Synchronous and asynchronous replication in PostgreSQL are two different approaches to replicating data from a primary (master) database to one or more standby (replica) databases. The choice between these two replication modes depends on the balance between data safety (durability) and performance (latency) you want to achieve. Below are the key differences between synchronous and asynchronous replication:
1. Data Durability and Safety
- Synchronous Replication:
- Guarantees that the data is written to both the primary and at least one standby before a transaction is considered committed.
- This ensures no data loss if the primary server fails, as the data is already synchronized with at least one replica.
- The primary waits for the confirmation (acknowledgment) from the standby before completing the transaction commit.
- Asynchronous Replication:
- The primary server does not wait for confirmation from the standby before committing a transaction.
- The transaction is considered committed as soon as it is written to the primary, but replication to the standby happens in the background.
- In case of a primary failure, some data loss can occur because the latest transactions may not yet have been replicated to the standby.
2. Performance
- Synchronous Replication:
- Slower than asynchronous replication because the primary must wait for acknowledgment from the standby before completing a transaction.
- The more standbys involved in synchronous replication, the more latency may increase, especially if network latency between the primary and standbys is high.
- Best suited for environments where data integrity is critical, and downtime/data loss is unacceptable.
- Asynchronous Replication:
- Faster performance since the primary server doesn’t need to wait for any confirmation from the standby.
- The commit latency is lower because replication happens in the background without delaying the primary server.
- Suitable for applications where performance is more important than zero data loss in the event of a failure.
3. Data Loss Risk
- Synchronous Replication:
- Almost no risk of data loss. If the primary server crashes, the standby is fully up to date with all committed transactions.
- The primary server is aware that the transaction has been successfully written to at least one standby.
- Asynchronous Replication:
- Potential for data loss. If the primary crashes, there is a risk that some recent transactions may not have been replicated to the standby.
- The time gap between the primary and standby is known as replication lag, which can result in missing the most recent data during failover.
4. Transaction Commit Behavior
- Synchronous Replication:
- The transaction on the primary is considered complete only after receiving confirmation from a standby, ensuring that the transaction has been persisted on both the primary and at least one replica.
- Provides strong consistency across the primary and the replica(s).
- Asynchronous Replication:
- The transaction is committed immediately on the primary without waiting for acknowledgment from the standby.
- Eventually consistent, meaning that the replica will catch up and reflect the data committed on the primary, but not in real time.
5. Failover Scenarios
- Synchronous Replication:
- During a failover, the standby will have all the committed transactions from the primary, allowing for a smooth and data-safe failover with minimal to no data loss.
- Failover is safer but may require a more complex configuration because you need to ensure that the replica is fully synchronized.
- Asynchronous Replication:
- Failover can result in some data loss, as the standby may not have received or applied all recent changes from the primary.
- After failover, the standby will continue from the last received transaction, and missing transactions may not be recoverable.
6. Use Cases
- Synchronous Replication:
- Ideal for critical systems where high availability and data integrity are crucial, such as financial systems, e-commerce platforms, and other applications where losing data would have severe consequences.
- It is often used in systems that require a high level of data consistency and durability.
- Asynchronous Replication:
- Suitable for systems where performance and throughput are prioritized over strict data consistency, such as reporting databases, analytics, or scenarios where slight data loss is acceptable.
- Often used when replication is needed across regions or where network latency may introduce delays that would impact synchronous replication performance.
7. Network Latency Impact
- Synchronous Replication:
- More sensitive to network latency, as the primary must wait for the standby to acknowledge the transaction. If there is high latency between the primary and standby, it can significantly slow down transaction processing.
- Asynchronous Replication:
- Less sensitive to network latency, as the primary server does not need to wait for the standby’s response. Replication happens asynchronously in the background, allowing the primary to continue operations without waiting.
Summary of Key Differences
| Aspect | Synchronous Replication | Asynchronous Replication |
|---|---|---|
| Data Durability | No data loss (safest option) | Possible data loss (depending on replication lag) |
| Performance | Slower (increased latency due to waiting for ACK) | Faster (no wait for standby acknowledgment) |
| Consistency | Strong consistency (all replicas are in sync) | Eventual consistency (replicas may lag) |
| Failover | Smooth failover, no data loss | Data loss possible during failover |
| Commit Behavior | Waits for acknowledgment from replica(s) | Commits immediately on the primary |
| Network Latency Sensitivity | High (latency can slow down commits) | Low (not impacted by network latency) |
| Use Case | Critical systems requiring high availability | High-performance systems with acceptable data loss |
| Configuration Complexity | Requires more careful configuration and monitoring | Easier to configure but with potential data risks |
When to Use Synchronous vs. Asynchronous Replication
- Use Synchronous Replication when:
- Data integrity and no data loss are top priorities.
- You need strong guarantees that data is written to at least one standby before considering the transaction complete.
- Latency due to waiting for acknowledgment from the replica is acceptable.
- You are dealing with critical applications such as financial services or healthcare systems.
- Use Asynchronous Replication when:
- Performance is more critical than 100% data durability.
- You can tolerate some data loss in case of primary server failure.
- You need to maintain high throughput and fast commits.
- The standby is geographically far from the primary, where synchronous replication would introduce unacceptable latency.
By choosing the appropriate replication mode, you can strike the right balance between performance, availability, and data safety for your application.
