Setting Up Failover Slots in PostgreSQL-17

PostgreSQL 17 introduces failover slots that enhance high-availability setups. A replication slot ensures that data remains reliable and consistent between nodes during replication, whereas a failover slot ensures consistency between nodes, specifically during and after a failover.

Failover slots are a powerful feature that ensures logical replication can continue seamlessly, even after a failover to a standby server. Using failover slots allows logical replication slots to be automatically synchronized across primary and standby nodes, significantly reducing downtime and the need for manual intervention during a failover.

This guide will walk you through setting up a high-availability PostgreSQL cluster using the new failover slots feature. By the end, you’ll have a robust replication setup capable of seamlessly handling a failover. 

Why Failover Slots Matter From a Historical Perspective

Challenges in PostgreSQL 15

• Replication Slots Tied to the Primary Node: In PostgreSQL 15, replication slots were only created on the primary server. All logical replication slots were lost if the primary server failed, leading to significant replication delays and data loss.
• Manual Failover Management: During failover scenarios, administrators manually recreated replication slots on the new primary server, which increased complexity, introduced errors, and prolonged downtime.
• No Slot Synchronization: Standby servers had no way of knowing about logical replication slots on the primary. This lack of synchronization led to a complete reset of replication streams if a failover occurred.

Improvements in PostgreSQL 16

Minimal Logical Decoding

PostgreSQL 16 introduced a feature called minimal logical decoding on standbys:

• Minimal Decoding on Standby: This allowed standby servers to decode WAL logs to prepare for logical replication, enabling pre-warmed slots for use if a failover occurred.
• Faster Failover: By pre-decoding WAL changes on the standby, it was possible to reduce replication lag when promoting a standby to the primary. However, this still required some manual configuration to ensure smooth failover.

PostgreSQL 17: The Game-Changer – Failover Slots

• Failover Slots: Introducing failover slots in PostgreSQL 17 eliminates the need for manual intervention by automatically synchronizing logical replication slots between the primary and standby servers.
• Automatic Synchronization: The new slot sync worker ensures that failover-enabled slots (failover = true) are always synchronized, even while the primary node is active.
• Seamless Transition: Upon failover, the standby server can take over as the primary without losing any replication slots, ensuring zero data loss and continuous replication.

Creating a High-Availability Cluster With Failover Slots

This section will walk you through creating a PostgreSQL high-availability cluster with failover slots. In our example, we’ll use the following nodes:

1. NodeA (Primary Server)
2. NodeB (Physical Standby)
3. NodeC (Logical Subscriber)

Prerequisites

Before we start, ensure you have:

• PostgreSQL 17 was installed on all three nodes.
• Passwordless SSH access between each node.
• A basic understanding of PostgreSQL, PostgreSQL replication, and PostgreSQL configuration files.

Step 1: Configuring the Primary Node (NodeA)

1.1 Initialize the cluster on NodeA

After installing PostgreSQL on the primary node, initialize the cluster; you can use the following commands:

mkdir -p /home/pgedge/nodeA

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initdb -D /home/pgedge/nodeA --no-locale -E UTF8 

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pg_ctl -D /home/pgedge/nodeA -l /home/pgedge/logs/nodeA.log start 

1.2 Configure replication in the postgresql.conf file

After initializing the cluster, edit the postgresql.conf file, located by default in /home/pgedge/nodeA/postgresql.conf. Set the following parameter values:

wal_level = logical

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max_replication_slots = 10

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max_wal_senders = 10

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synchronous_standby_names = '*'

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synchronized_standby_slots = 'sb1_slot'

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port = 5432

1.3 Update the pg_hba.conf file allowing for Replication Access

The pg_hba.conf file manages client authentication for the PostgreSQL server.  Add the following entry to /home/pgedge/nodeA/pg_hba.conf to ensure access for a replication user:

host replication replicator 127.0.0.1/32 md5

Then, reload the configuration:

pg_ctl -D /home/pgedge/nodeA reload 

1.4 Create a Replication User

Then, log into PostgreSQL and create the replication user:

psql -d postgres -p 5432 

CREATE ROLE replicator WITH REPLICATION LOGIN PASSWORD 'replicator_password';

1.5 Create a Table and Set Up a Publication

Next, you’ll need to create a table and create an associated publication:

CREATE TABLE foo (c1 INT PRIMARY KEY);

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GRANT SELECT ON foo TO replicator;

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CREATE PUBLICATION mypub FOR TABLE foo;

Step 2: Configuring the Physical Standby (NodeB)

2.1 Initialize NodeB

After installing PostgreSQL, initialize NodeB:

mkdir -p /home/pgedge/nodeB 

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initdb -D /home/pgedge/nodeB --no-locale -E UTF8 

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pg_ctl -D /home/pgedge/nodeB -l /home/pgedge/logs/nodeB.log start

2.1 Create a Base Backup

Then, use pg_basebackup to take a backup of the cluster:

mkdir -p /home/pgedge/nodeB 

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pg_basebackup -D /home/pgedge/nodeB -R -X stream -P -h localhost -p 5432 -U replicator

2.2 Configure postgresql.conf on Node-B

Modify the postgresql.conf file (located in /home/pgedge/nodeB/postgresql.conf), setting:

port = 5433

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primary_conninfo = 'host=localhost port=5432 user=replicator password=replicator_password dbname=postgres application_name=sb1_slot'

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primary_slot_name = 'sb1_slot'

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hot_standby_feedback = on

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sync_replication_slots = on

2.3 Enable Failover Slot Synchronization

Use the psql client to log in to NodeB:

psql -d postgres -p 5433 

Then, use the following statements to configure replication for NodeB:

ALTER SYSTEM SET sync_replication_slots = on;

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ALTER SYSTEM SET hot_standby_feedback = on;

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ALTER SYSTEM SET synchronized_standby_slots = 'sb1_slot';

Exit the psql client and restart NodeB:

pg_ctl -D /home/pgedge/nodeB restart 

2.4 Verify Slot Synchronization

Then, reconnect to NodeB with psql and verify that the slots are synchronized:

SELECT slot_name, failover, synced FROM pg_replication_slots;

Step 3: Setting Up the Logical Subscriber (NodeC)

3.1 Initialize the cluster and configure NodeC

After installing PostgreSQL, initialize the cluster; you can use the following commands:

mkdir -p /home/pgedge/nodeC

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initdb -D /home/pgedge/nodeC --no-locale -E UTF8

Then, edit the /home/pgedge/nodeC/postgresql.conf file, setting the following parameter values:

wal_level = logical

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max_replication_slots = 10

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max_wal_senders = 10

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sync_replication_slots = on

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port = 5444

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After editing the configuration file, start NodeC:

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pg_ctl -D /home/pgedge/nodeC -l /home/pgedge/logs/nodeC.log start

3.2 Create a Subscription on NodeC

Use the following command to create a subscription on NodeC:

CREATE SUBSCRIPTION foosub CONNECTION 'dbname=postgres host=localhost port=5432 user=replicator password=replicator_password' PUBLICATION mypub WITH (failover = true);

Step 4: Simulating Failover and Ensuring Continuity

You can use the following commands to simulate a failover and confirm that replication continues and data integrity is preserved.

4.1 Simulating a Failover

Use the following commands to simulate a failure of NodeA, followed by promotion from standby to primary of NodeB:

pg_ctl -D /home/pgedge/nodeA stop

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pg_ctl -D /home/pgedge/nodeB promote

4.2 Update the Subscription on NodeC

After promoting nodeB, log in to NodeC and update the connection to reflect that NodeB is now the primary node:

ALTER SUBSCRIPTION foosub DISABLE;

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ALTER SUBSCRIPTION foosub CONNECTION 'dbname=postgres host=localhost port=5433 user=replicator password=replicator_password';

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ALTER SUBSCRIPTION foosub ENABLE;

4.3 Verify Data Continuity

To test replication, use psql to log in to Node-B (now the primary):

INSERT INTO foo VALUES (3), (4);

Check replication on Node-C:

SELECT * FROM foo;

Conclusion

PostgreSQL 17’s failover slot feature allows for seamless failover in logical replication environments. Following the steps outlined in this guide, you can create a high-availability cluster that ensures uninterrupted data flow, even during a primary server failure.

By optimizing configurations and leveraging PostgreSQL 17’s new capabilities, you can create a resilient and efficient database infrastructure for your mission-critical applications.