Write-Ahead Log for fast strong consistency.

Replaces the heavy Three-Phase Commit (3PC) protocol with a lighter-weight WAL-based approach that:

1. Writes to local ETS immediately
2. Appends operation to WAL (in-memory ETS for speed)
3. Async replicates WAL entries to replicas
4. Tracks acknowledgments from replicas
5. Returns success once majority has acked

This reduces strong-mode latency from ~1500µs (3PC) to ~200µs (WAL).

Design

• WAL entries are stored in an ETS table for fast access
• Each entry has a monotonically increasing sequence number
• Replicas ack entries asynchronously via :erpc.cast
• Majority acknowledgment determines commit success
• Periodic cleanup of committed entries
• Recovery on node restart replays uncommitted entries

Usage

This module is called internally by the Replicator when replication_mode is set to :strong. You should not call it directly.

Configuration

The WAL uses sensible defaults but can be tuned via application config:

config :super_cache, :wal,
  majority_timeout: 2_000,  # ms to wait for majority ack
  cleanup_interval: 5_000,  # ms between cleanup cycles
  max_pending: 10_000       # max uncommitted entries before backpressure

Example

# Called internally by Replicator.replicate/3
SuperCache.Cluster.WAL.commit(2, [{:put, {:user, 1, "Alice"}}])
# => :ok