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43 lines
1.3 KiB
43 lines
1.3 KiB
5 years ago
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What is cereal?
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----
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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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----
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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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Pub Sub Backends
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----
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cereal supports two backends, one based on [zmq](https://zeromq.org/), the other called msgq, 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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# in publisher
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pm = messaging.PubMaster(['sensorEvents'])
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dat = messaging.new_message()
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dat.init('sensorEvents', 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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