Sync, Async, and Semi-Sync Commits

Sync, Async, and Semi-Sync Commits

Synchronous, asynchronous, and semi-synchronous commit — durability versus write latency.

Sync vs Async vs Sem-Sync Replication (Commit-type)

SYNCHRONOUS REPLICATION

Leader waits for follower to confirm before saying OK to client.

✓ Data is safe — follower always has a complete copy. ✗ Slow — you wait for network round trip on every write. One slow follower = entire system slow.

ASYNCHRONOUS REPLICATION

Leader says OK to client immediately. Sends to followers in the background.

✓ Super fast — no waiting. Write confirmed in milliseconds. ✗ Risk — if leader crashes before sending to followers, that data is GONE. You told user saved but it was not.

SEMI-SYNCHRONOUS — THE REAL WORLD CHOICE

This means you always have at least one up-to-date copy. If the synchronous follower slows down → promote an async one to sync. Most production systems (MySQL, PostgreSQL) use this by default.

Summary: Fully Sync → safe but slow. Almost nobody does this. Fully Async → fast but can lose data. Risky for critical systems. Semi-Sync → best of both. One safe copy guaranteed. Rest are fast.

One follower is synchronous. The rest are asynchronous.