Brian Hanson
5b67d4798b
Root cause of "TAKE CONTROL IMMEDIATELY / Communication Issue Between Processes" during driving: our 4Hz thermald and 2Hz gpsd changes pushed deviceState, managerState, and gpsLocation above their declared rate ceilings in cereal/services.py. SubMaster's freq_ok check requires observed rate within [0.8*min, 1.2*max]; publishing faster than max also fails the check — which trips all_freq_ok(), raises EventName.commIssue, and fires the banner. - deviceState: 2Hz → 4Hz (matches thermald DT_TRML=0.25) - gpsLocation: 1Hz → 2Hz (matches gpsd 2Hz polling) - managerState: 2Hz → 4Hz (gated on deviceState arrival, inherits rate) Also: - Re-enabled dashcamd in process_config.py (was disabled for the diagnostic test — dashcamd was innocent). - Added a CLAUDE.md section documenting this class of bug so the next rate change updates services.py too. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
What is cereal? 
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.
Imagine this use case:
- A sensor process reads gyro measurements directly from an IMU and publishes a
sensorEventspacket - A calibration process subscribes to the
sensorEventspacket to use the IMU - A localization process subscribes to the
sensorEventspacket to use the IMU also
Messaging Spec
You'll find the message types in log.capnp. It uses Cap'n proto and defines one struct called Event.
All Events have a logMonoTime and a valid. Then a big union defines the packet type.
Best Practices
- All fields must describe quantities in SI units, unless otherwise specified in the field name.
- In the context of the message they are in, field names should be completely unambiguous.
- All values should be easy to plot and be human-readable with minimal parsing.
Maintaining backwards-compatibility
When making changes to the messaging spec you want to maintain backwards-compatibility, such that old logs can be parsed with a new version of cereal. Adding structs and adding members to structs is generally safe, most other things are not. Read more details here.
Custom forks
Forks of openpilot might want to add things to the messaging spec, however this could conflict with future changes made in mainline cereal/openpilot. Rebasing against mainline openpilot then means breaking backwards-compatibility with all old logs of your fork. So we added reserved events in custom.capnp that we will leave empty in mainline cereal/openpilot. If you only modify those, you can ensure your fork will remain backwards-compatible with all versions of mainline cereal/openpilot and your fork.
Pub Sub Backends
cereal supports two backends, one based on zmq and another called msgq, a custom pub sub based on shared memory that doesn't require the bytes to pass through the kernel.
Example
import cereal.messaging as messaging
# in subscriber
sm = messaging.SubMaster(['sensorEvents'])
while 1:
sm.update()
print(sm['sensorEvents'])
# in publisher
pm = messaging.PubMaster(['sensorEvents'])
dat = messaging.new_message('sensorEvents', size=1)
dat.sensorEvents[0] = {"gyro": {"v": [0.1, -0.1, 0.1]}}
pm.send('sensorEvents', dat)