To reduce the risk of losing committed transactions during database failover, choose a replication mode that matches your recovery point objective (RPO), verify the standby has reached the required synchronization state, and promote it only through the database platform’s supported procedure. Replication durability does not by itself coordinate promotion: quorum and fencing must also prevent the old primary from continuing to accept writes.
Can database failover lose committed transactions?
Yes. With asynchronous replication, a primary can acknowledge a commit before a replica receives it. If the primary fails and an operator promotes a lagging replica, that replica may not contain the latest committed transactions. PostgreSQL streaming replication and ordinary MySQL replication are asynchronous by default in the cited PostgreSQL 16 and MySQL 8.4 documentation.
Synchronous replication changes the acknowledgment path: a commit waits for acknowledgments from configured standby or replica targets. This reduces the risk that an acknowledged change is absent from an eligible replica, but it does not make every failover automatically lossless. The result depends on the product’s acknowledgment semantics, the configured targets, which target is promoted, the failure scenario, and whether the old primary is prevented from writing.
There is a cost. Waiting for acknowledgments can increase commit latency, and writes may wait or stop when the required standby or quorum is unavailable. Asynchronous replication can preserve write availability or suit a distant disaster-recovery site, but it leaves a window in which a replica may be behind.
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Choose the recovery point before choosing replication settings
Set the workload’s acceptable RPO—the amount of recent data the business can tolerate losing—and recovery time objective (RTO), the time it can tolerate before service is restored. If the requirement is no loss of acknowledged transactions, design for that explicitly; do not infer it from a “synchronous” label or from a replica being connected.
- Prioritize write availability or distance: asynchronous replication may fit, provided stakeholders accept the possible gap and the failover runbook checks the promoted copy’s position.
- Prioritize acknowledged-transaction durability: use the engine’s supported synchronous or quorum mechanism and define what the system does when acknowledgment targets are missing.
- Balance both: place synchronous targets where the latency and failure-domain trade-off is acceptable, and use additional asynchronous replicas for other recovery needs when appropriate.
Network distance matters because acknowledgments have to travel between sites before a commit can complete. A remote target can improve geographic separation, but the extra round trip can affect write latency. The exact impact and guarantees depend on topology and workload; the cited product documentation does not establish a universal latency figure.
What do the replication options protect—and what do they not?
| Option | Transaction-loss exposure | Write behavior if a target is unavailable | Promotion and read considerations |
|---|---|---|---|
| Asynchronous replication | A lagging target may omit recently committed transactions at promotion. | Commits do not wait for replica acknowledgment under the asynchronous behavior described in PostgreSQL 16 and MySQL 8.4 documentation. | Check lag and the platform-specific log or transaction position before promotion; promoting the most recently connected replica is not sufficient evidence of synchronization. |
| Synchronous acknowledgment | Reduces the risk of losing changes acknowledged by the configured target or targets; exact protection depends on acknowledgment level and promotion choice. | Commits can wait or stall if the required acknowledgments cannot be obtained. | Confirm the intended standby is synchronized and use supported membership and promotion controls. Acknowledgment alone does not fence the former primary. |
| Quorum-based synchronous selection | Can require acknowledgments from a configured number of eligible standbys; which failure cases are covered depends on the configured quorum and eligible members. | Availability depends on whether enough eligible targets remain reachable to satisfy the configured requirement. | Verify the quorum and candidate set before relying on it for failover. PostgreSQL 16 supports `ANY` quorum-based and `FIRST` priority-based synchronous standby selection. |
| Forced promotion of an unsynchronized target | Can lose data that had not reached that target. | May restore writes sooner by accepting a weaker recovery point. | Use only as an explicit recovery decision with the loss risk understood. SQL Server Always On documentation warns that forced failover to an unsynchronized asynchronous target can lose data. |
Configure the database-native durability mechanism
PostgreSQL streaming replication
PostgreSQL 16 documentation describes streaming replication as asynchronous by default. For synchronous behavior, review `synchronous_commit` together with `synchronous_standby_names`; the setting and standby selection jointly determine which acknowledgments a transaction waits for. `FIRST` selects standbys by priority, while `ANY` uses a quorum-style count of acknowledgments. Choose enough eligible standbys for both the desired durability and the availability behavior when one is unreachable.
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Use PostgreSQL’s `pg_stat_replication` view to inspect standby state rather than treating a live connection as readiness to promote. A standby that is still catching up is not a synchronized promotion target. Confirm the platform-required synchronization and transaction position for the intended failover procedure.
MySQL replication and Group Replication
MySQL 8.4 ordinary replication is asynchronous by default. Its semisynchronous option confirms that at least one replica received and logged events, which is a different condition from confirming that a chosen failover target has applied all relevant changes. Assess the acknowledgment semantics and the actual candidate’s state before promotion.
MySQL Group Replication has separate consistency controls. Its consistency behavior can affect when a newly elected primary becomes usable: access may be allowed before backlog application completes, creating a period when reads can be stale, or access may wait for backlog application and delay availability. Decide which outcome fits application read-after-write requirements and recovery targets.
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SQL Server Always On availability groups
For SQL Server Always On, lossless planned or automatic failover requires a synchronized secondary. Automatic failover also has mode and quorum prerequisites; confirm those for the deployed version and operating system rather than assuming that enabling availability groups is enough. A forced failover to an unsynchronized asynchronous target can lose data.
The cited documentation includes a SQL Server 2017 overview and Linux guidance for SQL Server 17.x as accessed on October 4, 2026. Configuration options and prerequisites vary by version, platform, cluster manager, and topology, so use the documentation for the exact deployment.
How to verify a standby before promotion
- Check the candidate, not just replication connectivity. Confirm that the intended standby is current and in the platform’s required synchronized or ready state. For PostgreSQL 16, inspect `pg_stat_replication` and the documented state/position indicators. Use the corresponding health and synchronization indicators for other engines.
- Compare replication progress with the primary’s durable position. Use the database platform’s documented transaction or log-position mechanism. A generic “connected” or “replicating” status does not establish that all transactions required for the chosen RPO have arrived or been applied.
- Check quorum and membership. Confirm that the cluster has the members and quorum required by its failover policy. If synchronous acknowledgment depends on a standby that is currently unavailable, determine whether commits are waiting, whether policy permits a degraded mode, and what that mode means for durability.
- Check catch-up backlog and read policy. For systems that can expose a new primary before backlog application finishes, determine whether clients may read stale data and whether they must wait for catch-up before serving reads.
- Record the recovery point decision. If no candidate meets the required state, treat promotion as a conscious RPO exception rather than describing it as lossless.
Promote one primary and prevent split-brain
Replication moves data; it does not by itself establish that only one server can accept writes. Use one authoritative, database-supported promotion procedure that coordinates membership, quorum, and routing. Before sending clients to the new primary, fence the old primary—make it unable to accept writes—using the deployment’s supported mechanism. A network partition can leave nodes unable to see one another, so do not treat loss of connectivity as proof that the former primary is stopped.
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In cluster-managed deployments, validate quorum and fencing configuration as separate safeguards. If the old primary cannot be reliably fenced or the cluster cannot establish a single authoritative membership, avoid routing writers to a second node until the split-brain risk is resolved.
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Failover speed can conflict with read consistency. In MySQL Group Replication, consistency settings can allow a new primary to serve requests before backlog application completes, so reads may temporarily be stale; waiting for the backlog can delay access. Apply the same operational question to the application’s requirements: can it tolerate stale reads during recovery, or must reads wait until the promoted node has applied the necessary changes?
Make the choice explicit in the runbook and client behavior. Applications with read-after-write expectations should not assume that a newly promoted server is immediately current for every read path. Route or gate reads according to the platform’s documented readiness condition.
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Test the failure paths and keep independent recovery options
Exercise the complete procedure in a controlled environment, including planned switchover, primary failure, network partition, and loss of a synchronous standby. Measure actual RPO and RTO, and verify that applications reconnect, writes reach only the new primary, and reads meet the intended consistency policy. Confirm alerts fire for replication staleness, unavailable synchronous targets, quorum loss, and growing backlog.
Maintain independent backups and point-in-time recovery as protection against logical corruption and operator error. Replication can reproduce unwanted changes; a replicated copy is not a substitute for a recoverable backup.
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