durability

Durability primitives for local persistence.

Quick start

use durability::storage::MemoryDirectory;
use durability::walog::{WalWriter, WalReader, WalEntry};

// MemoryDirectory::arc() returns Arc<dyn Directory> directly
let dir = MemoryDirectory::arc();

// Write
let mut w = WalWriter::<WalEntry>::new(dir.clone());
w.append(&WalEntry::AddSegment { segment_id: 1, doc_count: 100 }).unwrap();
w.append(&WalEntry::DeleteDocuments { deletes: vec![(1, 42)] }).unwrap();
w.flush().unwrap();

assert_eq!(w.last_entry_id(), Some(2));

// Recover
let records = WalReader::<WalEntry>::new(dir).replay().unwrap();
assert_eq!(records.len(), 2);
assert_eq!(records[0].entry_id, 1); // entry_id assigned by writer

WalWriter<E> and WalReader<E> are generic -- define your own entry type with #[derive(Serialize, Deserialize)] and use WalWriter::<YourType>::new(dir). Entry IDs are assigned by the writer and stored in the frame header, not in your payload.

Important: WalWriter::new() creates a fresh WAL and errors if segments already exist. Use WalWriter::resume() to continue an existing WAL (it handles both empty and non-empty directories).

For batch writes (amortize flush cost across multiple entries):

let ids = w.append_batch(&[
    WalEntry::AddSegment { segment_id: 3, doc_count: 50 },
    WalEntry::DeleteDocuments { deletes: vec![(1, 10)] },
]).unwrap();
// Single flush for both entries

For large WALs, use streaming replay to avoid collecting into a Vec:

reader.replay_each(|record| {
    println!("entry {}: {:?}", record.entry_id, record.payload);
    Ok(())
})?;

Not provided (and why)

• Multi-process locking: This crate does not manage flock or IPC locks. Single-writer-per-directory is assumed. Multiple writers silently corrupt data. WalWriter creates an advisory lockfile (wal/.lock) to catch accidental double-instantiation within a process, but this does not guarantee cross-process exclusion.
• Strong consistency by default: write calls are buffered. Use flush_and_sync() when you need a durability barrier.
• fsync failure recovery: A failed fsync on Linux clears dirty pages; retrying reports false success. This crate propagates IO errors as fatal. Callers should treat fsync failure as unrecoverable and restart from WAL.

What really matters (failure model)

• Crash / torn writes: partial writes at the tail (e.g. process crash mid-record).
• Corruption detection: CRC/magic/version/type mismatches are treated as errors (even in "best-effort" modes).
• Stable storage vs "reported success": unless you add explicit barriers, a successful write may still be only in OS caches.

Contract surface (what you get)

• Prefix property: Best-effort replay returns a prefix of the valid operation stream. No garbage, no reordering.
• Narrow best-effort scope: Tolerance applies only to the final WAL segment's torn tail records, and also tolerates a torn header in the final segment (crash during segment creation). Corruption in non-final segments is an error.
• Deterministic checkpoints: Checkpoint payloads are written deterministically (stable ordering).

Stable-storage durability (opt-in)

If you need "survives power loss after success", add explicit barriers:

• WalWriter::flush_and_sync() / RecordLogWriter::flush_and_sync()
• durability::storage::sync_file(dir, path) -- fsync the file
• durability::storage::sync_parent_dir(dir, path) -- sync the parent directory (needed for durable create/rename)
• DurableDirectory trait provides atomic_write_durable / atomic_rename_durable

If the backend cannot map paths to the OS filesystem (Directory::file_path() returns None), these operations return NotSupported.

Checkpoint publishing + WAL truncation

To truncate old WAL segments safely:

1. Write a durable checkpoint
2. Append WalEntry::Checkpoint to the WAL and make it durable
3. Only then delete WAL segments covered by the checkpoint

Use CheckpointPublisher for this pattern. After truncation, recovery should start from the latest checkpoint marker (see RecoveryManager::recover_latest).

Modules at a glance

• storage: Directory abstraction + FsDirectory/MemoryDirectory + sync helpers.
• recordlog: append-only log with CRC framing + strict/best-effort replay.
• walog: generic multi-segment WAL (WalWriter<E> / WalReader<E>) with strict/best-effort replay and resume repair. Includes WalEntry for segment-index use cases.
• checkpoint: CRC-validated snapshot files (postcard payloads).
• recover: generic recover_with_wal() for any checkpoint + WAL entry types, plus segment-specific RecoveryManager.
• publish: crash-safe checkpoint publish + WAL truncation.

Running

• Tests: cargo test
• Heavier property runs: PROPTEST_CASES=512 cargo test --test prop_wal_resume
• Benches: cargo bench

Fuzzing (opt-in)

Property tests cover semantic invariants; fuzzing covers "never panic on weird bytes".

• Install: cargo install cargo-fuzz
• Run (see fuzz/):
  • cargo fuzz run fuzz_wal_entry_decode
  • cargo fuzz run fuzz_wal_dir_replay
  • cargo fuzz run fuzz_checkpoint_read
  • cargo fuzz run fuzz_recordlog_read