Question: How does compression work in PostgreSQL replication?

Answer

PostgreSQL replication involves copying and maintaining database objects and data from one database server (the primary) to another (the standby). This process is crucial for ensuring high availability, load balancing, and disaster recovery. Given the potential volume of data transferred during replication, compression can play a significant role in reducing bandwidth usage and improving performance.

Logical and Physical Replication

PostgreSQL supports two main types of replication: logical and physical. Both can benefit from compression, but the mechanisms and configurations may differ.

Physical Replication

Physical replication copies the binary data from the primary to the standby server. It can be synchronous or asynchronous and is typically used for creating exact copies of the database for high availability or backup purposes.

Compression in Physical Replication

In physical replication, compression isn't directly controlled within PostgreSQL itself but can be managed through external tools or by configuring the underlying network infrastructure. For instance, you could use SSH tunneling with compression enabled (ssh -C) for the replication connection or configure a VPN with compression capabilities.

Logical Replication

Logical replication sends changes at the logical transaction level, allowing for more flexibility, such as replicating between different versions of PostgreSQL or selective table replication.

Compression in Logical Replication

While PostgreSQL does not natively compress logical replication traffic directly, logical replication slots can leverage third-party tools or custom scripts that compress data before sending it over the network.

WAL Compression

WAL (Write-Ahead Logging) files are crucial for both physical and logical replication. Starting from PostgreSQL 9.5, it's possible to enable WAL compression (wal_compression parameter), which can reduce the size of WAL data needing to be transferred during replication. This setting, however, compresses WAL files stored on disk and indirectly benefits replication by reducing the volume of data to transfer.

-- Enable WAL compression
ALTER SYSTEM SET wal_compression = on;

Ensuring efficient replication, especially over limited bandwidth connections, might also involve tuning other parameters like max_wal_size, min_wal_size, and wal_keep_segments, depending on your specific replication setup and PostgreSQL version.

Summary

Compression in PostgreSQL replication is an indirect process, relying more on external tools or network configurations for physical replication, and through WAL compression settings that affect both logical and physical replication. Properly configuring these aspects can lead to significant performance improvements and reduced bandwidth usage.