Synchronous Replication and Quorum Commit
By default a primary says 'committed' as soon as its own log is safe, without waiting for any replica.
If it then dies, the last few transactions the replica never received are simply gone. Synchronous replication makes the primary wait for a replica to confirm first, closing that data-loss gap in exchange for slightly slower commits.
Pro members see the rest of this lesson
- A 4-step state diagram: Flush → Wait → Confirm → Starve
- The full mechanism: what PostgreSQL does internally, in source terms
- 1 SQL query you can run, each labelled by how it was verified
- 1 transcript captured on PostgreSQL 17.10
- The closing insight, the mistake it prevents, and what it changes in your work
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Check it against the source2 citations in postgres/postgres · file, symbol and line verified on REL_17_STABLE
Primary symbol: SyncRepWaitForLSN · line 148
Primary symbol: WalSndLoop · line 2831
Anchored to postgres/postgres on REL_17_STABLE and cross-checked against the manual for PostgreSQL 15–18. Every query here was run and its output captured on a throwaway PostgreSQL 17.10 lab; corrections are noted inline. §27.2.8 Synchronous Replication in the official docs →
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