Automating slot creation with Ansible

Logical replication slot provisioning for Change Data Capture (CDC) pipelines demands deterministic, idempotent execution to eliminate WAL retention gaps, consumer desynchronization, and configuration drift across distributed environments. Manual psql execution introduces operational risk, particularly when scaling Python ETL consumers or orchestrating multi-tenant data platform deployments. Ansible provides a declarative control plane for slot lifecycle management, enabling database engineers and DevOps teams to enforce consistent configuration, capture precise restart_lsn anchors, and integrate slot metadata directly into pipeline orchestration workflows.

Architectural Alignment & Idempotent Execution #

Automated slot provisioning operates as a foundational component within the broader Logical Replication Setup & Management framework, replacing ad-hoc administrative commands with version-controlled infrastructure-as-code. The operational intent is strictly isolated to provisioning persistent logical decoding slots bound to the pgoutput plugin, enforcing deterministic naming conventions, and exposing slot state to downstream CDC consumers. This approach eliminates race conditions during consumer startup and guarantees that WAL segments are retained until explicitly acknowledged by the replication stream.

Idempotency is enforced by querying pg_replication_slots prior to creation. The Ansible controller evaluates slot_name, plugin, and slot_type against the desired state. If the slot exists and matches the expected configuration, the task returns changed: false. If the slot is absent or misconfigured, the playbook executes a CREATE_REPLICATION_SLOT statement with explicit parameters. This workflow aligns with standardized Initializing Replication Slots procedures while shifting execution from interactive terminals to auditable automation pipelines.

Production-Grade Playbook Implementation #

The following Ansible tasks leverage community.postgresql.postgresql_query (v2.3+) and PostgreSQL 14+ syntax to guarantee atomic slot creation, precise restart_lsn capture, and robust error handling. Variables are injected via Ansible Vault or external inventory, ensuring sensitive credentials never appear in playbooks.

- name: Query existing replication slots
  community.postgresql.postgresql_query:
    db: "{{ target_db }}"
    query: >
      SELECT slot_name, plugin, slot_type, active, restart_lsn
      FROM pg_replication_slots
      WHERE slot_name = '{{ slot_name }}'
  register: slot_state
  retries: 3
  delay: 2
  until: slot_state is not failed
  changed_when: false

- name: Create persistent logical replication slot
  community.postgresql.postgresql_query:
    db: "{{ target_db }}"
    query: >
      SELECT slot_name, restart_lsn
      FROM pg_create_logical_replication_slot('{{ slot_name }}', 'pgoutput', false, false)
  register: slot_creation
  when: slot_state.query_result | length == 0
  retries: 3
  delay: 5
  until: slot_creation is not failed

- name: Register slot metadata for downstream consumers
  ansible.builtin.set_fact:
    cdc_slot_restart_lsn: "{{ slot_creation.query_result[0].restart_lsn }}"
    cdc_slot_active: false
  when: slot_creation is changed

The pg_create_logical_replication_slot call explicitly passes temporary := false and two_phase := false (adjust two_phase to true only if your CDC pipeline requires atomic distributed transaction decoding). The false flag for temporary behavior ensures WAL retention survives consumer restarts, network partitions, or rolling deployments. Data platform teams must enforce naming conventions that encode environment, consumer group, and pipeline identifier (e.g., cdc_prod_etl_orders_v2) to prevent namespace collisions during parallel deployments.

WAL Retention & Configuration Alignment #

Slot creation must align with PostgreSQL server configuration to prevent unbounded WAL growth. The following parameters require explicit validation before slot provisioning:

Unacknowledged slots cause pg_wal to accumulate indefinitely. Implement automated lag monitoring via pg_stat_replication and pg_replication_slots views. When restart_lsn diverges significantly from confirmed_flush_lsn, trigger alerting and evaluate slot reset or consumer restart.

Pipeline Integration & Operational Resilience #

Subscription Sync & Publication Binding #

Logical slots operate independently of publications until a subscription attaches. After slot provisioning, Python ETL developers or orchestration tools must bind the slot to a publication using CREATE SUBSCRIPTION ... WITH (slot_name = '{{ slot_name }}', create_slot = false, copy_data = false). This prevents duplicate slot creation and aligns with standard subscription sync procedures. The copy_data = false flag ensures the pipeline begins streaming from the exact restart_lsn captured during Ansible execution, eliminating redundant historical data transfer.

Async Monitoring Integration #

Expose slot state to observability platforms by querying pg_replication_slots at regular intervals. Key metrics include:

• slot_lag_bytes: Difference between current WAL position and restart_lsn
• active: Boolean indicating consumer connectivity
• confirmed_flush_lsn: Last LSN acknowledged by the consumer

Integrate these metrics into Prometheus exporters or cloud-native monitoring stacks. Configure thresholds to trigger automated remediation when lag exceeds acceptable bounds (e.g., > 5GB or > 2 hours).

Error Handling & Retry Logic #

Network partitions, connection pool exhaustion, or transient PostgreSQL restarts can cause slot creation to fail. Ansible’s retries, delay, and until directives handle transient failures gracefully. For persistent failures, implement fallback logic that logs the exact PostgreSQL error code (SQLSTATE), notifies the on-call rotation, and halts pipeline deployment until manual intervention or automated recovery completes.

Emergency Failover Procedures #

When a consumer crashes irrecoverably or a slot becomes permanently inactive, immediate intervention is required to prevent WAL bloat. Execute the following sequence:

1. Verify consumer termination: SELECT active FROM pg_replication_slots WHERE slot_name = '{{ slot_name }}';
2. Drop the stale slot: SELECT pg_drop_replication_slot('{{ slot_name }}');
3. Re-provision via Ansible with a fresh restart_lsn anchor.
4. Re-attach the publication and resume streaming from the new checkpoint.

Document slot lifecycle transitions in runbooks and automate failover detection using external health checks. Never drop an active slot during peak ingestion windows; schedule maintenance during low-throughput periods or implement graceful consumer shutdown protocols.