Overview
Synchronous and asynchronous replication differ in exactly one moment: when the primary tells the client a write succeeded. Sync replication waits for the replica to confirm before acknowledging, while async replication acknowledges immediately and copies the data afterward. That single timing difference cascades into everything else — latency, throughput, and how much data you can lose on failover.
Comparison Diagram
Comparison Table
| Aspect | Sync Replication | Async Replication |
|---|---|---|
| Write acknowledgment | Waits for replica confirmation before committing | Commits on primary alone, replicates after |
| Commit latency | Includes network round-trip to replica | Bound only by primary’s local write |
| Data consistency | Replica is always up to date at commit time | Replica can lag behind primary momentarily |
| Throughput under load | Degrades as replica distance or count grows | Unaffected by replica speed or distance |
| Replica or network failure | Writes block or fail until replica responds | Writes continue uninterrupted on primary |
| Failover data loss | None — replica always has the committed write | Possible — unreplicated writes are lost |
| Replication lag monitoring | Not applicable — lag is structurally zero | Critical — must track and alert on lag |
Key Differences
- Commit timing is the root difference: sync waits, async doesn’t
- Sync trades latency for a zero-data-loss guarantee on failover
- Async trades durability for consistently fast local commits
- Multi-region setups favor async since round-trip time would make sync commits too slow
- Async requires active lag monitoring that sync simply doesn’t need
When to Use Each
Sync Replication
- Financial transactions: Sync guarantees the replica has every committed write, so a primary failure never loses a confirmed balance change.
- Same-datacenter replicas: Low round-trip time keeps the sync latency penalty small enough to be worth the durability guarantee.
- Regulatory durability requirements: Compliance rules that mandate zero data loss on failover are only satisfiable with synchronous acknowledgment.
Async Replication
- Cross-region disaster recovery: Async avoids forcing every write to pay the cost of a continent-spanning round trip.
- High-throughput write workloads: Removing the replica wait lets the primary sustain far higher write rates.
- Read replica scaling: Read-only replicas serving cached or slightly-stale queries don’t need every write instantly present.