# C13 — Flight Data Recorder (FDR)

## 1. High-Level Overview

**Purpose**: persist a per-flight ≤ 64 GB record of every payload class onboard (estimates, IMU traces, emitted MAVLink, system health, mid-flight tiles, ≤0.1 Hz failed-tile thumbnails) without silently dropping data (AC-NEW-3). Exclude raw nav/AI-cam frames (AC-8.5; only the failed-tile thumbnail forensic exception is allowed). The FDR is the system's audit log: every safety-critical decision, every emitted frame, every signing key rotation, every spoof-promotion-block lands here.

**Architectural Pattern**: single concrete `FileFdrWriter` behind a `FdrWriter` interface. Single writer thread fed by lock-free in-process queues from every component. Lossy on writer-thread overrun **only by logging the rollover event**, never silently.

**Upstream dependencies**: every component publishes to C13 via in-process pub/sub (drop-oldest-with-rollover-log on overrun).

**Downstream consumers**:
- Post-flight: operator workstation (read via C12 retrieval).
- Real-time: nothing — C13 is write-only at runtime.

## 2. Internal Interfaces

### Interface: `FdrWriter`

| Method | Input | Output | Async | Error Types |
|--------|-------|--------|-------|-------------|
| `open_flight` | `FlightHeader` | `None` | No (called once at takeoff) | `FdrOpenError` |
| `write_record` | `FdrRecord` | `None` | No (lock-free enqueue) | `FdrQueueOverrunError` (logged but does not raise) |
| `close_flight` | `()` | `FlightFooter` | No (called once at landing) | — |
| `current_size_bytes` | `()` | `int` | No | — |
| `is_rolling` | `()` | `bool` | No | — |

**Input/Output DTOs**:
```
FlightHeader:
  flight_id:                       uuid
  flight_started_at:               ISO 8601 + monotonic_ns
  config_snapshot:                 JSON
  signing_key_rotation_event:      record
  manifest_content_hashes:         dict[Path, sha256]

FdrRecord:                       see data_model.md (FdrRecord; tagged union over payload classes)

FlightFooter:
  flight_ended_at:                 ISO 8601 + monotonic_ns
  records_written:                 int
  records_dropped_overrun:         int
  bytes_written:                   int
  rollover_count:                  int
```

## 3. External API Specification

Not applicable.

## 4. Data Access Patterns

| Query | Frequency | Hot Path | Index Needed |
|-------|-----------|----------|--------------|
| `write_record` from every component | up to ~100 Hz aggregate | Yes | n/a |
| Post-flight read (operator retrieval) | once per flight | No | filesystem layout per `(flight_id, segment)` |

### Caching Strategy

| Data | Cache Type | TTL | Invalidation |
|------|-----------|-----|-------------|
| In-process queue from each producer | bounded ring (drop-oldest with rollover log) | flight lifetime | per-record write |
| Writer-thread buffer | sized for ≥1 s of typical write load | flight lifetime | flush on segment rollover |

### Storage Estimates

| Table/Collection | Est. Row Count (1yr) | Row Size | Total Size | Growth Rate |
|-----------------|---------------------|----------|------------|-------------|
| Per-flight record file (segmented, oldest-segment-dropped policy) | bounded by 64 GB per AC-NEW-3 | varies per payload class | ≤ 64 GB / flight | bounded by AC-NEW-3 |
| Per-flight tile snapshots (mid-flight tiles) | ~few hundred / flight | 50–200 KB each | up to ~50 MB / flight | bounded by F4 mid-flight gen |
| Per-flight failed-tile thumbnails (AC-8.5 forensic exception) | ≤ 0.1 Hz × 8 h = ≤ 2880 thumbnails / flight | small JPEG | <50 MB | bounded by ≤ 0.1 Hz cap |

### Data Management

**Seed data**: none.

**Rollback**: per-segment file layout makes per-segment deletion safe. The writer never overwrites a closed segment; it only appends to the current open segment, then opens a new segment when the previous reaches a configurable size cap.

## 5. Implementation Details

**Algorithmic Complexity**: per-record cost is `O(record_size)` for serialisation + write. Aggregate throughput sized for the worst-case AC-NEW-3 cap.

**State Management**:
- Owns the open per-flight segment file handle.
- Owns the writer thread and the in-process producer queues.
- Owns the rollover policy (oldest-segment-dropped first when total reaches 64 GB).

**Key Dependencies**:

| Library | Version | Purpose |
|---------|---------|---------|
| orjson / msgpack | per project pin | Record serialisation (serialised format choice during decompose phase) |
| atomicwrites | latest | Segment file rotation (atomic open of new segment + close of previous) |
| filelock | per project pin | Cross-process safety for the FDR root (operator-tool reads while companion writes — companion-only access during flight) |

**Error Handling Strategy**:
- `FdrOpenError` at takeoff: refuse takeoff (per AC-NEW-3 every payload class must be present from t=0).
- `FdrQueueOverrunError`: per-producer drop-oldest, but the rollover event itself is ALWAYS logged (a separate "overrun" record in the FDR records the dropped count and producer-id). Never silent.
- Filesystem write failure mid-flight: log to stdout/stderr (since we can't log to FDR at this point) + STATUSTEXT to GCS; the system continues to emit external positions because losing the audit log doesn't compromise navigation, but the operator must be alerted.

## 6. Extensions and Helpers

| Helper | Purpose | Used By |
|--------|---------|---------|
| `RecordSchema` | versioned record schema for cross-version FDR compatibility | C13 only — this is internal |

## 7. Caveats & Edge Cases

**Known limitations**:
- 64 GB cap is per AC-NEW-3. If payload-class throughput grows beyond what the cap supports for an 8 h flight, the producers MUST throttle or accept oldest-dropped — the FDR will not silently exceed the cap.
- Failed-tile thumbnail forensic exception is the ONLY raw-imagery-adjacent persistence; AC-8.5 must be re-asserted if any new payload class is added.

**Potential race conditions**:
- The writer thread is the single writer; producers enqueue lock-free. No filesystem contention from within the companion. Operator-tool reads happen post-landing only.

**Performance bottlenecks**:
- Writer-thread serialisation throughput must exceed peak producer throughput. NFT-LIM-02 (8 h synthetic AC-NEW-3) validates.

## 8. Dependency Graph

**Must be implemented after**: nothing internal — C13 is foundational along with C7.

**Can be implemented in parallel with**: every other component.

**Blocks**: every component (every component logs to C13).

## 9. Logging Strategy

| Log Level | When | Example |
|-----------|------|---------|
| ERROR | `FdrOpenError`, mid-flight filesystem write failure | `C13 segment write failure: errno=ENOSPC; STATUSTEXT to GCS` |
| WARN | queue overrun (any producer) | `C13 queue overrun: producer=c5_state; dropped_count=23` |
| INFO | open/close flight; segment rollover | `C13 flight opened: flight_id=…; segment=0` |
| DEBUG | per-write timing (only in dev tier) | `C13 record written: kind=estimate; bytes=412; took=0.1ms` |

**Log format**: structured JSON to stdout/journald.
**Log storage**: stdout / journald — but not C13 itself for ERROR (we'd be writing to the broken thing). FDR records are the project-level "logs" for everything except C13's own operational status.
