---
type: guidelines
---
# Contract guidelines

## 1. `signals.json` is the single source for a signal's identity
- Topic name and the envelope's `unit` are both **derived** from a registry entry.
  Neither is authored by hand anywhere else.  
- ⚠ **The unit appears in both the topic name and the payload, so they can
  contradict each other.** Generating both from one entry is what stops that; a
  hand-written `"unit"` reintroduces the failure it was meant to prevent.
- Signal names carry their unit (`vehicle_speed_kph`, `coolant_temp_c`), so a unit
  mismatch is visible at the call site rather than one registry lookup away.

## 2. Two clocks, and they answer different questions
- `monotonic` is authoritative for **ordering and intervals**. It cannot step.  
- `timestamp` is the **absolute reference** and can step: the car boots with no
  network, LTE connects several minutes in, and NTP corrects the clock mid-drive.  
- ⚠ **Never derive a duration from `timestamp`.** A step lands inside the interval
  and the result is silently wrong, most likely during exactly the drive worth
  keeping. Durations come from `monotonic`.

## 3. Raw CAN is not the envelope
- `yukart/can/raw` is binary. A four-key JSON object at 1000–3000 frames/sec is
  not a reasonable encoding.
- The envelope is for decoded signals, which are low-rate by comparison.

## 4. Absent means default
- `quality` is omitted when `ok`; `mock` is omitted when false. A consumer treats
  absence as the default, never as an error.

## 5. This repo is versioned by git tag
- There is no version in any topic or payload. A recorded session names the
  contracts tag it was captured under, which is what keeps old captures readable.

## 6. A decoded signal's layout lives only in its registry entry
- Every `source: z27ag` signal has a `can` block in `signals.json`: frame ID, byte,
  bit or length, byte order, scale and offset. The decoder reads those blocks and
  nothing else, so a correction lands in one file and every consumer sees it.
- ⚠ **No CAN ID or bit position is written in any service's code.** A layout
  hard-coded downstream is a second source that silently disagrees the day the
  registry is corrected.
- Each block names the report with its evidence (`evidence`). A layout without
  evidence is a guess and does not go in.

## 7. To be added