2026-10-09 20:15:13 +09:00
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---
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type: guidelines
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---
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# Core services guidelines
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2026-10-12 23:25:03 +09:00
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## 1. One CAN writer: `obd-poller`
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- `obd-poller` is the only thing that transmits on the car's bus (project-yukart
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decision 3): OBD-II requests, one at a time, only while the bus is visibly
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alive. Anything else that transmits is a bug; the recorder only reads.
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- `can_pcan` is no longer listen-only (2026-10-12), so the adapter acknowledges
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frames like any scan tool. Going back is one line in orin-nano-env-setup's
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`/etc/default/can-can_pcan`.
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2026-10-09 20:15:13 +09:00
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## 2. Capture never waits
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- A frame missed is gone. Producers hand frames to the writer without blocking;
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what doesn't fit is **counted**, recorded as an event, and never silently lost.
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- Capture reads SocketCAN and gpsd directly. It does not depend on the broker, a
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display, or any other service being up.
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## 3. Every failure leaves complete files
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- Append, flush and `fdatasync` about once a second. Power loss costs at most the
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last second.
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- Replace files by write-temp, fsync, rename. Never rewrite in place.
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- Anything found half-done at startup is repaired or marked, not deleted.
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## 4. Raw stays raw
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- Recordings are `candump -L` text plus the gpsd JSON as received, so `can-utils`,
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`canplayer` and anything already written against the first captures keep working.
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- Decoding happens downstream of the recording, never instead of it.
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## 5. Time is evidence, not truth
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- The car unit's clock can boot stale and be stepped mid-drive. Record what is
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needed to correct it later (sync state, monotonic time, observed steps, GNSS
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time beside local receive time) rather than trusting it.
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## 6. To be added
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