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Synchronous replication makes a primary wait for a configured confirmation from one or more replicas before it acknowledges a write. Asynchronous replication lets the primary acknowledge the write without waiting. That choice affects commit latency, how current replica reads are, and whether a promoted replica might be missing acknowledged changes.
Neither mode is universally better. Choose based on the data loss you can tolerate during failover, your write-latency budget, replica placement and network reliability, and how your database engine defines confirmation.
What changes at commit time?
The key difference is what the primary waits for before reporting a successful commit to the client.
- Synchronous: The primary waits for the configured remote confirmation. The confirmation might mean a replica received the change, wrote or flushed it, or applied it; the exact point depends on the database and its settings.
- Asynchronous: The primary acknowledges the commit without waiting for a replica to confirm it. Replication continues separately, so a replica may temporarily lag behind.
“Synchronous” alone is not a complete durability guarantee. You also need to know how many replicas must confirm, which replicas qualify, what their acknowledgment represents, and which replica is eligible for promotion.
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Compare the tradeoffs
| Decision factor | Synchronous replication | Asynchronous replication |
|---|---|---|
| Primary commit | Waits for the configured remote confirmation before returning success. | Returns success without waiting for replica acknowledgment. |
| Failover exposure | Better protects acknowledged writes if the promotion target is among the required confirming replicas and the configured acknowledgment level was met. | A replica promoted before catching up may be missing recently acknowledged primary changes. |
| Replica read freshness | A confirmation does not necessarily mean the change has been applied and is visible to queries; that depends on configuration. | Lag can cause reads from replicas to return stale data. |
| Write responsiveness | Adds the time needed for network communication and remote confirmation. PostgreSQL warns that transactions retain locks while waiting, which can increase response times and contention. | Usually reduces primary commit latency because the primary does not wait for the replica. |
| Network and placement | Standbys need to be placed and connected so remote acknowledgments meet the application’s performance needs. | Can better accommodate distant or intermittently connected replicas, but lag and recovery exposure can grow. |
Understand failover risk and stale reads separately
Failover can lose acknowledged writes
With asynchronous replication, the primary may acknowledge a write before it has reached the replica chosen for promotion. If the primary fails in that interval, the promoted replica may not contain that write. The size of the exposure depends on replication delay and failover timing; asynchronous replication does not by itself promise a fixed recovery-point gap.
Synchronous confirmation can reduce this exposure, but only if the replica that confirmed the write is usable in the failover plan and the configured confirmation rule was actually satisfied. Promotion policy therefore matters as much as the mode name.
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Confirmation is not always read visibility
A replica may acknowledge that it received or persisted a change before applying it for queries. PostgreSQL makes this distinction explicit: its remote_apply commit setting waits for the standby to apply the transaction. Do not assume that every synchronous configuration guarantees immediate read-after-write visibility on a replica.
For asynchronous replicas, route reads that require current data to the primary or use an application strategy that accounts for lag. Monitor replica delay and define what the application should do when a replica is behind.
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Semi-synchronous replication is an implementation-specific middle ground
MySQL 8.4 is asynchronous by default. Its semisynchronous mode blocks a source commit until at least one replica confirms receipt and logging of the transaction events. This adds a remote wait and can provide stronger protection than ordinary asynchronous replication, but the confirmation point is specifically receipt and logging; it is not interchangeable with every product’s synchronous mode or with a guarantee that the replica has applied the transaction for reads.
How database engines implement these terms
PostgreSQL
PostgreSQL’s synchronous standby configuration can select standbys by priority with a FIRST list or use a quorum with an ANY list. Commits wait for the configured number of synchronous standbys to confirm; other standbys can remain asynchronous. The required standbys and confirmation behavior should be evaluated alongside network placement, since a slow network can substantially reduce performance.
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PostgreSQL also warns that waiting for synchronous confirmation keeps transaction locks held, potentially increasing contention and response times. Its documentation distinguishes standby confirmation from application on the standby: remote_apply is the setting that waits for application.
MySQL 8.4
MySQL 8.4 replication is asynchronous by default. Semisynchronous replication waits for at least one replica to receive and log transaction events before the source returns commit to the client. MySQL points to NDB Cluster for use cases requiring synchronous replication. GTID-based replication can establish consistency once all source-committed transactions have been applied on the replica; it does not make a lagging asynchronous replica current in advance.
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Microsoft SQL Server database mirroring
In SQL Server database mirroring, high-safety mode is synchronous and high-performance mode is asynchronous. Synchronous mirroring commits the transaction on both partners, increasing transaction latency. In asynchronous high-performance mode, the primary does not wait for the mirror to write the log, reducing transaction latency while allowing possible data loss.
Automatic failover in database mirroring requires high-safety mode, a synchronized database, a mirror, and a witness. These mode names and conditions describe database mirroring specifically; they should not be generalized to every SQL Server availability feature.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choose a mode against your recovery and performance needs
- Set the recovery point objective (RPO): Decide how much acknowledged data, if any, your organization can tolerate losing after a failure. If the acceptable loss is effectively zero, verify that the intended promotion target participates in the required synchronous acknowledgments.
- Set the commit-latency budget: Account for the network round trip and remote work required by the engine’s confirmation rule. Include the effect of longer-held locks where applicable.
- Assess replica geography and network behavior: A distant or unstable connection can make synchronous waits costly or unavailable. Asynchronous replication tolerates lag more readily but creates more uncertainty about replica freshness and failover state.
- Define read freshness requirements: Identify which reads can tolerate delay, how lag is measured, and whether read-after-write requests must go to the primary or wait for replica application.
- Document the confirmation and promotion rules: Record the acknowledgment point, number and selection of required replicas, what happens if no qualifying standby is available, and which replica can be promoted.
Synchronous replication is a fit when stronger protection for acknowledged writes is worth remote-acknowledgment latency and the deployment can sustain it. Asynchronous replication is a fit when low-latency commits, distant replicas, or tolerance for a bounded recovery-point gap matter more. Whatever you choose, test failover behavior against the actual configuration rather than relying on the label alone.
Quick Recap
Official documentation
- PostgreSQL 17: Warm Standby
- PostgreSQL 18: Synchronous Replication
- MySQL 8.4: Replication
- Microsoft SQL Server: Database Mirroring Operating Modes
- PostgreSQL 18: synchronous_commit
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