yet-unknown result of an ongoing operation and will eventually be resolved to some other
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# object once that operation completes. Calls made to a promise will be forwarded to the eventual
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# target once it is known. The eventual replacement object does *not* get the same ID as the
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# promise, as it may turn out to be an object that is already exported (so already has an ID) or
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# may even live in a completely different vat (and so won't get an ID on the same export table
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# at all).
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#
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# IDs can be reused over time. To make this safe, we carefully define the lifetime of IDs. Since
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# messages using the ID could be traveling in both directions simultaneously, we must define the
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# end of life of each ID _in each direction_. The ID is only safe to reuse once it has been
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# released by both sides.
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#
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# When a Cap'n Proto connection is lost, everything on the four tables is lost. All questions are
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# canceled and throw exceptions. All imports become broken (all future calls to them throw
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# exceptions). All exports and answers are implicitly released. The only things not lost are
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# persistent capabilities (`SturdyRef`s). The application must plan for this and should respond by
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# establishing a new connection and restoring from these persistent capabilities.
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using QuestionId = UInt32;
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# **(level 0)**
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#
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# Identifies a question in the sender's question table (which corresponds to the receiver's answer
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# table). The questioner (caller) chooses an ID when making a call. The ID remains valid in
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# caller -> callee messages until a Finish message is sent, and remains valid in callee -> caller
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# messages until a Return message is sent.
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using AnswerId = QuestionId;
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# **(level 0)**
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#
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# Identifies an answer in the sender's answer table (which corresponds to the receiver's question
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# table).
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#
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# AnswerId is physically equivalent to QuestionId, since the question and answer tables correspond,
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# but we define a separate type for documentation purposes: we always use the type representing
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# the sender's point of view.
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using ExportId = UInt32;
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# **(level 1)**
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#
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# Identifies an exported capability or promise in the sender's export table (which corresponds
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# to the receiver's import table). The exporter chooses an ID before sending a capability over the
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# wire. If the capability is already in the table, the exporter should reuse the same ID. If the
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# ID is a promise (as opposed to a settled capability), this must be indicated at the time the ID
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# is introduced (e.g. by using `senderPromise` instead of `senderHosted` in `CapDescriptor`); in
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# this case, the importer shall expect a later `Resolve` message that replaces the promise.
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#
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# ExportId/ImportIds are subject to reference counting. Whenever an `ExportId` is sent over the
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# wire (from the exporter to the importer), the export's reference count is incremented (unless
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# otherwise specified). The reference count is later decremented by a `Release` message. Since
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# the `Release` message can specify an arbitrary number by which to reduce the reference count, the
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# importer should usually batch reference decrements and only send a `Release` when it believes the
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# reference count has hit zero. Of course, it is possible that a new reference to the export is
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# in-flight at the time that the `Release` message is sent, so it is necessary for the exporter to
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# keep track of the reference count on its end as well to avoid race conditions.
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#
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# When a connection is lost, all exports are implicitly released. It is not possible to restore
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# a connection state after disconnect (although a transport layer could implement a concept of
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# persistent connections if it is transparent to the RPC layer).
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using ImportId = ExportId;
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# **(level 1)**
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#
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# Identifies an imported capability or promise in the sender's import table (which corresponds to
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# the receiver's export table).
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#
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# ImportId is physically equivalent to ExportId, since the export and import tables correspond,
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# but we define a separate type for documentation purposes: we always use the type representing
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# the sender's point of view.
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#
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# An `ImportId` remains valid in importer -> exporter messages until the importer has sent
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# `Release` messages that (it believes) have reduced the reference count to zero.
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# ========================================================================================
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# Messages
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struct Message {
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# An RPC connection is a bi-directional stream of Messages.
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union {
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unimplemented @0 :Message;
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# The sender previously received this message from the peer but didn't understand it or doesn't
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# yet implement the functionality that was requested. So, the sender is echoing the message
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# back. In some cases, the receiver may be able to recover from this by pretending the sender
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# had taken some appropriate "null" action.
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#
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# For example, say `resolve` is received by a level 0 implementation (because a previous call
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# or return happened to contain a promise). The level 0 implementation will echo it back as
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# `unimplemented`. The original sender can then simply release the cap to which the promise
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# had resolved, thus avoiding a leak.
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#
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# For any message type that introduces a question, if the message comes back unimplemented,
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# the original sender may simply treat it as if the question failed with an exception.
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#
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# In cases where there is no sensible way to react to an `unimplemented` message (without
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# resource leaks or other serious problems), the connection may need to be aborted. This is
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# a gray area; different implementations may take different approaches.
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abort @1 :Exception;
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# Sent when a connection is being aborted due to an unrecoverable error. This could be e.g.
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# because the sender received an invalid or nonsensical message (`isCallersFault` is true) or
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# because the sender had an internal error (`isCallersFault` is false). The sender will shut
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# down the outgoing half of the connection after `abort` and will completely close the
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# connection shortly thereafter (it's up to the sender how much of a time buffer they want to
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# offer for the client to receive the `abort` before the connection is reset).
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# Level 0 features -----------------------------------------------
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bootstrap @8 :Bootstrap; # Request the peer's bootstrap interface.
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call @2 :Call; # Begin a method call.
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return @3 :Return; # Complete a method call.
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finish @4 :Finish; # Release a returned answer / cancel a call.
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# Level 1 features -----------------------------------------------
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resolve @5 :Resolve; # Resolve a previously-sent promise.
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release @6 :Release; # Release a capability so that the remote object can be deallocated.
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disembargo @13 :Disembargo; # Lift an embargo used to enforce E-order over promise resolution.
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# Level 2 features -----------------------------------------------
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obsoleteSave @7 :AnyPointer;
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# Obsolete request to save a capability, resulting in a SturdyRef. This has been replaced
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# by the `Persistent` interface defined in `persistent.capnp`. This operation was never
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# implemented.
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obsoleteDelete @9 :AnyPointer;
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# Obsolete way to delete a SturdyRef. This operation was never implemented.
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# Level 3 features -----------------------------------------------
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provide @10 :Provide; # Provide a capability to a third party.
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accept @11 :Accept; # Accept a capability provided by a third party.
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# Level 4 features -----------------------------------------------
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join @12 :Join; # Directly connect to the common root of two or more proxied caps.
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}
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}
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# Level 0 message types ----------------------------------------------
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struct Bootstrap {
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# **(level 0)**
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#
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# Get the "bootstrap" interface exported by the remote vat.
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#
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# For level 0, 1, and 2 implementations, the "bootstrap" interface is simply the main interface
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# exported by a vat. If the vat acts as a server fielding connections from clients, then the
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# bootstrap interface defines the basic functionality available to a client when it connects.
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# The exact interface definition obviously depends on the application.
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#
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# We call this a "bootstrap" because in an ideal Cap'n Proto world, bootstrap interfaces would
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# never be used. In such a world, any time you connect to a new vat, you do so because you
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# received an introduction from some other vat (see `ThirdPartyCapId`). Thus, the first message
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# you send is `Accept`, and further communications derive from there. `Bootstrap` is not used.
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#
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# In such an ideal world, DNS itself would support Cap'n Proto -- performing a DNS lookup would
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# actually return a new Cap'n Proto capability, thus introducing you to the target system via
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# level 3 RPC. Applications would receive the capability to talk to DNS in the first place as
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# an initial endowment or part of a Powerbox interaction. Therefore, an app can form arbitrary
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# connections without ever using `Bootstrap`.
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#
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# Of course, in the real world, DNS is not Cap'n-Proto-based, and we don't want Cap'n Proto to
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# require a whole new internet infrastructure to be useful. Therefore, we offer bootstrap
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# interfaces as a way to get up and running without a level 3 introduction. Thus, bootstrap
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# interfaces are used to "bootstrap" from other, non-Cap'n-Proto-based means of service discovery,
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# such as legacy DNS.
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#
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# Note that a vat need not provide a bootstrap interface, and in fact many vats (especially those
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# acting as clients) do not. In this case, the vat should either reply to `Bootstrap` with a
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# `Return` indicating an exception, or should return a dummy capability with no methods.
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questionId @0 :QuestionId;
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# A new question ID identifying this request, which will eventually receive a Return message
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# containing the restored capability.
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deprecatedObjectId @1 :AnyPointer;
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# ** DEPRECATED **
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#
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# A Vat may export multiple bootstrap interfaces. In this case, `deprecatedObjectId` specifies
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# which one to return. If this pointer is null, then the default bootstrap interface is returned.
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#
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# As of verison 0.5, use of this field is deprecated. If a service wants to export multiple
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# bootstrap interfaces, it should instead define a single bootstarp interface that has methods
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# that return each of the other interfaces.
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#
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# **History**
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#
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# In the first version of Cap'n Proto RPC (0.4.x) the `Bootstrap` message was called `Restore`.
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# At the time, it was thought that this would eventually serve as the way to restore SturdyRefs
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# (level 2). Meanwhile, an application could offer its "main" interface on a well-known
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# (non-secret) SturdyRef.
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#
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# Since level 2 RPC was not implemented at the time, the `Restore` message was in practice only
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# used to obtain the main interface. Since most applications had only one main interface that
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# they wanted to restore, they tended to designate this with a null `objectId`.
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#
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# Unfortunately, the earliest version of the EZ RPC interfaces set a precedent of exporting
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# multiple main interfaces by allowing them to be exported under string names. In this case,
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# `objectId` was a Text value specifying the name.
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#
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# All of this proved problematic for several reasons:
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#
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# - The arrangement assumed that a client wishing to restore a SturdyRef would know exactly what
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# machine to connect to and would be able to immediately restore a SturdyRef on connection.
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# However, in practice, the ability to restore SturdyRefs is itself a capability that may
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# require going through an authentication process to obtain. Thus, it makes more sense to
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# define a "restorer service" as a full Cap'n Proto interface. If this restorer interface is
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# offered as the vat's bootstrap interface, then this is equivalent to the old arrangement.
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#
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# - Overloading "Restore" for the purpose of obtaining well-known capabilities encouraged the
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# practice of exporting singleton services with string names. If singleton services are desired,
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# it is better to have one main interface that has methods that can be used to obtain each
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# service, in order to get all the usual benefits of schemas and type checking.
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#
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# - Overloading "Restore" also had a security problem: Often, "main" or "well-known"
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# capabilities exported by a vat are in fact not public: they are intended to be accessed only
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# by clients who are capable of forming a connection to the vat. This can lead to trouble if
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# the client itself has other clients and wishes to foward some `Restore` requests from those
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# external clients -- it has to be very careful not to allow through `Restore` requests
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# addressing the default capability.
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#
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# For example, consider the case of a sandboxed Sandstorm application and its supervisor. The
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# application exports a default capability to its supervisor that provides access to
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# functionality that only the supervisor is supposed to access. Meanwhile, though, applications
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# may publish other capabilities that may be persistent, in which case the application needs
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# to field `Restore` requests that could come from anywhere. These requests of course have to
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# pass through the supervisor, as all communications with the outside world must. But, the
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# supervisor has to be careful not to honor an external request addressing the application's
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# default capability, since this capability is privileged. Unfortunately, the default
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# capability cannot be given an unguessable name, because then the supervisor itself would not
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# be able to address it!
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#
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# As of Cap'n Proto 0.5, `Restore` has been renamed to `Bootstrap` and is no longer planned for
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# use in restoring SturdyRefs.
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#
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# Note that 0.4 also defined a message type called `Delete` that, like `Restore`, addressed a
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# SturdyRef, but indicated that the client would not restore the ref again in the future. This
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# operation was never implemented, so it was removed entirely. If a "delete" operation is desired,
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# it should exist as a method on the same interface that handles restoring SturdyRefs. However,
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# the utility of such an operation is questionable. You wouldn't be able to rely on it for
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# garbage collection since a client could always disappear permanently without remembering to
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# delete all its SturdyRefs, thus leaving them dangling forever. Therefore, it is advisable to
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# design systems such that SturdyRefs never represent "owned" pointers.
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#
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# For example, say a SturdyRef points to an image file hosted on some server. That image file
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# should also live inside a collection (a gallery, perhaps) hosted on the same server, owned by
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# a user who can delete the image at any time. If the user deletes the image, the SturdyRef
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# stops working. On the other hand, if the SturdyRef is discarded, this has no effect on the
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# existence of the image in its collection.
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}
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struct Call {
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# **(level 0)**
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#
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# Message type initiating a method call on a capability.
|
|
|
|
questionId @0 :QuestionId;
|
|
# A number, chosen by the caller, that identifies this call in future messages. This number
|
|
# must be different from all other calls originating from the same end of the connection (but
|
|
# may overlap with question IDs originating from the opposite end). A fine strategy is to use
|
|
# sequential question IDs, but the recipient should not assume this.
|
|
#
|
|
# A question ID can be reused once both:
|
|
# - A matching Return has been received from the callee.
|
|
# - A matching Finish has been sent from the caller.
|
|
|
|
target @1 :MessageTarget;
|
|
# The object that should receive this call.
|
|
|
|
interfaceId @2 :UInt64;
|
|
# The type ID of the interface being called. Each capability may implement multiple interfaces.
|
|
|
|
methodId @3 :UInt16;
|
|
# The ordinal number of the method to call within the requested interface.
|
|
|
|
allowThirdPartyTailCall @8 :Bool = false;
|
|
# Indicates whether or not the receiver is allowed to send a `Return` containing
|
|
# `acceptFromThirdParty`. Level 3 implementations should set this true. Otherwise, the callee
|
|
# will have to proxy the return in the case of a tail call to a third-party vat.
|
|
|
|
params @4 :Payload;
|
|
# The call parameters. `params.content` is a struct whose fields correspond to the parameters of
|
|
# the method.
|
|
|
|
sendResultsTo :union {
|
|
# Where should the return message be sent?
|
|
|
|
caller @5 :Void;
|
|
# Send the return message back to the caller (the usual).
|
|
|
|
yourself @6 :Void;
|
|
# **(level 1)**
|
|
#
|
|
# Don't actually return the results to the sender. Instead, hold on to them and await
|
|
# instructions from the sender regarding what to do with them. In particular, the sender
|
|
# may subsequently send a `Return` for some other call (which the receiver had previously made
|
|
# to the sender) with `takeFromOtherQuestion` set. The results from this call are then used
|
|
# as the results of the other call.
|
|
#
|
|
# When `yourself` is used, the receiver must still send a `Return` for the call, but sets the
|
|
# field `resultsSentElsewhere` in that `Return` rather than including the results.
|
|
#
|
|
# This feature can be used to implement tail calls in which a call from Vat A to Vat B ends up
|
|
# returning the result of a call from Vat B back to Vat A.
|
|
#
|
|
# In particular, the most common use case for this feature is when Vat A makes a call to a
|
|
# promise in Vat B, and then that promise ends up resolving to a capability back in Vat A.
|
|
# Vat B must forward all the queued calls on that promise back to Vat A, but can set `yourself`
|
|
# in the calls so that the results need not pass back through Vat B.
|
|
#
|
|
# For example:
|
|
# - Alice, in Vat A, call foo() on Bob in Vat B.
|
|
# - Alice makes a pipelined call bar() on the promise returned by foo().
|
|
# - Later on, Bob resolves the promise from foo() to point at Carol, who lives in Vat A (next
|
|
# to Alice).
|
|
# - Vat B dutifully forwards the bar() call to Carol. Let us call this forwarded call bar'().
|
|
# Notice that bar() and bar'() are travelling in opposite directions on the same network
|
|
# link.
|
|
# - The `Call` for bar'() has `sendResultsTo` set to `yourself`, with the value being the
|
|
# question ID originally assigned to the bar() call.
|
|
# - Vat A receives bar'() and delivers it to Carol.
|
|
# - When bar'() returns, Vat A immediately takes the results and returns them from bar().
|
|
# - Meanwhile, Vat A sends a `Return` for bar'() to Vat B, with `resultsSentElsewhere` set in
|
|
# place of results.
|
|
# - Vat A sends a `Finish` for that call to Vat B.
|
|
# - Vat B receives the `Return` for bar'() and sends a `Return` for bar(), with
|
|
# `receivedFromYourself` set in place of the results.
|
|
# - Vat B receives the `Finish` for bar() and sends a `Finish` to bar'().
|
|
|
|
thirdParty @7 :RecipientId;
|
|
# **(level 3)**
|
|
#
|
|
# The call's result should be returned to a different vat. The receiver (the callee) expects
|
|
# to receive an `Accept` message from the indicated vat, and should return the call's result
|
|
# to it, rather than to the sender of the `Call`.
|
|
#
|
|
# This operates much like `yourself`, above, except that Carol is in a separate Vat C. `Call`
|
|
# messages are sent from Vat A -> Vat B and Vat B -> Vat C. A `Return` message is sent from
|
|
# Vat B -> Vat A that contains `acceptFromThirdParty` in place of results. When Vat A sends
|
|
# an `Accept` to Vat C, it receives back a `Return` containing the call's actual result. Vat C
|
|
# also sends a `Return` to Vat B with `resultsSentElsewhere`.
|
|
}
|
|
}
|
|
|
|
struct Return {
|
|
# **(level 0)**
|
|
#
|
|
# Message type sent from callee to caller indicating that the call has completed.
|
|
|
|
answerId @0 :AnswerId;
|
|
# Equal to the QuestionId of the corresponding `Call` message.
|
|
|
|
releaseParamCaps @1 :Bool = true;
|
|
# If true, all capabilities that were in the params should be considered released. The sender
|
|
# must not send separate `Release` messages for them. Level 0 implementations in particular
|
|
# should always set this true. This defaults true because if level 0 implementations forget to
|
|
# set it they'll never notice (just silently leak caps), but if level >=1 implementations forget
|
|
# to set it to false they'll quickly get errors.
|
|
|
|
union {
|
|
results @2 :Payload;
|
|
# The result.
|
|
#
|
|
# For regular method calls, `results.content` points to the result struct.
|
|
#
|
|
# For a `Return` in response to an `Accept`, `results` contains a single capability (rather
|
|
# than a struct), and `results.content` is just a capability pointer with index 0. A `Finish`
|
|
# is still required in this case.
|
|
|
|
exception @3 :Exception;
|
|
# Indicates that the call failed and explains why.
|
|
|
|
canceled @4 :Void;
|
|
# Indicates that the call was canceled due to the caller sending a Finish message
|
|
# before the call had completed.
|
|
|
|
resultsSentElsewhere @5 :Void;
|
|
# This is set when returning from a `Call` that had `sendResultsTo` set to something other
|
|
# than `caller`.
|
|
|
|
takeFromOtherQuestion @6 :QuestionId;
|
|
# The sender has also sent (before this message) a `Call` with the given question ID and with
|
|
# `sendResultsTo.yourself` set, and the results of that other call should be used as the
|
|
# results here.
|
|
|
|
acceptFromThirdParty @7 :ThirdPartyCapId;
|
|
# **(level 3)**
|
|
#
|
|
# The caller should contact a third-party vat to pick up the results. An `Accept` message
|
|
# sent to the vat will return the result. This pairs with `Call.sendResultsTo.thirdParty`.
|
|
# It should only be used if the corresponding `Call` had `allowThirdPartyTailCall` set.
|
|
}
|
|
}
|
|
|
|
struct Finish {
|
|
# **(level 0)**
|
|
#
|
|
# Message type sent from the caller to the callee to indicate:
|
|
# 1) The questionId will no longer be used in any messages sent by the callee (no further
|
|
# pipelined requests).
|
|
# 2) If the call has not returned yet, the caller no longer cares about the result. If nothing
|
|
# else cares about the result either (e.g. there are no other outstanding calls pipelined on
|
|
# the result of this one) then the callee may wish to immediately cancel the operation and
|
|
# send back a Return message with "canceled" set. However, implementations are not required
|
|
# to support premature cancellation -- instead, the implementation may wait until the call
|
|
# actually completes and send a normal `Return` message.
|
|
#
|
|
# TODO(someday): Should we separate (1) and implicitly releasing result capabilities? It would be
|
|
# possible and useful to notify the server that it doesn't need to keep around the response to
|
|
# service pipeline requests even though the caller still wants to receive it / hasn't yet
|
|
# finished processing it. It could also be useful to notify the server that it need not marshal
|
|
# the results because the caller doesn't want them anyway, even if the caller is still sending
|
|
# pipelined calls, although this seems less useful (just saving some bytes on the wire).
|
|
|
|
questionId @0 :QuestionId;
|
|
# ID of the call whose result is to be released.
|
|
|
|
releaseResultCaps @1 :Bool = true;
|
|
# If true, all capabilities that were in the results should be considered released. The sender
|
|
# must not send separate `Release` messages for them. Level 0 implementations in particular
|
|
# should always set this true. This defaults true because if level 0 implementations forget to
|
|
# set it they'll never notice (just silently leak caps), but if level >=1 implementations forget
|
|
# set it false they'll quickly get errors.
|
|
}
|
|
|
|
# Level 1 message types ----------------------------------------------
|
|
|
|
struct Resolve {
|
|
# **(level 1)**
|
|
#
|
|
# Message type sent to indicate that a previously-sent promise has now been resolved to some other
|
|
# object (possibly another promise) -- or broken, or canceled.
|
|
#
|
|
# Keep in mind that it's possible for a `Resolve` to be sent to a level 0 implementation that
|
|
# doesn't implement it. For example, a method call or return might contain a capability in the
|
|
# payload. Normally this is fine even if the receiver is level 0, because they will implicitly
|
|
# release all such capabilities on return / finish. But if the cap happens to be a promise, then
|
|
# a follow-up `Resolve` may be sent regardless of this release. The level 0 receiver will reply
|
|
# with an `unimplemented` message, and the sender (of the `Resolve`) can respond to this as if the
|
|
# receiver had immediately released any capability to which the promise resolved.
|
|
#
|
|
# When implementing promise resolution, it's important to understand how embargos work and the
|
|
# tricky case of the Tribble 4-way race condition. See the comments for the Disembargo message,
|
|
# below.
|
|
|
|
promiseId @0 :ExportId;
|
|
# The ID of the promise to be resolved.
|
|
#
|
|
# Unlike all other instances of `ExportId` sent from the exporter, the `Resolve` message does
|
|
# _not_ increase the reference count of `promiseId`. In fact, it is expected that the receiver
|
|
# will release the export soon after receiving `Resolve`, and the sender will not send this
|
|
# `ExportId` again until it has been released and recycled.
|
|
#
|
|
# When an export ID sent over the wire (e.g. in a `CapDescriptor`) is indicated to be a promise,
|
|
# this indicates that the sender will follow up at some point with a `Resolve` message. If the
|
|
# same `promiseId` is sent again before `Resolve`, still only one `Resolve` is sent. If the
|
|
# same ID is sent again later _after_ a `Resolve`, it can only be because the export's
|
|
# reference count hit zero in the meantime and the ID was re-assigned to a new export, therefore
|
|
# this later promise does _not_ correspond to the earlier `Resolve`.
|
|
#
|
|
# If a promise ID's reference count reaches zero before a `Resolve` is sent, the `Resolve`
|
|
# message may or may not still be sent (the `Resolve` may have already been in-flight when
|
|
# `Release` was sent, but if the `Release` is received before `Resolve` then there is no longer
|
|
# any reason to send a `Resolve`). Thus a `Resolve` may be received for a promise of which
|
|
# the receiver has no knowledge, because it already released it earlier. In this case, the
|
|
# receiver should simply release the capability to which the promise resolved.
|
|
|
|
union {
|
|
cap @1 :CapDescriptor;
|
|
# The object to which the promise resolved.
|
|
#
|
|
# The sender promises that from this point forth, until `promiseId` is released, it shall
|
|
# simply forward all messages to the capability designated by `cap`. This is true even if
|
|
# `cap` itself happens to desigate another promise, and that other promise later resolves --
|
|
# messages sent to `promiseId` shall still go to that other promise, not to its resolution.
|
|
# This is important in the case that the receiver of the `Resolve` ends up sending a
|
|
# `Disembargo` message towards `promiseId` in order to control message ordering -- that
|
|
# `Disembargo` really needs to reflect back to exactly the object designated by `cap` even
|
|
# if that object is itself a promise.
|
|
|
|
exception @2 :Exception;
|
|
# Indicates that the promise was broken.
|
|
}
|
|
}
|
|
|
|
struct Release {
|
|
# **(level 1)**
|
|
#
|
|
# Message type sent to indicate that the sender is done with the given capability and the receiver
|
|
# can free resources allocated to it.
|
|
|
|
id @0 :ImportId;
|
|
# What to release.
|
|
|
|
referenceCount @1 :UInt32;
|
|
# The amount by which to decrement the reference count. The export is only actually released
|
|
# when the reference count reaches zero.
|
|
}
|
|
|
|
struct Disembargo {
|
|
# **(level 1)**
|
|
#
|
|
# Message sent to indicate that an embargo on a recently-resolved promise may now be lifted.
|
|
#
|
|
# Embargos are used to enforce E-order in the presence of promise resolution. That is, if an
|
|
# application makes two calls foo() and bar() on the same capability reference, in that order,
|
|
# the calls should be delivered in the order in which they were made. But if foo() is called
|
|
# on a promise, and that promise happens to resolve before bar() is called, then the two calls
|
|
# may travel different paths over the network, and thus could arrive in the wrong order. In
|
|
# this case, the call to `bar()` must be embargoed, and a `Disembargo` message must be sent along
|
|
# the same path as `foo()` to ensure that the `Disembargo` arrives after `foo()`. Once the
|
|
# `Disembargo` arrives, `bar()` can then be delivered.
|
|
#
|
|
# There are two particular cases where embargos are important. Consider object Alice, in Vat A,
|
|
# who holds a promise P, pointing towards Vat B, that eventually resolves to Carol. The two
|
|
# cases are:
|
|
# - Carol lives in Vat A, i.e. next to Alice. In this case, Vat A needs to send a `Disembargo`
|
|
# message that echos through Vat B and back, to ensure that all pipelined calls on the promise
|
|
# have been delivered.
|
|
# - Carol lives in a different Vat C. When the promise resolves, a three-party handoff occurs
|
|
# (see `Provide` and `Accept`, which constitute level 3 of the protocol). In this case, we
|
|
# piggyback on the state that has already been set up to handle the handoff: the `Accept`
|
|
# message (from Vat A to Vat C) is embargoed, as are all pipelined messages sent to it, while
|
|
# a `Disembargo` message is sent from Vat A through Vat B to Vat C. See `Accept.embargo` for
|
|
# an example.
|
|
#
|
|
# Note that in the case where Carol actually lives in Vat B (i.e., the same vat that the promise
|
|
# already pointed at), no embargo is needed, because the pipelined calls are delivered over the
|
|
# same path as the later direct calls.
|
|
#
|
|
# Keep in mind that promise resolution happens both in the form of Resolve messages as well as
|
|
# Return messages (which resolve PromisedAnswers). Embargos apply in both cases.
|
|
#
|
|
# An alternative strategy for enforcing E-order over promise resolution could be for Vat A to
|
|
# implement the embargo internally. When Vat A is notified of promise resolution, it could
|
|
# send a dummy no-op call to promise P and wait for it to complete. Until that call completes,
|
|
# all calls to the capability are queued locally. This strategy works, but is pessimistic:
|
|
# in the three-party case, it requires an A -> B -> C -> B -> A round trip before calls can start
|
|
# being delivered directly to from Vat A to Vat C. The `Disembargo` message allows latency to be
|
|
# reduced. (In the two-party loopback case, the `Disembargo` message is just a more explicit way
|
|
# of accomplishing the same thing as a no-op call, but isn't any faster.)
|
|
#
|
|
# *The Tribble 4-way Race Condition*
|
|
#
|
|
# Any implementation of promise resolution and embargos must be aware of what we call the
|
|
# "Tribble 4-way race condition", after Dean Tribble, who explained the problem in a lively
|
|
# Friam meeting.
|
|
#
|
|
# Embargos are designed to work in the case where a two-hop path is being shortened to one hop.
|
|
# But sometimes there are more hops. Imagine that Alice has a reference to a remote promise P1
|
|
# that eventually resolves to _another_ remote promise P2 (in a third vat), which _at the same
|
|
# time_ happens to resolve to Bob (in a fourth vat). In this case, we're shortening from a 3-hop
|
|
# path (with four parties) to a 1-hop path (Alice -> Bob).
|
|
#
|
|
# Extending the embargo/disembargo protocol to be able to shorted multiple hops at once seems
|
|
# difficult. Instead, we make a rule that prevents this case from coming up:
|
|
#
|
|
# One a promise P has been resolved to a remove object reference R, then all further messages
|
|
# received addressed to P will be forwarded strictly to R. Even if it turns out later that R is
|
|
# itself a promise, and has resolved to some other object Q, messages sent to P will still be
|
|
# forwarded to R, not directly to Q (R will of course further forward the messages to Q).
|
|
#
|
|
# This rule does not cause a significant performance burden because once P has resolved to R, it
|
|
# is expected that people sending messages to P will shortly start sending them to R instead and
|
|
# drop P. P is at end-of-life anyway, so it doesn't matter if it ignores chances to further
|
|
# optimize its path.
|
|
|
|
target @0 :MessageTarget;
|
|
# What is to be disembargoed.
|
|
|
|
using EmbargoId = UInt32;
|
|
# Used in `senderLoopback` and `receiverLoopback`, below.
|
|
|
|
context :union {
|
|
senderLoopback @1 :EmbargoId;
|
|
# The sender is requesting a disembargo on a promise that is known to resolve back to a
|
|
# capability hosted by the sender. As soon as the receiver has echoed back all pipelined calls
|
|
# on this promise, it will deliver the Disembargo back to the sender with `receiverLoopback`
|
|
# set to the same value as `senderLoopback`. This value is chosen by the sender, and since
|
|
# it is also consumed be the sender, the sender can use whatever strategy it wants to make sure
|
|
# the value is unambiguous.
|
|
#
|
|
# The receiver must verify that the target capability actually resolves back to the sender's
|
|
# vat. Otherwise, the sender has committed a protocol error and should be disconnected.
|
|
|
|
receiverLoopback @2 :EmbargoId;
|
|
# The receiver previously sent a `senderLoopback` Disembargo towards a promise resolving to
|
|
# this capability, and that Disembargo is now being echoed back.
|
|
|
|
accept @3 :Void;
|
|
# **(level 3)**
|
|
#
|
|
# The sender is requesting a disembargo on a promise that is known to resolve to a third-party
|
|
# capability that the sender is currently in the process of accepting (using `Accept`).
|
|
# The receiver of this `Disembargo` has an outstanding `Provide` on said capability. The
|
|
# receiver should now send a `Disembargo` with `provide` set to the question ID of that
|
|
# `Provide` message.
|
|
#
|
|
# See `Accept.embargo` for an example.
|
|
|
|
provide @4 :QuestionId;
|
|
# **(level 3)**
|
|
#
|
|
# The sender is requesting a disembargo on a capability currently being provided to a third
|
|
# party. The question ID identifies the `Provide` message previously sent by the sender to
|
|
# this capability. On receipt, the receiver (the capability host) shall release the embargo
|
|
# on the `Accept` message that it has received from the third party. See `Accept.embargo` for
|
|
# an example.
|
|
}
|
|
}
|
|
|
|
# Level 2 message types ----------------------------------------------
|
|
|
|
# See persistent.capnp.
|
|
|
|
# Level 3 message types ----------------------------------------------
|
|
|
|
struct Provide {
|
|
# **(level 3)**
|
|
#
|
|
# Message type sent to indicate that the sender wishes to make a particular capability implemented
|
|
# by the receiver available to a third party for direct access (without the need for the third
|
|
# party to proxy through the sender).
|
|
#
|
|
# (In CapTP, `Provide` and `Accept` are methods of the global `NonceLocator` object exported by
|
|
# every vat. In Cap'n Proto, we bake this into the core protocol.)
|
|
|
|
questionId @0 :QuestionId;
|
|
# Question ID to be held open until the recipient has received the capability. A result will be
|
|
# returned once the third party has successfully received the capability. The sender must at some
|
|
# point send a `Finish` message as with any other call, and that message can be used to cancel the
|
|
# whole operation.
|
|
|
|
target @1 :MessageTarget;
|
|
# What is to be provided to the third party.
|
|
|
|
recipient @2 :RecipientId;
|
|
# Identity of the third party that is expected to pick up the capability.
|
|
}
|
|
|
|
struct Accept {
|
|
# **(level 3)**
|
|
#
|
|
# Message type sent to pick up a capability hosted by the receiving vat and provided by a third
|
|
# party. The third party previously designated the capability using `Provide`.
|
|
#
|
|
# This message is also used to pick up a redirected return -- see `Return.redirect`.
|
|
|
|
questionId @0 :QuestionId;
|
|
# A new question ID identifying this accept message, which will eventually receive a Return
|
|
# message containing the provided capability (or the call result in the case of a redirected
|
|
# return).
|
|
|
|
provision @1 :ProvisionId;
|
|
# Identifies the provided object to be picked up.
|
|
|
|
embargo @2 :Bool;
|
|
# If true, this accept shall be temporarily embargoed. The resulting `Return` will not be sent,
|
|
# and any pipelined calls will not be delivered, until the embargo is released. The receiver
|
|
# (the capability host) will expect the provider (the vat that sent the `Provide` message) to
|
|
# eventually send a `Disembargo` message with the field `context.provide` set to the question ID
|
|
# of the original `Provide` message. At that point, the embargo is released and the queued
|
|
# messages are delivered.
|
|
#
|
|
# For example:
|
|
# - Alice, in Vat A, holds a promise P, which currently points toward Vat B.
|
|
# - Alice calls foo() on P. The `Call` message is sent to Vat B.
|
|
# - The promise P in Vat B ends up resolving to Carol, in Vat C.
|
|
# - Vat B sends a `Provide` message to Vat C, identifying Vat A as the recipient.
|
|
# - Vat B sends a `Resolve` message to Vat A, indicating that the promise has resolved to a
|
|
# `ThirdPartyCapId` identifying Carol in Vat C.
|
|
# - Vat A sends an `Accept` message to Vat C to pick up the capability. Since Vat A knows that
|
|
# it has an outstanding call to the promise, it sets `embargo` to `true` in the `Accept`
|
|
# message.
|
|
# - Vat A sends a `Disembargo` message to Vat B on promise P, with `context.accept` set.
|
|
# - Alice makes a call bar() to promise P, which is now pointing towards Vat C. Alice doesn't
|
|
# know anything about the mechanics of promise resolution happening under the hood, but she
|
|
# expects that bar() will be delivered after foo() because that is the order in which she
|
|
# initiated the calls.
|
|
# - Vat A sends the bar() call to Vat C, as a pipelined call on the result of the `Accept` (which
|
|
# hasn't returned yet, due to the embargo). Since calls to the newly-accepted capability
|
|
# are embargoed, Vat C does not deliver the call yet.
|
|
# - At some point, Vat B forwards the foo() call from the beginning of this example on to Vat C.
|
|
# - Vat B forwards the `Disembargo` from Vat A on to vat C. It sets `context.provide` to the
|
|
# question ID of the `Provide` message it had sent previously.
|
|
# - Vat C receives foo() before `Disembargo`, thus allowing it to correctly deliver foo()
|
|
# before delivering bar().
|
|
# - Vat C receives `Disembargo` from Vat B. It can now send a `Return` for the `Accept` from
|
|
# Vat A, as well as deliver bar().
|
|
}
|
|
|
|
# Level 4 message types ----------------------------------------------
|
|
|
|
struct Join {
|
|
# **(level 4)**
|
|
#
|
|
# Message type sent to implement E.join(), which, given a number of capabilities that are
|
|
# expected to be equivalent, finds the underlying object upon which they all agree and forms a
|
|
# direct connection to it, skipping any proxies that may have been constructed by other vats
|
|
# while transmitting the capability. See:
|
|
# http://erights.org/elib/equality/index.html
|
|
#
|
|
# Note that this should only serve to bypass fully-transparent proxies -- proxies that were
|
|
# created merely for convenience, without any intention of hiding the underlying object.
|
|
#
|
|
# For example, say Bob holds two capabilities hosted by Alice and Carol, but he expects that both
|
|
# are simply proxies for a capability hosted elsewhere. He then issues a join request, which
|
|
# operates as follows:
|
|
# - Bob issues Join requests on both Alice and Carol. Each request contains a different piece
|
|
# of the JoinKey.
|
|
# - Alice is proxying a capability hosted by Dana, so forwards the request to Dana's cap.
|
|
# - Dana receives the first request and sees that the JoinKeyPart is one of two. She notes that
|
|
# she doesn't have the other part yet, so she records the request and responds with a
|
|
# JoinResult.
|
|
# - Alice relays the JoinAswer back to Bob.
|
|
# - Carol is also proxying a capability from Dana, and so forwards her Join request to Dana as
|
|
# well.
|
|
# - Dana receives Carol's request and notes that she now has both parts of a JoinKey. She
|
|
# combines them in order to form information needed to form a secure connection to Bob. She
|
|
# also responds with another JoinResult.
|
|
# - Bob receives the responses from Alice and Carol. He uses the returned JoinResults to
|
|
# determine how to connect to Dana and attempts to form the connection. Since Bob and Dana now
|
|
# agree on a secret key that neither Alice nor Carol ever saw, this connection can be made
|
|
# securely even if Alice or Carol is conspiring against the other. (If Alice and Carol are
|
|
# conspiring _together_, they can obviously reproduce the key, but this doesn't matter because
|
|
# the whole point of the join is to verify that Alice and Carol agree on what capability they
|
|
# are proxying.)
|
|
#
|
|
# If the two capabilities aren't actually proxies of the same object, then the join requests
|
|
# will come back with conflicting `hostId`s and the join will fail before attempting to form any
|
|
# connection.
|
|
|
|
questionId @0 :QuestionId;
|
|
# Question ID used to respond to this Join. (Note that this ID only identifies one part of the
|
|
# request for one hop; each part has a different ID and relayed copies of the request have
|
|
# (probably) different IDs still.)
|
|
#
|
|
# The receiver will reply with a `Return` whose `results` is a JoinResult. This `JoinResult`
|
|
# is relayed from the joined object's host, possibly with transformation applied as needed
|
|
# by the network.
|
|
#
|
|
# Like any return, the result must be released using a `Finish`. However, this release
|
|
# should not occur until the joiner has either successfully connected to the joined object.
|
|
# Vats relaying a `Join` message similarly must not release the result they receive until the
|
|
# return they relayed back towards the joiner has itself been released. This allows the
|
|
# joined object's host to detect when the Join operation is canceled before completing -- if
|
|
# it receives a `Finish` for one of the join results before the joiner successfully
|
|
# connects. It can then free any resources it had allocated as part of the join.
|
|
|
|
target @1 :MessageTarget;
|
|
# The capability to join.
|
|
|
|
keyPart @2 :JoinKeyPart;
|
|
# A part of the join key. These combine to form the complete join key, which is used to establish
|
|
# a direct connection.
|
|
|
|
# TODO(before implementing): Change this so that multiple parts can be sent in a single Join
|
|
# message, so that if multiple join parts are going to cross the same connection they can be sent
|
|
# together, so that the receive can potentially optimize its handling of them. In the case where
|
|
# all parts are bundled together, should the recipient be expected to simply return a cap, so
|
|
# that the caller can immediately start pipelining to it?
|
|
}
|
|
|
|
# ========================================================================================
|
|
# Common structures used in messages
|
|
|
|
struct MessageTarget {
|
|
# The target of a `Call` or other messages that target a capability.
|
|
|
|
union {
|
|
importedCap @0 :ImportId;
|
|
# This message is to a capability or promise previously imported by the caller (exported by
|
|
# the receiver).
|
|
|
|
promisedAnswer @1 :PromisedAnswer;
|
|
# This message is to a capability that is expected to be returned by another call that has not
|
|
# yet been completed.
|
|
#
|
|
# At level 0, this is supported only for addressing the result of a previous `Bootstrap`, so
|
|
# that initial startup doesn't require a round trip.
|
|
}
|
|
}
|
|
|
|
struct Payload {
|
|
# Represents some data structure that might contain capabilities.
|
|
|
|
content @0 :AnyPointer;
|
|
# Some Cap'n Proto data structure. Capability pointers embedded in this structure index into
|
|
# `capTable`.
|
|
|
|
capTable @1 :List(CapDescriptor);
|
|
# Descriptors corresponding to the cap pointers in `content`.
|
|
}
|
|
|
|
struct CapDescriptor {
|
|
# **(level 1)**
|
|
#
|
|
# When an application-defined type contains an interface pointer, that pointer contains an index
|
|
# into the message's capability table -- i.e. the `capTable` part of the `Payload`. Each
|
|
# capability in the table is represented as a `CapDescriptor`. The runtime API should not reveal
|
|
# the CapDescriptor directly to the application, but should instead wrap it in some kind of
|
|
# callable object with methods corresponding to the interface that the capability implements.
|
|
#
|
|
# Keep in mind that `ExportIds` in a `CapDescriptor` are subject to reference counting. See the
|
|
# description of `ExportId`.
|
|
|
|
union {
|
|
none @0 :Void;
|
|
# There is no capability here. This `CapDescriptor` should not appear in the payload content.
|
|
# A `none` CapDescriptor can be generated when an application inserts a capability into a
|
|
# message and then later changes its mind and removes it -- rewriting all of the other
|
|
# capability pointers may be hard, so instead a tombstone is left, similar to the way a removed
|
|
# struct or list instance is zeroed out of the message but the space is not reclaimed.
|
|
# Hopefully this is unusual.
|
|
|
|
senderHosted @1 :ExportId;
|
|
# A capability newly exported by the sender. This is the ID of the new capability in the
|
|
# sender's export table (receiver's import table).
|
|
|
|
senderPromise @2 :ExportId;
|
|
# A promise that the sender will resolve later. The sender will send exactly one Resolve
|
|
# message at a future point in time to replace this promise. Note that even if the same
|
|
# `senderPromise` is received multiple times, only one `Resolve` is sent to cover all of
|
|
# them. If `senderPromise` is released before the `Resolve` is sent, the sender (of this
|
|
# `CapDescriptor`) may choose not to send the `Resolve` at all.
|
|
|
|
receiverHosted @3 :ImportId;
|
|
# A capability (or promise) previously exported by the receiver (imported by the sender).
|
|
|
|
receiverAnswer @4 :PromisedAnswer;
|
|
# A capability expected to be returned in the results of a currently-outstanding call posed
|
|
# by the sender.
|
|
|
|
thirdPartyHosted @5 :ThirdPartyCapDescriptor;
|
|
# **(level 3)**
|
|
#
|
|
# A capability that lives in neither the sender's nor the receiver's vat. The sender needs
|
|
# to form a direct connection to a third party to pick up the capability.
|
|
#
|
|
# Level 1 and 2 implementations that receive a `thirdPartyHosted` may simply send calls to its
|
|
# `vine` instead.
|
|
}
|
|
}
|
|
|
|
struct PromisedAnswer {
|
|
# **(mostly level 1)**
|
|
#
|
|
# Specifies how to derive a promise from an unanswered question, by specifying the path of fields
|
|
# to follow from the root of the eventual result struct to get to the desired capability. Used
|
|
# to address method calls to a not-yet-returned capability or to pass such a capability as an
|
|
# input to some other method call.
|
|
#
|
|
# Level 0 implementations must support `PromisedAnswer` only for the case where the answer is
|
|
# to a `Bootstrap` message. In this case, `path` is always empty since `Bootstrap` always returns
|
|
# a raw capability.
|
|
|
|
questionId @0 :QuestionId;
|
|
# ID of the question (in the sender's question table / receiver's answer table) whose answer is
|
|
# expected to contain the capability.
|
|
|
|
transform @1 :List(Op);
|
|
# Operations / transformations to apply to the result in order to get the capability actually
|
|
# being addressed. E.g. if the result is a struct and you want to call a method on a capability
|
|
# pointed to by a field of the struct, you need a `getPointerField` op.
|
|
|
|
struct Op {
|
|
union {
|
|
noop @0 :Void;
|
|
# Does nothing. This member is mostly defined so that we can make `Op` a union even
|
|
# though (as of this writing) only one real operation is defined.
|
|
|
|
getPointerField @1 :UInt16;
|
|
# Get a pointer field within a struct. The number is an index into the pointer section, NOT
|
|
# a field ordinal, so that the receiver does not need to understand the schema.
|
|
|
|
# TODO(someday): We could add:
|
|
# - For lists, the ability to address every member of the list, or a slice of the list, the
|
|
# result of which would be another list. This is useful for implementing the equivalent of
|
|
# a SQL table join (not to be confused with the `Join` message type).
|
|
# - Maybe some ability to test a union.
|
|
# - Probably not a good idea: the ability to specify an arbitrary script to run on the
|
|
# result. We could define a little stack-based language where `Op` specifies one
|
|
# "instruction" or transformation to apply. Although this is not a good idea
|
|
# (over-engineered), any narrower additions to `Op` should be designed as if this
|
|
# were the eventual goal.
|
|
}
|
|
}
|
|
}
|
|
|
|
struct ThirdPartyCapDescriptor {
|
|
# **(level 3)**
|
|
#
|
|
# Identifies a capability in a third-party vat that the sender wants the receiver to pick up.
|
|
|
|
id @0 :ThirdPartyCapId;
|
|
# Identifies the third-party host and the specific capability to accept from it.
|
|
|
|
vineId @1 :ExportId;
|
|
# A proxy for the third-party object exported by the sender. In CapTP terminology this is called
|
|
# a "vine", because it is an indirect reference to the third-party object that snakes through the
|
|
# sender vat. This serves two purposes:
|
|
#
|
|
# * Level 1 and 2 implementations that don't understand how to connect to a third party may
|
|
# simply send calls to the vine. Such calls will be forwarded to the third-party by the
|
|
# sender.
|
|
#
|
|
# * Level 3 implementations must release the vine once they have successfully picked up the
|
|
# object from the third party. This ensures that the capability is not released by the sender
|
|
# prematurely.
|
|
#
|
|
# The sender will close the `Provide` request that it has sent to the third party as soon as
|
|
# it receives either a `Call` or a `Release` message directed at the vine.
|
|
}
|
|
|
|
struct Exception {
|
|
# **(level 0)**
|
|
#
|
|
# Describes an arbitrary error that prevented an operation (e.g. a call) from completing.
|
|
#
|
|
# Cap'n Proto exceptions always indicate that something went wrong. In other words, in a fantasy
|
|
# world where everything always works as expected, no exceptions would ever be thrown. Clients
|
|
# should only ever catch exceptions as a means to implement fault-tolerance, where "fault" can
|
|
# mean:
|
|
# - Bugs.
|
|
# - Invalid input.
|
|
# - Configuration errors.
|
|
# - Network problems.
|
|
# - Insufficient resources.
|
|
# - Version skew (unimplemented functionality).
|
|
# - Other logistical problems.
|
|
#
|
|
# Exceptions should NOT be used to flag application-specific conditions that a client is expected
|
|
# to handle in an application-specific way. Put another way, in the Cap'n Proto world,
|
|
# "checked exceptions" (where an interface explicitly defines the exceptions it throws and
|
|
# clients are forced by the type system to handle those exceptions) do NOT make sense.
|
|
|
|
reason @0 :Text;
|
|
# Human-readable failure description.
|
|
|
|
type @3 :Type;
|
|
# The type of the error. The purpose of this enum is not to describe the error itself, but
|
|
# rather to describe how the client might want to respond to the error.
|
|
|
|
enum Type {
|
|
failed @0;
|
|
# A generic problem occurred, and it is believed that if the operation were repeated without
|
|
# any change in the state of the world, the problem would occur again.
|
|
#
|
|
# A client might respond to this error by logging it for investigation by the developer and/or
|
|
# displaying it to the user.
|
|
|
|
overloaded @1;
|
|
# The request was rejected due to a temporary lack of resources.
|
|
#
|
|
# Examples include:
|
|
# - There's not enough CPU time to keep up with incoming requests, so some are rejected.
|
|
# - The server ran out of RAM or disk space during the request.
|
|
# - The operation timed out (took significantly longer than it should have).
|
|
#
|
|
# A client might respond to this error by scheduling to retry the operation much later. The
|
|
# client should NOT retry again immediately since this would likely exacerbate the problem.
|
|
|
|
disconnected @2;
|
|
# The method failed because a connection to some necessary capability was lost.
|
|
#
|
|
# Examples include:
|
|
# - The client introduced the server to a third-party capability, the connection to that third
|
|
# party was subsequently lost, and then the client requested that the server use the dead
|
|
# capability for something.
|
|
# - The client previously requested that the server obtain a capability from some third party.
|
|
# The server returned a capability to an object wrapping the third-party capability. Later,
|
|
# the server's connection to the third party was lost.
|
|
# - The capability has been revoked. Revocation does not necessarily mean that the client is
|
|
# no longer authorized to use the capability; it is often used simply as a way to force the
|
|
# client to repeat the setup process, perhaps to efficiently move them to a new back-end or
|
|
# get them to recognize some other change that has occurred.
|
|
#
|
|
# A client should normally respond to this error by releasing all capabilities it is currently
|
|
# holding related to the one it called and then re-creating them by restoring SturdyRefs and/or
|
|
# repeating the method calls used to create them originally. In other words, disconnect and
|
|
# start over. This should in turn cause the server to obtain a new copy of the capability that
|
|
# it lost, thus making everything work.
|
|
#
|
|
# If the client receives another `disconnencted` error in the process of rebuilding the
|
|
# capability and retrying the call, it should treat this as an `overloaded` error: the network
|
|
# is currently unreliable, possibly due to load or other temporary issues.
|
|
|
|
unimplemented @3;
|
|
# The server doesn't implement the requested method. If there is some other method that the
|
|
# client could call (perhaps an older and/or slower interface), it should try that instead.
|
|
# Otherwise, this should be treated like `failed`.
|
|
}
|
|
|
|
obsoleteIsCallersFault @1 :Bool;
|
|
# OBSOLETE. Ignore.
|
|
|
|
obsoleteDurability @2 :UInt16;
|
|
# OBSOLETE. See `type` instead.
|
|
}
|
|
|
|
# ========================================================================================
|
|
# Network-specific Parameters
|
|
#
|
|
# Some parts of the Cap'n Proto RPC protocol are not specified here because different vat networks
|
|
# may wish to use different approaches to solving them. For example, on the public internet, you
|
|
# may want to authenticate vats using public-key cryptography, but on a local intranet with trusted
|
|
# infrastructure, you may be happy to authenticate based on network address only, or some other
|
|
# lightweight mechanism.
|
|
#
|
|
# To accommodate this, we specify several "parameter" types. Each type is defined here as an
|
|
# alias for `AnyPointer`, but a specific network will want to define a specific set of types to use.
|
|
# All vats in a vat network must agree on these parameters in order to be able to communicate.
|
|
# Inter-network communication can be accomplished through "gateways" that perform translation
|
|
# between the primitives used on each network; these gateways may need to be deeply stateful,
|
|
# depending on the translations they perform.
|
|
#
|
|
# For interaction over the global internet between parties with no other prior arrangement, a
|
|
# particular set of bindings for these types is defined elsewhere. (TODO(someday): Specify where
|
|
# these common definitions live.)
|
|
#
|
|
# Another common network type is the two-party network, in which one of the parties typically
|
|
# interacts with the outside world entirely through the other party. In such a connection between
|
|
# Alice and Bob, all objects that exist on Bob's other networks appear to Alice as if they were
|
|
# hosted by Bob himself, and similarly all objects on Alice's network (if she even has one) appear
|
|
# to Bob as if they were hosted by Alice. This network type is interesting because from the point
|
|
# of view of a simple application that communicates with only one other party via the two-party
|
|
# protocol, there are no three-party interactions at all, and joins are unusually simple to
|
|
# implement, so implementing at level 4 is barely more complicated than implementing at level 1.
|
|
# Moreover, if you pair an app implementing the two-party network with a container that implements
|
|
# some other network, the app can then participate on the container's network just as if it
|
|
# implemented that network directly. The types used by the two-party network are defined in
|
|
# `rpc-twoparty.capnp`.
|
|
#
|
|
# The things that we need to parameterize are:
|
|
# - How to store capabilities long-term without holding a connection open (mostly level 2).
|
|
# - How to authenticate vats in three-party introductions (level 3).
|
|
# - How to implement `Join` (level 4).
|
|
#
|
|
# Persistent references
|
|
# ---------------------
|
|
#
|
|
# **(mostly level 2)**
|
|
#
|
|
# We want to allow some capabilities to be stored long-term, even if a connection is lost and later
|
|
# recreated. ExportId is a short-term identifier that is specific to a connection, so it doesn't
|
|
# help here. We need a way to specify long-term identifiers, as well as a strategy for
|
|
# reconnecting to a referenced capability later.
|
|
#
|
|
# Three-party interactions
|
|
# ------------------------
|
|
#
|
|
# **(level 3)**
|
|
#
|
|
# In cases where more than two vats are interacting, we have situations where VatA holds a
|
|
# capability hosted by VatB and wants to send that capability to VatC. This can be accomplished
|
|
# by VatA proxying requests on the new capability, but doing so has two big problems:
|
|
# - It's inefficient, requiring an extra network hop.
|
|
# - If VatC receives another capability to the same object from VatD, it is difficult for VatC to
|
|
# detect that the two capabilities are really the same and to implement the E "join" operation,
|
|
# which is necessary for certain four-or-more-party interactions, such as the escrow pattern.
|
|
# See: http://www.erights.org/elib/equality/grant-matcher/index.html
|
|
#
|
|
# Instead, we want a way for VatC to form a direct, authenticated connection to VatB.
|
|
#
|
|
# Join
|
|
# ----
|
|
#
|
|
# **(level 4)**
|
|
#
|
|
# The `Join` message type and corresponding operation arranges for a direct connection to be formed
|
|
# between the joiner and the host of the joined object, and this connection must be authenticated.
|
|
# Thus, the details are network-dependent.
|
|
|
|
using SturdyRef = AnyPointer;
|
|
# **(level 2)**
|
|
#
|
|
# Identifies a persisted capability that can be restored in the future. How exactly a SturdyRef
|
|
# is restored to a live object is specified along with the SturdyRef definition (i.e. not by
|
|
# rpc.capnp).
|
|
#
|
|
# Generally a SturdyRef needs to specify three things:
|
|
# - How to reach the vat that can restore the ref (e.g. a hostname or IP address).
|
|
# - How to authenticate the vat after connecting (e.g. a public key fingerprint).
|
|
# - The identity of a specific object hosted by the vat. Generally, this is an opaque pointer whose
|
|
# format is defined by the specific vat -- the client has no need to inspect the object ID.
|
|
# It is important that the objec ID be unguessable if the object is not public (and objects
|
|
# should almost never be public).
|
|
#
|
|
# The above are only suggestions. Some networks might work differently. For example, a private
|
|
# network might employ a special restorer service whose sole purpose is to restore SturdyRefs.
|
|
# In this case, the entire contents of SturdyRef might be opaque, because they are intended only
|
|
# to be forwarded to the restorer service.
|
|
|
|
using ProvisionId = AnyPointer;
|
|
# **(level 3)**
|
|
#
|
|
# The information that must be sent in an `Accept` message to identify the object being accepted.
|
|
#
|
|
# In a network where each vat has a public/private key pair, this could simply be the public key
|
|
# fingerprint of the provider vat along with the question ID used in the `Provide` message sent from
|
|
# that provider.
|
|
|
|
using RecipientId = AnyPointer;
|
|
# **(level 3)**
|
|
#
|
|
# The information that must be sent in a `Provide` message to identify the recipient of the
|
|
# capability.
|
|
#
|
|
# In a network where each vat has a public/private key pair, this could simply be the public key
|
|
# fingerprint of the recipient. (CapTP also calls for a nonce to identify the object. In our
|
|
# case, the `Provide` message's `questionId` can serve as the nonce.)
|
|
|
|
using ThirdPartyCapId = AnyPointer;
|
|
# **(level 3)**
|
|
#
|
|
# The information needed to connect to a third party and accept a capability from it.
|
|
#
|
|
# In a network where each vat has a public/private key pair, this could be a combination of the
|
|
# third party's public key fingerprint, hints on how to connect to the third party (e.g. an IP
|
|
# address), and the question ID used in the corresponding `Provide` message sent to that third party
|
|
# (used to identify which capability to pick up).
|
|
|
|
using JoinKeyPart = AnyPointer;
|
|
# **(level 4)**
|
|
#
|
|
# A piece of a secret key. One piece is sent along each path that is expected to lead to the same
|
|
# place. Once the pieces are combined, a direct connection may be formed between the sender and
|
|
# the receiver, bypassing any men-in-the-middle along the paths. See the `Join` message type.
|
|
#
|
|
# The motivation for Joins is discussed under "Supporting Equality" in the "Unibus" protocol
|
|
# sketch: http://www.erights.org/elib/distrib/captp/unibus.html
|
|
#
|
|
# In a network where each vat has a public/private key pair and each vat forms no more than one
|
|
# connection to each other vat, Joins will rarely -- perhaps never -- be needed, as objects never
|
|
# need to be transparently proxied and references to the same object sent over the same connection
|
|
# have the same export ID. Thus, a successful join requires only checking that the two objects
|
|
# come from the same connection and have the same ID, and then completes immediately.
|
|
#
|
|
# However, in networks where two vats may form more than one connection between each other, or
|
|
# where proxying of objects occurs, joins are necessary.
|
|
#
|
|
# Typically, each JoinKeyPart would include a fixed-length data value such that all value parts
|
|
# XOR'd together forms a shared secret that can be used to form an encrypted connection between
|
|
# the joiner and the joined object's host. Each JoinKeyPart should also include an indication of
|
|
# how many parts to expect and a hash of the shared secret (used to match up parts).
|
|
|
|
using JoinResult = AnyPointer;
|
|
# **(level 4)**
|
|
#
|
|
# Information returned as the result to a `Join` message, needed by the joiner in order to form a
|
|
# direct connection to a joined object. This might simply be the address of the joined object's
|
|
# host vat, since the `JoinKey` has already been communicated so the two vats already have a shared
|
|
# secret to use to authenticate each other.
|
|
#
|
|
# The `JoinResult` should also contain information that can be used to detect when the Join
|
|
# requests ended up reaching different objects, so that this situation can be detected easily.
|
|
# This could be a simple matter of including a sequence number -- if the joiner receives two
|
|
# `JoinResult`s with sequence number 0, then they must have come from different objects and the
|
|
# whole join is a failure.
|
|
|
|
# ========================================================================================
|
|
# Network interface sketch
|
|
#
|
|
# The interfaces below are meant to be pseudo-code to illustrate how the details of a particular
|
|
# vat network might be abstracted away. They are written like Cap'n Proto interfaces, but in
|
|
# practice you'd probably define these interfaces manually in the target programming language. A
|
|
# Cap'n Proto RPC implementation should be able to use these interfaces without knowing the
|
|
# definitions of the various network-specific parameters defined above.
|
|
|
|
# interface VatNetwork {
|
|
# # Represents a vat network, with the ability to connect to particular vats and receive
|
|
# # connections from vats.
|
|
# #
|
|
# # Note that methods returning a `Connection` may return a pre-existing `Connection`, and the
|
|
# # caller is expected to find and share state with existing users of the connection.
|
|
#
|
|
# # Level 0 features -----------------------------------------------
|
|
#
|
|
# connect(vatId :VatId) :Connection;
|
|
# # Connect to the given vat. The transport should return a promise that does not
|
|
# # resolve until authentication has completed, but allows messages to be pipelined in before
|
|
# # that; the transport either queues these messages until authenticated, or sends them encrypted
|
|
# # such that only the authentic vat would be able to decrypt them. The latter approach avoids a
|
|
# # round trip for authentication.
|
|
#
|
|
# accept() :Connection;
|
|
# # Wait for the next incoming connection and return it. Only connections formed by
|
|
# # connect() are returned by this method.
|
|
#
|
|
# # Level 4 features -----------------------------------------------
|
|
#
|
|
# newJoiner(count :UInt32) :NewJoinerResponse;
|
|
# # Prepare a new Join operation, which will eventually lead to forming a new direct connection
|
|
# # to the host of the joined capability. `count` is the number of capabilities to join.
|
|
#
|
|
# struct NewJoinerResponse {
|
|
# joinKeyParts :List(JoinKeyPart);
|
|
# # Key parts to send in Join messages to each capability.
|
|
#
|
|
# joiner :Joiner;
|
|
# # Used to establish the final connection.
|
|
# }
|
|
#
|
|
# interface Joiner {
|
|
# addJoinResult(result :JoinResult) :Void;
|
|
# # Add a JoinResult received in response to one of the `Join` messages. All `JoinResult`s
|
|
# # returned from all paths must be added before trying to connect.
|
|
#
|
|
# connect() :ConnectionAndProvisionId;
|
|
# # Try to form a connection to the joined capability's host, verifying that it has received
|
|
# # all of the JoinKeyParts. Once the connection is formed, the caller should send an `Accept`
|
|
# # message on it with the specified `ProvisionId` in order to receive the final capability.
|
|
# }
|
|
#
|
|
# acceptConnectionFromJoiner(parts :List(JoinKeyPart), paths :List(VatPath))
|
|
# :ConnectionAndProvisionId;
|
|
# # Called on a joined capability's host to receive the connection from the joiner, once all
|
|
# # key parts have arrived. The caller should expect to receive an `Accept` message over the
|
|
# # connection with the given ProvisionId.
|
|
# }
|
|
#
|
|
# interface Connection {
|
|
# # Level 0 features -----------------------------------------------
|
|
#
|
|
# send(message :Message) :Void;
|
|
# # Send the message. Returns successfully when the message (and all preceding messages) has
|
|
# # been acknowledged by the recipient.
|
|
#
|
|
# receive() :Message;
|
|
# # Receive the next message, and acknowledges receipt to the sender. Messages are received in
|
|
# # the order in which they are sent.
|
|
#
|
|
# # Level 3 features -----------------------------------------------
|
|
#
|
|
# introduceTo(recipient :Connection) :IntroductionInfo;
|
|
# # Call before starting a three-way introduction, assuming a `Provide` message is to be sent on
|
|
# # this connection and a `ThirdPartyCapId` is to be sent to `recipient`.
|
|
#
|
|
# struct IntroductionInfo {
|
|
# sendToRecipient :ThirdPartyCapId;
|
|
# sendToTarget :RecipientId;
|
|
# }
|
|
#
|
|
# connectToIntroduced(capId :ThirdPartyCapId) :ConnectionAndProvisionId;
|
|
# # Given a ThirdPartyCapId received over this connection, connect to the third party. The
|
|
# # caller should then send an `Accept` message over the new connection.
|
|
#
|
|
# acceptIntroducedConnection(recipientId :RecipientId) :Connection;
|
|
# # Given a RecipientId received in a `Provide` message on this `Connection`, wait for the
|
|
# # recipient to connect, and return the connection formed. Usually, the first message received
|
|
# # on the new connection will be an `Accept` message.
|
|
# }
|
|
#
|
|
# struct ConnectionAndProvisionId {
|
|
# # **(level 3)**
|
|
#
|
|
# connection :Connection;
|
|
# # Connection on which to issue `Accept` message.
|
|
#
|
|
# provision :ProvisionId;
|
|
# # `ProvisionId` to send in the `Accept` message.
|
|
# }
|
|
|