Understanding and Reducing PostgreSQL Replication Lag

Replication lag in PostgreSQL occurs when changes made on the primary server take time to reflect on the replica server. Whether you use streaming or logical replication, lag can impact performance, consistency, and system availability. This post covers the types of replication, their differences, lag causes, mathematical formulas for lag estimation, monitoring techniques, and strategies to minimize replication lag.

Streaming replication continuously sends WAL (Write-Ahead Log) changes from the primary to one or more replica servers in near real-time. The replica applies the changes sequentially, as they're received. This method replicates the entire database and ensures replicas stay synchronized.

Logical replication transfers data-level changes rather than low-level WAL data. It enables selective replication, where only specific tables or parts of a database are replicated. Logical replication uses a logical decoding process to convert WAL changes into SQL-like changes.

Replication lag occurs when the rate at which changes are generated on the primary server exceeds the rate at which they can be processed and applied to the replica server. This imbalance can occur due to various underlying factors, each contributing to delays in data synchronization. The most common causes of replication lag are:

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