PostgreSQL, a widely used open-source relational database system, faces a common challenge called replication lag. This blog explores what replication lag is, why it happens, and provides professional insights on how to address it effectively in PostgreSQL.

So, let’s talk about a client setup involving TB’s of data, using a primary data center (DC) and a distant disaster recovery (DR) setup. Unfortunately, the DR node faced continuous replication lag issues, leading to frequent breaks in replication. This meant we had to spend a lot of time recreating the DR server. To get to the bottom of the problem, we conducted a thorough investigation, addressing each aspect one by one. It’s worth noting that different environments may have unique challenges, so understanding the specific nature of the environment is crucial when dealing with such situations. Here, I’ll share the steps we took to investigate and resolve the replication lag issue in simpler terms.

Let’s see what is replication lag?

Replication lag is the time delay between data being written in the main database and its replication to backup databases in PostgreSQL. This delay can happen for reasons like slow network, sluggish disk performance, or prolonged transactions.
In systems that depend on backup databases for failover, having too much replication lag is a big problem. If the lag takes too much time, there’s a chance of losing data when failover events occur.

You can check the lag using below query on Primary Node:
SELECT pid,application_name,client_addr,client_hostname,state,sync_state,replay_lag
postgres-# FROM pg_stat_replication

While on Standby use below:
select pg_is_in_recovery(),pg_is_wal_replay_paused(), pg_last_wal_receive_lsn(), pg_last_wal_replay_lsn(), pg_last_xact_replay_timestamp();

What causes replication lag?

Replication lag can arise from several factors, including:

• Configuration issues: Poor configurations, like setting low values for max_wal_senders while handling a high volume of transaction requests, can contribute to lag.
• Long-running transactions: Replication can be delayed when transactions take a long time to complete, as changes are not replicated until the transaction is committed.
• Network latency: The delay in data transfer between the primary and standby databases, influenced by factors like distance and network congestion.
• Slow disk I/O: Issues like disk fragmentation or insufficient disk space can slow down writing data to standby databases.
• Checkpointing behavior: Sometimes, the server recycles old Write-Ahead Logging (WAL) segments before the backup completes, creating difficulties in finding the required WAL segment from the primary. This is often related to checkpointing practices where WAL segments are rotated or recycled.

At times, the server might reuse old Write-Ahead Logging (WAL) segments before the backup process completes, leading to difficulties in locating the required WAL segment from the primary server. This situation is typically influenced by the checkpointing behavior, where WAL segments undergo rotation or recycling.

Addressing Replication Lag in PostgreSQLx

How to Tackle Replication Lag in PostgreSQL! Here we go.

Boosting the Connection: Amp up your PostgreSQL game by beefing up the network bandwidth between primary and standby databases to kick network latency out of the equation.

Sync with Style: Embrace the asynchronous replication charm to let standby databases groove at their own pace, without waiting for the primary database to finish its transactions.

Fine-Tuning Symphony: Elevate your PostgreSQL experience by fine-tuning configuration parameters like wal_buffers and max_wal_senders, setting the stage for optimal performance and minimal replication lag.

Keep an Eye on the Beat: Stay in control by monitoring replication lag using PostgreSQL tools like pg_stat_replication and pg_wal_receiver_stats, ensuring a smooth performance rhythm.

In wrapping up, dealing with replication lag is a familiar challenge in PostgreSQL, and it can significantly impact systems striving for high availability.

To safeguard the reliability of the database system, it’s crucial to comprehend the root causes of replication lag and take targeted actions. Elevating network bandwidth, adopting asynchronous replication, fine-tuning PostgreSQL settings, and vigilant monitoring with tools like pg_stat_replication are proactive steps. These measures empower administrators to tackle replication lag, fostering a more resilient and dependable database environment.