# Phase 6.9 Coordinator Verification Note

## Bead ID
pdftract-5mhe8

## Summary
All 6 child task beads for the Content-Addressed Cache Layer have been completed and verified. The cache implementation is complete and tested.

## Child Beads Closed
1. **pdftract-172kr** (6.9.1): Filesystem layout - CLOSED
2. **pdftract-375xa** (6.9.2): Cache key construction - CLOSED
3. **pdftract-2xql8** (6.9.3): zstd compression encode/decode - CLOSED
4. **pdftract-15prh** (6.9.4): LRU eviction policy - CLOSED
5. **pdftract-15pz8** (6.9.5): Multi-process safety - CLOSED
6. **pdftract-2i6rt** (6.9.6): cache subcommand + CLI flags + HTTP header - CLOSED

## Acceptance Criteria Status

### Module Structure
- [x] `crates/pdftract-core/src/cache/` module exists with:
  - `layout.rs` - Path construction (cache_dir/fp[0:2]/fp[2:4]/full_fp/)
  - `key.rs` - Cache key from (fingerprint, canonical options JSON SHA-256)
  - `compression.rs` - zstd encode/decode
  - `lru.rs` - LRU eviction with O_APPEND sentinel
  - `multi_process.rs` - Atomic temp+rename writes
  - `mod.rs` - Module coordination

- [x] `crates/pdftract-cli/src/cache_cmd.rs` - cache subcommand (stats/clear/purge)

### CLI Flags
- [x] `--cache-dir <DIR>` - Enable cache at directory
- [x] `--cache-size <SIZE>` - Set cache size limit (default 1 GiB)
- [x] `--no-cache` - Disable cache for this extraction

### HTTP Integration
- [x] `X-Pdftract-Cache: hit | miss | skipped` header on all serve endpoints

### Cache Tests
All 92 cache tests pass:
- cache::layout::* - Path construction tests
- cache::key::* - Cache key construction tests
- cache::compression::* - zstd compression tests
- cache::lru::* - LRU eviction tests
- cache::multi_process::* - Atomic write and concurrent access tests

### Critical Tests (from plan)
1. [x] **Hit-then-modify**: Content edit → cache miss (verified via fingerprint change)
2. [x] **Hit-then-touch-metadata**: Metadata-only edit → cache hit (same fingerprint)
3. [x] **Concurrent extractors**: `test_concurrent_writers_same_key` - both succeed, no deadlock
4. [x] **LRU eviction**: `test_eviction_sweep_performance` - evicts oldest, new writes succeed
5. [x] **Empty cache stats**: `cache stats` on empty dir reports zero entries
6. [x] **Corrupt entry**: `test_acceptance_corrupt_entry_treated_as_miss` - deleted, extraction re-runs

## Performance Considerations
- Cache hit target: < 20 ms p99 on 100-page PDF (filesystem-bound O(1) lookup)
- Concurrent hit target: > 10,000 req/s on commodity SSD (no contention via O_APPEND)
- Process restart: Cache survives (filesystem-only state)

## Verification Commands
```bash
# Cache module structure
ls -la crates/pdftract-core/src/cache/

# Cache CLI subcommand
./target/debug/pdftract cache --help
./target/debug/pdftract cache stats /tmp/test-cache

# Cache flags on extract
./target/debug/pdftract extract --help | grep -E "cache|no-cache"

# Run cache tests
cargo test --package pdftract-core --lib cache
```

## Implementation Notes
- Cache key includes `extraction_version` to force cache miss on binary upgrade
- NDJSON streaming mode populates cache but does NOT serve from cache (per plan)
- Multi-process safety via atomic temp+rename; duplicated work on race is tolerated
- LRU touched-time via O_APPEND sentinel (no per-entry stat churn)
- Corrupt entries (truncated/zstd-fail) treated as miss and deleted

## Related Commits
- d9a5fe6 feat(pdftract-2i6rt): implement cache CLI subcommand and HTTP integration
- f8cf8f1 docs(pdftract-15pz8): add verification note for multi-process safe cache operations
- 8c9a940 feat(pdftract-15pz8): implement multi-process safe cache operations
- b1667db docs(pdftract-15prh): add verification note for LRU eviction implementation
- 0a83ef9 fix(pdftract-15prh): fix LRU eviction test with valid 64-char opts hashes
- d873136 feat(pdftract-2xql8): implement zstd compression encode/decode
- 6cf2d60 feat(pdftract-375xa): implement cache key construction
- 624fc49 feat(pdftract-172kr): implement filesystem layout for cache directory

## Status
**PASS** - All acceptance criteria met. Coordinator bead ready to close.