How Do You Drop a Replication Slot in PostgreSQL?

Replication slots are a crucial feature in PostgreSQL to ensure that changes in the database are delivered reliably to replicas. They prevent the removal of write-ahead logs (WAL) that are still needed by replicas, maintaining synchronization across the system.

However, there may be scenarios where you need to drop a replication slot, such as cleaning up unused slots, resolving replication issues, or reconfiguring replication setups. Dropping a replication slot removes its associated WAL retention, freeing up storage and system resources.

This article explains how to enable the Postgres drop replication slot safely.

What are Replication Slots in PostgreSQL?

Replication slots are a mechanism introduced in the PostgreSQL 9.4 version, along with logical decoding, to ensure reliable replication. This mechanism helps you track the replication status for downstream clients like standby servers or logical replication subscribers. As a result, replication slots enable you to make sure no changes are lost during replication, even if there is a delay in processing them.

Purpose of PostgreSQL Replication Slots

In PostgreSQL, replication relies on several key internal components, most notably the WAL. The WAL allows you to record all the database changes in the memory via the WAL buffer. When the buffer reaches its configured size set by the wal_buffers setting, the data is flushed to disk as a WAL segment.

Each entry in the WAL is identified by a Log Sequence Number (LSN), a byte offset that helps you increase with each new record. These WAL records are stored in the pg_wal directory under the PostgreSQL data directory, with a default maximum size of 16 MB.

The WAL files are then decoded using logical decoding. It allows you to convert the raw WAL data into an easy-to-understand format representing high-level database operations like INSERT, UPDATE, and DELETE.

The logical change records can then be published by a PostgreSQL instance and subscribed to by other servers. The WAL Sender process helps you stream these changes to subscribed standby servers after flushing the WAL records to disk. This process shows how PostgreSQL CDC replication works.

To manage this replication, PostgreSQL uses replication slots, which are persistent data structures that help you track the replication progress for each subscriber. Although WAL files are continuously generated, PostgreSQL’s wal_keep_size or checkpoint settings may cause older files to be deleted.

By leveraging replication slots, you can prevent WAL files required by subscribers (replicas) from being removed, regardless of these settings. This way, you can keep the necessary WAL files until the connected standby server or replica no longer needs them.

Why Drop PostgreSQL Replication Slots?

Some of the reasons for Postgres drop replication slots are:

Disk Space Consumption

If a replica is shut down or fails and the replication slot associated with it is not removed, the slot becomes an orphan record. It can lead to unbounded growth in the disk usage on the master server. This excessive disk space consumption can result from the WAL segments not being removed until the replication slot is cleared.

To prevent this, drop the replication slots that are no longer in use, particularly when a replica is offline for an extended period or permanently down.

Performance Implications

Orphaned replication slots can negatively impact server performance. Since the server keeps unnecessary WAL files, the system has to track and manage slots that are no longer needed. It can increase disk I/O and memory usage. Over time, this can cause resource contention, affecting the overall performance of the PostgreSQL instance.

Dropping unused replication slots enables you to solve these performance issues, ensuring that the server resources are allocated efficiently for active replication tasks. 

How to Drop Replication Slots in PostgreSQL?

Replication slots need to be created and deleted manually. It is important not to leave inactive slots, as the master server will retain the WAL files needed by those inactive slots indefinitely, potentially filling up disk space. The command for the Postgres drop replication slot is:

select pg_drop_replication_slot(‘name_of_replication_slot’);

Where pg_drop_replication_slot is a system function that helps you remove a physical or logical replication slot.

Before you drop the replication slot in PostgreSQL, you must follow the below practices to not affect your replication setup:

Verify Replication Slot Status

Initially, you must verify the status of the replication slot to ensure it is no longer used. With the following query, you can list all active replication slots:

SELECT * FROM pg_replication_slots;

The output will display information about all replication slots, including the active column value, which indicates whether a slot is currently in use. If the active column shows f (false), the slot is inactive and can be safely removed.

Check If There Are Any Dependent Applications

You should verify that no active replication clients or applications depend on the replication slot you plan to drop. Dropping an in-use slot could disrupt replication and cause data inconsistency.

Check If You Have All the Permissions

Ensure you have sufficient privileges to drop the replication slot in PostgreSQL. You must be a superuser or the owner of the replication slot.

How Does Airbyte Manage Replication Slots in Postgres CDC?

Replicating data between PostgreSQL servers through the master-replica (primary-standby) pattern using logical replication is an efficient and reliable approach. In this method, the replication slots allow you to ensure that the master (primary) server retains enough WAL segments for all replicas.

However, this approach is well-suited for PostgreSQL-to-PostgreSQL replication. If you need to replicate data from PostgreSQL to other target systems, leveraging the logical replication method alone would not be adequate. A no-code data movement platform like Airbyte helps you streamline this process. It offers 550+ pre-built connectors, including PostgreSQL, to help you extract data from various sources and load it into your chosen destination.

Airbyte uses logical replication of Postgres WAL to incrementally capture changes through a replication plugin. To start replicating your data from the PostgreSQL database to a preferred destination, follow these steps:

• Begin by configuring PostgreSQL as a source in Airbyte Cloud and setting the necessary permissions.
• Enable logical replication in the Postgres database.
• Create a replication slot on your Postgres database using pgoutput replication plugin.
• Create publication and replication identities for each Postgres table.
• Enable CDC in Airbyte UI.

To enable Postgres CDC with Airbyte, go through this in-detailed guide.

Troubleshooting Issues While Dropping Postgres Replication Slots

Let’s address a few challenges encountered when attempting to drop the replication slot in PostgreSQL:

Why Aren't You Able to Drop a Replication Slot?

You might not have sufficient permissions, or the replication slot may still be in use by an active replication client.

How to Stop an Active Replication Slot?

To stop an active replication slot, you must first ensure it is not active on the replica side. If you cannot disable it directly on the replica, you must terminate the WAL sender process using the query given:

SELECT pg_terminate_backend(backend_process_id);

Now, you can drop the slot before it restarts:

SELECT pg_drop_replication_slot('slot_name');

Why Do Replication Slots Keep Recreating After You Drop Them?

Replication slots may be recreated automatically if they are associated with an active publication or replication subscription.

How Can You Drop Orphaned Replication Slots?

Orphaned slots can be dropped using the pg_drop_replication_slot() function if they are inactive.

Can't Able to Drop a Replication Slot After Dropping Publication?

Dropping a publication alone automatically drops replication slots. You must manually drop the replication using the pg_drop_replication_slot() function.

Conclusion

Replication slots are critical for managing the consistency of streaming replication. Postgres drop replication slot is a simple process but requires caution. Before removing a replication slot, you must ensure that it is unused. Following the steps and best practices given in this guide can help you handle replication slots without affecting your database’s replication setup.