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Pg Create Physical Replication SlotEdit

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Pg Create Physical Replication Slot

In PostgreSQL, a physical replication slot is a server-side object that helps manage the flow of WAL (Write-Ahead Log) data to a standby server in a streaming replication setup. By anchoring on a specific WAL position for a given replication client, the primary node retains the WAL segments needed by the standby (or backup process) instead of recycling them immediately. This mechanism ensures that a replica can catch up and remain consistent with the primary, even if the standby experiences temporary disconnections or slowdowns. See PostgreSQL for the broader database system context and WAL for the logging mechanism that underpins this feature.

Creating a physical replication slot

A physical replication slot can be created in PostgreSQL with SQL or via the catalog function. The primary purposes are to preserve WAL for a particular replication consumer and to provide a deterministic point from which a standby can resume replication.

  • SQL command (typical syntax):

    • CREATE REPLICATION SLOT slot_name TYPE physical;

  • Alternative function (also widely used):

    • SELECT pg_create_physical_replication_slot('slot_name');

In both cases, the slot name is a persistent object stored in the cluster’s catalog, and it remains until it is explicitly dropped or the cluster no longer needs it. The slot will be visible in the system catalogs, such as the view pg_replication_slots.

Notes on permissions and scope: - Creating replication slots generally requires a role with REPLICATION privileges or a superuser. - A slot is a cluster-wide object, not per-user, and it is usable by a single standby at a time with physical replication (as opposed to logical replication, which uses logical slots).

How slots interact with replication and backups

  • The primary writes WAL as part of normal operation. When a physical replication slot exists for a standby, the primary will retain WAL files up to the point that the standby has confirmed receipt (via the slot’s position) rather than aging them out immediately.
  • Slots are especially important in streaming replication, where a standby may temporarily lag or reconnect after a network interruption. By preserving the WAL, the standby can resume from the correct location without requiring a fresh base backup.
  • Slots can be helpful during base backups. Tools like pg_basebackup can leverage slots to ensure that the backup process has the necessary WAL history for a consistent restore. For example, you can use a slot with pg_basebackup to ensure the backup stream has the required WAL available during the operation. See pg_basebackup for more details on base backups and streaming.

slot lifecycle and maintenance: - To remove a slot when it is no longer needed, you use DROP_REPLICATION_SLOT or the corresponding function (e.g., SELECT pg_drop_replication_slot('slot_name')). - If a standby that relied on a slot is permanently removed, leaving the slot in place can cause the primary to retain WAL longer than necessary, potentially consuming disk space. Regular review of active slots and their usage is prudent. - The slot’s activity state is reported in pg_replication_slots; an active slot indicates that a replication connection is currently using or streaming through that slot.

Operational considerations

  • Disk usage: A slot prevents WAL recycling until the consumer has advanced beyond the WAL it requires. If a standby is down for a long period or lags severely, WAL files can accumulate, consuming disk space on the primary.
  • Recovery and consistency: Replication slots help guarantee that standby databases can rejoin replication without data loss due to missing WAL, assuming the slot is managed correctly and the standby is able to catch up.
  • Interaction with backups: Using slots during baselines or standby promotions can provide a stable point from which to resume replication or perform restores.

Best practices and common patterns

  • Monitor slot usage and disk space to avoid unexpected outages caused by WAL backlog. Alerts on WAL write-ahead log space and slot activity help maintain reliability.
  • Use slots in conjunction with automated failover planning. Slots simplify ensuring that a promoted standby has access to the required WAL history for a clean transition.
  • When a standby is decommissioned, drop its slot promptly to release retained WAL and reclaim space.
  • Consider combining slots with base backups or with streaming replication configurations to improve resilience and recovery options. See base backup for general backup concepts and streaming replication for the broader replication approach.

See also