Pg Replication SlotsEdit
Pg Replication Slots are a core reliability feature of PostgreSQL that manage the retention of write-ahead log (WAL) segments to support replication clients. By tying WAL retention to specific replication needs, slots help prevent data loss in streaming and logical replication scenarios, while also creating operational considerations that administrators must manage.
In PostgreSQL, the WAL is the durable record of all changes to the database. Replication clients, such as standby servers in a hot standby configuration or external systems consuming logical changes, rely on WAL data to replay and synchronize state. Replication slots ensure that WAL files are not discarded until the corresponding replica has confirmed receipt and processing, reducing the risk of gaps in replication. For readers familiar with the term, you can think of slots as a way to guarantee that the networked consumers have time to catch up without forcing aggressive retention policies elsewhere in the system. See also Write-Ahead Logging and PostgreSQL.
There are two primary kinds of replication slots in PostgreSQL: physical replication slots and logical replication slots. Physical slots are used with traditional streaming replication to one or more standby servers. Logical slots support logical decoding, enabling downstream consumers to receive a stream of changes in a structured format (for example, to feed into external systems or services). See physical replication slot and logical replication slot for more detail.
Overview
Replication slots work as follows: a slot records the point up to which a replica has consumed WAL. The primary must retain WAL files at least until all active slots have progressed beyond their respective consumption points. This model provides strong safety guarantees but introduces tradeoffs in disk usage and operational complexity. Administrators monitor the catalog view pg_replication_slots to see which slots exist, their type, and how far they’ve progressed, and they monitor pg_stat_replication on the primary to understand replication activity and lag. For management tasks, you’ll typically interact with the built-in functions like pg_create_physical_replication_slot and pg_create_logical_replication_slot to create slots, and pg_drop_replication_slot to remove them when they are no longer needed.
Types of replication slots
- Physical replication slots: Used for streaming replication to physical standby servers. They keep WAL files until the standby has replayed transactions up to the slot’s confirmed position. This ensures the standby can recover if it falls behind or reconnects after an outage. See physical replication slot.
- Logical replication slots: Used with logical decoding to feed changes to downstream consumers in a logical format. This enables feeding data into downstream databases, queues, or event streams. See logical replication slot.
Creation and management of these slots is done through specific functions: - Create a physical slot: SELECT pg_create_physical_replication_slot('slot_name'); - Create a logical slot: SELECT pg_create_logical_replication_slot('slot_name', 'pgoutput'); - Drop a slot: SELECT pg_drop_replication_slot('slot_name'); - You can verify and inspect slots via SELECT * FROM pg_replication_slots;
See also pg_replication_slots for the catalog view and pg_stat_replication for monitoring replication on the primary.
How slots interact with WAL and replication
WAL files are generated as normal during database operation. When a replication slot exists, the primary treats WAL records differently: it will not recycle or remove WAL files that are still needed to satisfy the downstream consumers attached to the slots. The committed position for each slot is tracked in the slot’s state, which means the primary can safely discard WAL that is no longer required by any active slot.
This dynamic creates a safety buffer but also a management burden. If a replica (or a logical consumer) falls far behind or goes offline for an extended period, WAL files accumulate, consuming disk space. If disk space runs too low, the primary can stall, potentially impacting write performance for all users until the backlog is cleared. Therefore, slot configuration and monitoring are essential, particularly in environments with large writes or multiple replicas. See WAL and pg_replication_slots.
Operational considerations and best practices
- Plan for disk capacity: Because slots prevent WAL deletion, systems with long-lived slots or offline subscribers require careful provisioning of disk space or archiving solutions. Regularly assess the total WAL retention caused by active slots and the rate of change in the primary. See Disk space management if you want to explore related topics.
- Monitor lag and slot status: Use pg_stat_replication to observe replication lag and pg_replication_slots to verify active slots and their progress. Tuning alerting for slots that appear stalled can prevent unexpected disk usage spikes.
- Use dedicated slots for different consumers: In architectures with multiple downstream consumers (e.g., separate replicas or external event streams), separate slots help isolate progress and retention concerns per consumer.
- Align with backup and DR strategy: Replication slots can play a key role in disaster recovery plans by maintaining a consistent point to recover to, but they should be integrated with backup schedules and WAL archiving policies.
- Consider logical decoding implications: When using logical slots, ensure that the selected decoding output plugin (for example, pgoutput or other logical decoding plugins) is appropriate for downstream systems. Logical slots have their own retention semantics tied to the logical stream and consumer progress.
Risks and tradeoffs
- Disk usage risk: If a replica is slow or offline, WAL retention can grow significantly, increasing disk usage on the primary. Proper capacity planning and monitoring are essential.
- Potential for stalls: If the primary cannot keep up with the backlog, normal write throughput can be affected because WAL files must be retained longer. Efficient replication pipelines and scaled resources help mitigate this risk.
- Complexity in management: Slots add an extra layer of state that must be maintained. Administrators should incorporate slot health checks into routine maintenance and incident response.
Notable considerations in practice
- Slot cleanup: When a replica is decommissioned or a logical consumer is retired, drop the corresponding slot to release WAL retention pressure. This is done with pg_drop_replication_slot.
- Interaction with backup windows: Some backup strategies rely on WAL archiving and consistency guarantees; replication slots should be coordinated with your backup and DR plans to avoid unexpected retention behaviors.
- Version differences: The exact semantics and available features of physical vs logical slots can vary between PostgreSQL major versions; always consult the specific release notes for your version when planning deployment and maintenance.