Synchronous Replication Modes

PostgreSQL supports three modes of replication that determine how the primary waits for replicas before committing transactions.

Replication Modes Overview

Mode	REPLICATION_SYNCHRONOUS_MODE	REPLICATION_SYNCHRONOUS_COUNT	REPLICATION_SYNCHRONOUS_REPLICAS	Use Case
Single Sync	true	-	-	Basic HA, one sync replica
Quorum Sync	true	N	replica1,replica2,...	Critical data, multiple sync replicas
Async	false	-	-	Best performance, potential data loss

Single Sync Mode (Default)

The primary commits after waiting for one replica to acknowledge.

environment:
  HA_MODE: native
  REPLICATION_ROLE: primary
  REPLICATION_SYNCHRONOUS_MODE: "true"

Behavior

Primary: [writes] ──────► [sync replica]    (waits for 1 ack)
                   └─────► [potential replica] (standby)

• One replica becomes sync (primary waits for it)
• Other replicas become potential (will become sync if current sync fails)

Verification

SELECT client_addr, state, sync_state FROM pg_stat_replication;

 client_addr |   state   | sync_state
-------------+-----------+------------
 10.0.1.5    | streaming | sync
 10.0.1.6    | streaming | potential

Quorum Sync Mode

The primary commits after waiting for multiple replicas to acknowledge. Use this when data safety requires multiple replicas.

environment:
  HA_MODE: native
  REPLICATION_ROLE: primary
  REPLICATION_SYNCHRONOUS_MODE: "true"
  REPLICATION_SYNCHRONOUS_COUNT: "2"
  REPLICATION_SYNCHRONOUS_REPLICAS: "replica1,replica2"

Behavior

Primary: [writes] ──────► [replica1]  ┐
                   └─────► [replica2]  ├─► (waits for ALL N acks)
                   └─────► [replica3]  │

• All listed replicas must acknowledge before commit
• sync_state shows sync for all qualifying replicas

Verification

SELECT client_addr, state, sync_state FROM pg_stat_replication;

 client_addr |   state   | sync_state
-------------+-----------+------------
 10.0.1.5    | streaming | sync
 10.0.1.6    | streaming | sync
 10.0.1.7    | streaming | potential

Configuration Details

• REPLICATION_SYNCHRONOUS_COUNT: Number of replicas required (e.g., 2 means at least 2 must ack)
• REPLICATION_SYNCHRONOUS_REPLICAS: Comma-separated list of replica application names

Async Mode

The primary does not wait for replicas. Best performance but potential data loss on failover.

environment:
  HA_MODE: native
  REPLICATION_ROLE: primary
  REPLICATION_SYNCHRONOUS_MODE: "false"

Behavior

Primary: [writes] ──────► [async replica]  (no waiting)

• Replicas may lag behind primary
• On failover, recent transactions may be lost

Verification

SELECT client_addr, state, sync_state FROM pg_stat_replication;

 client_addr |   state   | sync_state
-------------+-----------+------------
 10.0.1.5    | streaming | async
 10.0.1.6    | streaming | async

sync_state Values Explained

Value	Meaning
sync	Synchronous - primary waits for this replica before committing
potential	Backup sync - will become sync if current sync fails
async	Async - primary does not wait for this replica

Choosing a Mode

Single Sync

Best for: Basic high availability where you need at least one synchronous copy.

• Pros: Simple configuration, one sync replica
• Cons: Only one replica guaranteed to be in sync

Quorum Sync

Best for: Critical data requiring multiple synchronous copies.

• Pros: Multiple replicas always in sync, better data safety
• Cons: Slower commits (must wait for multiple acks), more complex setup

Async

Best for: Read scaling where data loss on failover is acceptable.

• Pros: Fastest performance, no waiting for replicas
• Cons: Potential data loss on failover

Related

• Replica Names - Understanding application names for quorum mode
• Dynamic Configuration - Change sync settings at runtime