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Pg Create Logical Replication SlotEdit

PostgreSQL supports flexible replication through logical decoding, which enables feeding change streams to external systems. A core construct in this ecosystem is the logical replication slot, created by the function or SQL command that invokes the underlying logical decoding mechanism. A replication slot ensures that the WAL (Write-Ahead Log) records needed by a logical decoding consumer are retained until the consumer has processed them. This feature is central to building reliable streaming replication pipelines, data integration jobs, and real-time analytics that run off a primary database. For practical use, see the built-in plugins such as pgoutput and test_decoding as common sources of decoding output.

One key distinction in PostgreSQL replication is between physical replication slots and logical replication slots. Physical slots preserve the exact order and integrity of physical streaming replication, while logical slots allow a higher-level view of changes (inserts, updates, deletes, and DDL) that can be consumed by external subscribers. The logical slot mechanism is implemented to coordinate between the primary cluster and one or more subscribers, ensuring that WAL changes are retained long enough for downstream processes to read them. See replication slot for a broader treatment of this concept.

Overview

  • Logical replication slots are per-database objects maintained by the server. They track a decoding position in the WAL and keep WAL segments needed by the connected logical decoding output plugin until those segments have been consumed.
  • The creation and management of logical replication slots typically require a user with the REPLICATION privilege or superuser status. This permission model is designed to prevent unauthorized consumption of WAL data.
  • A slot’s progress is expressed in terms of Log Sequence Numbers (LSNs). The slot has values such as the slot’s name, the plugin in use, and the current restart/confirmed positions that indicate how far downstream consumers have acknowledged processing.
  • Plugins are the payloads that interpret the raw WAL changes. The built-in options include pgoutput for the standard logical replication flow and test_decoding for debugging and demonstrations.

Creating and configuring slots

  • A slot is created to begin logical decoding for a given stream. The typical creation path is through a function call or a SQL command that specifies the slot name and the decoding plugin. For example, a common pattern is to create a slot with a call to the function that handles logical replication slot creation, such as pg_create_logical_replication_slot. The command associates the slot with a decoding plugin and sets the initial consumption point.
  • The command can be invoked from a privileged user session, and the resulting slot will appear in the server’s catalog of replication slots. Typical queries to inspect slots include retrieving data from pg_replication_slots to see the current configuration, slot type, active test, and the restart/confirmed points.
  • In practice, you might see a slot created with a command like: SELECT * FROM pg_create_logical_replication_slot('my_slot', 'test_decoding'); This creates a logical slot named my_slot using the test_decoding output plugin. For production pipelines that use a more structured feed, a built-in plugin such as pgoutput can be chosen to support the standard logical replication workflow.

Usage patterns and considerations

  • Practical use cases include streaming changes to an external data warehouse, feeding a real-time cache invalidation system, or powering cross-cluster synchronization via Subscriptions in a logical replication topology.
  • When a subscriber falls behind, the server preserves WAL for as long as the slot requires, up to configured limits. If a slot is not consumed, disk usage in the WAL area can grow; administrators must monitor the growth of pg_replication_slots and related WAL retention settings to avoid storage problems.
  • Dropping a replication slot removes the bookkeeping for that decoding stream. The command to drop a slot is commonly issued as pg_drop_replication_slot for clean removal when a consumer is decommissioned or moved to a new slot. After dropping, the server will no longer retain WAL for that stream based on that slot.

Management and monitoring

  • Slots are visible in system catalogs and can be queried to assess their status, including the plugin in use and the current restart point. Administrators typically monitor the wellbeing of logical replication through views and metrics such as pg_stat_replication and the replication slot catalog.
  • It is important to coordinate slot creation with the lifecycle of publications and subscriptions in the cluster. In PostgreSQL, logical replication uses Publications and Subscriptions to manage which tables are published and consumed, respectively.
  • Security considerations include restricting slot creation to trusted roles and ensuring that only authorized processes can connect to the replication stream. The replication mechanism is designed to protect integrity and ensure that only legitimate consumers can participate in change-data capture workflows.

Architecture and performance implications

  • A logical replication slot provides a retention point in the WAL stream. The presence of an active slot prevents the WAL from being truncated, which guarantees that attendees downstream have access to the required changes.
  • The choice of decoding plugin influences both the format of the output and the ease of integration with downstream systems. For a streaming pipeline that adheres to the PostgreSQL streaming replication model, the built-in pgoutput plugin is a natural fit. For testing and prototyping, test_decoding offers a human-readable textual representation of changes.
  • System administrators should consider the balance between WAL retention (to prevent data loss for slow subscribers) and disk usage (to avoid filling storage). Tuning strategies may involve monitoring replication lag, adjusting slot usage, and ensuring subscribers maintain throughput that keep pace with WAL generation.

See also