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61 lines
2.8 KiB
61 lines
2.8 KiB
11 months ago
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# What is cereal? [](https://github.com/commaai/cereal/actions) [](https://codecov.io/gh/commaai/cereal)
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cereal is both a messaging spec for robotics systems as well as generic high performance IPC pub sub messaging with a single publisher and multiple subscribers.
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Imagine this use case:
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* A sensor process reads gyro measurements directly from an IMU and publishes a `sensorEvents` packet
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* A calibration process subscribes to the `sensorEvents` packet to use the IMU
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* A localization process subscribes to the `sensorEvents` packet to use the IMU also
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## Messaging Spec
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You'll find the message types in [log.capnp](log.capnp). It uses [Cap'n proto](https://capnproto.org/capnp-tool.html) and defines one struct called `Event`.
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All `Events` have a `logMonoTime` and a `valid`. Then a big union defines the packet type.
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### Best Practices
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- **All fields must describe quantities in SI units**, unless otherwise specified in the field name.
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- In the context of the message they are in, field names should be completely unambiguous.
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- All values should be easy to plot and be human-readable with minimal parsing.
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### Maintaining backwards-compatibility
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When making changes to the messaging spec you want to maintain backwards-compatibility, such that old logs can
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be parsed with a new version of cereal. Adding structs and adding members to structs is generally safe, most other
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things are not. Read more details [here](https://capnproto.org/language.html).
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### Custom forks
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Forks of [openpilot](https://github.com/commaai/openpilot) might want to add things to the messaging
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spec, however this could conflict with future changes made in mainline cereal/openpilot. Rebasing against mainline openpilot
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then means breaking backwards-compatibility with all old logs of your fork. So we added reserved events in
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[custom.capnp](custom.capnp) that we will leave empty in mainline cereal/openpilot. **If you only modify those, you can ensure your
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fork will remain backwards-compatible with all versions of mainline cereal/openpilot and your fork.**
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## Pub Sub Backends
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cereal supports two backends, one based on [zmq](https://zeromq.org/) and another called [msgq](messaging/msgq.cc), a custom pub sub based on shared memory that doesn't require the bytes to pass through the kernel.
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Example
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---
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```python
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import cereal.messaging as messaging
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# in subscriber
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sm = messaging.SubMaster(['sensorEvents'])
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while 1:
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sm.update()
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print(sm['sensorEvents'])
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```
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```python
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# in publisher
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pm = messaging.PubMaster(['sensorEvents'])
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dat = messaging.new_message('sensorEvents', size=1)
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dat.sensorEvents[0] = {"gyro": {"v": [0.1, -0.1, 0.1]}}
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pm.send('sensorEvents', dat)
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```
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