PostgreSQL Logical Replication: A Comprehensive Guide

Introduction

PostgreSQL has established itself as one of the most powerful and versatile open-source database management systems. One of its many strengths lies in the support for logical replication, a mechanism that allows for the selective replication of data and schema changes between databases. This feature enables users to scale out their database architecture, improve performance, and enhance data availability. In this blog post, we will dive into the world of PostgreSQL logical replication, providing you with a comprehensive guide to understanding and implementing this powerful feature.

What is Logical Replication?

Logical replication in PostgreSQL refers to the process of replicating specific data changes and schema alterations from one database to another. Unlike physical replication, which copies the entire database cluster, logical replication enables you to selectively replicate particular tables, columns, or even rows of data. This granular control over data replication is particularly useful in scenarios where you want to:

1. Scale out read-heavy workloads by creating read replicas.
2. Implement data sharding to distribute data across multiple nodes.
3. Perform zero-downtime upgrades of PostgreSQL instances.
4. Integrate with other databases or external systems.

Getting Started with Logical Replication

1. Prepare Primary Server:

   • Ensure that your PostgreSQL version is 10 or higher, as logical replication is supported from this version onwards.
   • Modify the postgresql.conf configuration file on the primary server to include the following settings:
     wal_level = logical max_replication_slots = 5 max_wal_senders = 5

     1 2 3 4 5

     wal_level = logical max_replication_slots = 5 max_wal_senders = 5

     Adjust the values according to your requirements. These settings enable logical replication and specify the maximum number of replication slots and WAL senders allowed.
   • Restart the PostgreSQL service to apply the changes.

2. Create Replication Slot:

   • Create a replication slot on the primary server using the pg_create_logical_replication_slot function. This reserves WAL segments for replication.
     SELECT pg_create_logical_replication_slot('slot_name', 'pgoutput');

     1 2 3

     SELECT pg_create_logical_replication_slot('slot_name', 'pgoutput');

     Replace 'slot_name' with the name you choose for the replication slot. The 'pgoutput' parameter specifies the output plugin used for logical replication.

3. Create Publication:

   • Create a publication on the primary server to define which tables or database objects to replicate.
     CREATE PUBLICATION my_publication FOR TABLE my_table;

     1 2 3

     CREATE PUBLICATION my_publication FOR TABLE my_table;

     Replace 'my_publication' with the name of your publication and 'my_table' with the name of the table you want to replicate. You can add multiple tables to a publication.

4. Prepare Replica Server:

   • On the replica server, ensure that your PostgreSQL version is compatible with logical replication.
   • Modify the recovery.conf or postgresql.conf file on the replica server to include the following setting:
     primary_conninfo = 'host=primary_host port=5432 dbname=mydb user=myuser password=mypassword'

     1 2 3

     primary_conninfo = 'host=primary_host port=5432 dbname=mydb user=myuser password=mypassword'

     Replace the connection string with the appropriate values for your primary server.

5. Create Subscription:

   • Create a subscription on the replica server to subscribe to the publication on the primary server.
     CREATE SUBSCRIPTION my_subscription CONNECTION 'host=primary_host port=5432 dbname=mydb user=myuser password=mypassword' PUBLICATION my_publication;

     1 2 3

     CREATE SUBSCRIPTION my_subscription CONNECTION 'host=primary_host port=5432 dbname=mydb user=myuser password=mypassword' PUBLICATION my_publication;

     Replace 'my_subscription' with the name of your subscription, and adjust the connection string and publication name as needed.

6. Start Replication:

   • Start the replication process by enabling the subscription.
     ALTER SUBSCRIPTION my_subscription ENABLE;

     1 2 3

     ALTER SUBSCRIPTION my_subscription ENABLE;

7. Monitor and Manage Replication:

   • Use SQL queries to monitor replication status, lag, and manage replication settings as needed.
     • Check replication status: SELECT * FROM pg_stat_replication;
     • Monitor replication lag: SELECT pg_current_wal_lsn() - replay_lsn AS replication_lag FROM pg_stat_replication;
     • Add or remove tables from the publication: ALTER PUBLICATION my_publication ADD TABLE new_table;
     • Refresh the subscription: ALTER SUBSCRIPTION my_subscription REFRESH PUBLICATION;
     • Drop subscription or publication: DROP SUBSCRIPTION my_subscription; DROP PUBLICATION my_publication;

Monitoring and Managing Logical Replication

Once your logical replication is up and running, it’s essential to monitor its progress and performance. Here are some useful queries and tools:

1. 1. Check replication status:

1. 2. Monitor replication lag:

1. 3. Add or remove tables from the publication:

1. 4. Refresh the subscription to synchronize schema changes:

1. 5. Drop a subscription or publication:

Conclusion

PostgreSQL’s logical replication is a powerful feature that allows you to scale your database, improve performance, and ensure data availability. By understanding its capabilities and implementing the steps outlined in this comprehensive guide, you can harness the full potential of logical replication in your PostgreSQL environment.