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backendcritical PriorityID: postgres-17-logical-replication-failover-slots

PostgreSQL 17 Synchronized Logical Replication Failover Slots

Enforces failover-enabled logical replication slots and standby slot synchronization introduced in PostgreSQL 17. Guarantees that downstream CDC consumers, Debezium instances, and transactional outbox workers survive physical primary failover without silent data loss, split-brain LSN divergence, or massive duplicate event replays.

Prohibited Anti-Pattern: Unsynchronized logical slots on HA clusters causing catastrophic event loss or missing outbox CDC feeds during primary failover.

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---
description: PostgreSQL 17 Synchronized Logical Replication Failover Slots - Enforces failover-enabled logical replication slots and standby slot synchronization introduced in PostgreSQL 17. Guarantees that downstream CDC consumers, Debezium instances, and transactional outbox workers survive physical primary failover without silent data loss, split-brain LSN divergence, or massive duplicate event replays.
globs: src/**/*.ts, migrations/**/*.sql, deploy/**/*.sql, infra/**/*.tf
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/postgres-17-logical-replication-failover-slots"
---

# PostgreSQL 17 Synchronized Logical Replication Failover Slots

> **Severity**: CRITICAL | **Category**: backend
> **Anti-Pattern Warning**: Unsynchronized logical slots on HA clusters causing catastrophic event loss or missing outbox CDC feeds during primary failover.

## System Constraints
You are a Principal Database Reliability Engineer enforcing PostgreSQL 17 high-availability standards. Mandate failover-enabled logical replication slots (failover = true) and physical standby synchronization (synchronized_standby_slots) to safeguard CDC pipelines and transactional outbox architectures against automatic multi-AZ failovers.

## Strict Guidelines
- Engineers MUST ALWAYS set `failover = true` when invoking `pg_create_logical_replication_slot()` or `CREATE SUBSCRIPTION ... WITH (failover = true)` on PostgreSQL 17+.
- Engineers MUST configure `synchronized_standby_slots` on the primary instance containing the physical replication slot names of HA standbys, ensuring the primary does not emit logical WAL records past the physical standby sync point.
- Standby replicas MUST configure `sync_replication_slots = true` in postgresql.conf to allow automatic background logical slot tracking via the slotsync worker.
- CDC consumer initialization code MUST verify `synced = true` from `pg_replication_slots` on the newly promoted primary before resuming consumption after failover.
- Application pipelines MUST NEVER recreate missing replication slots blindly upon disconnect; doing so resets the confirmed flush LSN to current WAL end and permanently drops uncommitted outbox events.
- Subscribers and outbox processors MUST maintain idempotent consumer deduplication because synchronized failover guarantees at-least-once delivery with bounded replay from the synchronized confirmed LSN.

## Bad vs Good Implementations
### Configuring PG17 Failover-Enabled Logical Replication Slots

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Legacy logical replication slots lack failover persistence in PostgreSQL. During an unexpected primary node failover or Patroni switchover, standby instances do not have this slot, causing the CDC pipeline to break or skip unconsumed outbox events.
```typescript
import { Client } from 'pg';

export async function setupOutboxSlot(client: Client, slotName: string): Promise<void> {
  // Anti-Pattern: Creating legacy logical slot without failover flag in Postgres 17.
  // When the physical primary fails over to a multi-AZ standby, this slot does not exist
  // on the new primary, causing unacknowledged outbox events to be permanently lost.
  await client.query(
    `SELECT pg_create_logical_replication_slot($1, 'pgoutput');`,
    [slotName]
  );
}
```

#### Production Standard (Correct)
> **Standard**: PostgreSQL 17 allows the fifth argument `failover = true` in `pg_create_logical_replication_slot()`. Combined with `synchronized_standby_slots`, this guarantees the slot is synced to all standby nodes, preserving consistent LSN positions during automatic failovers.
```typescript
import { Client } from 'pg';

export async function setupOutboxSlot(client: Client, slotName: string): Promise<void> {
  // Production Standard: Explicitly configure PG17 failover-enabled logical replication slot
  // and assert synchronized standby guardrails are present on the primary node.
  const checkResult = await client.query<{ server_version_num: string; synchronized_standby_slots: string }>(
    `SELECT current_setting('server_version_num')::int AS server_version_num,
            current_setting('synchronized_standby_slots', true) AS synchronized_standby_slots;`
  );

  const version = parseInt(checkResult.rows[0].server_version_num, 10);
  if (version < 170000) {
    throw new Error(`Postgres 17+ required for failover slots. Current version num: ${version}`);
  }

  if (!checkResult.rows[0].synchronized_standby_slots) {
    throw new Error('Safety guardrail violation: synchronized_standby_slots must be configured on primary');
  }

  // pg_create_logical_replication_slot(slot_name, plugin, temporary, two_phase, failover)
  await client.query(
    `SELECT pg_create_logical_replication_slot(
       $1,
       'pgoutput',
       false,
       false,
       true
     );`,
    [slotName]
  );
}
```
Recommended MCP Servers (mcp-serverlist.com)

Equip agent with system tools to verify code for this stack.

PostgreSQL Diagnostic MCP Server
Provides schema inspection and replication slot diagnostics directly inside AI agent workflows.
Recommended Agent Skills (askills.dev)

Ready-to-use executable skill functions tailored to this stack.

Postgres Replication Health Check CLI
Inspects pg_replication_slots and pg_stat_replication for synchronized failover slot compatibility and lag.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: PostgreSQL 17 Synchronized Logical Replication Failover Slots

PostgreSQL 17 Synchronized Logical Replication Failover Slots defines an industrial-grade behavioral standard for backend engineering. Enforces failover-enabled logical replication slots and standby slot synchronization introduced in PostgreSQL 17. Guarantees that downstream CDC consumers, Debezium instances, and transactional outbox workers survive physical primary failover without silent data loss, split-brain LSN divergence, or massive duplicate event replays. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: Unsynchronized logical slots on HA clusters causing catastrophic event loss or missing outbox CDC feeds during primary failover. Without an explicit system instruction file, developers spend hours debugging silent failures.
Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure ModeAI Symptom (The Anti-Pattern)SeverityProduction Impact & Remedy
AI Hallucination & Deprecation DriftUnsynchronized logical slots on HA clusters causing catastrophic event loss or missing outbox CDC feeds during primary failover.Critical
Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production.
Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture InconsistencyAI assistants produce mixed paradigms across files, creating fragmented codebase conventions.Medium
Increases cognitive load during code reviews and leads to maintenance debt across engineering teams.
Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.
Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

1

Engineers MUST ALWAYS set `failover = true` when invoking `pg_create_logical_replication_slot()` or `CREATE SUBSCRIPTION ... WITH (failover = true)` on PostgreSQL 17+.

Engineers MUST ALWAYS set `failover = true` when invoking `pg_create_logical_replication_slot()` or `CREATE SUBSCRIPTION ... WITH (failover = true)` on PostgreSQL 17+.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
2

Engineers MUST configure `synchronized_standby_slots` on the primary instance containing the physical replication slot names of HA standbys, ensuring the primary does not emit logical WAL records past the physical standby sync point.

Engineers MUST configure `synchronized_standby_slots` on the primary instance containing the physical replication slot names of HA standbys, ensuring the primary does not emit logical WAL records past the physical standby sync point.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
3

Standby replicas MUST configure `sync_replication_slots = true` in postgresql.conf to allow automatic background logical slot tracking via the slotsync worker.

Standby replicas MUST configure `sync_replication_slots = true` in postgresql.conf to allow automatic background logical slot tracking via the slotsync worker.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
4

CDC consumer initialization code MUST verify `synced = true` from `pg_replication_slots` on the newly promoted primary before resuming consumption after failover.

CDC consumer initialization code MUST verify `synced = true` from `pg_replication_slots` on the newly promoted primary before resuming consumption after failover.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
5

Application pipelines MUST NEVER recreate missing replication slots blindly upon disconnect

Application pipelines MUST NEVER recreate missing replication slots blindly upon disconnect; doing so resets the confirmed flush LSN to current WAL end and permanently drops uncommitted outbox events.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
6

Subscribers and outbox processors MUST maintain idempotent consumer deduplication because synchronized failover guarantees at-least-once delivery with bounded replay from the synchronized confirmed LSN.

Subscribers and outbox processors MUST maintain idempotent consumer deduplication because synchronized failover guarantees at-least-once delivery with bounded replay from the synchronized confirmed LSN.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
Technical Questions & Answers

Frequently Asked Technical Questions

Common questions developers face when configuring AI coding assistants for this stack:

Q:Why is PostgreSQL 17 Synchronized Logical Replication Failover Slots critical for AI coding assistants?

Enforces failover-enabled logical replication slots and standby slot synchronization introduced in PostgreSQL 17. Guarantees that downstream CDC consumers, Debezium instances, and transactional outbox workers survive physical primary failover without silent data loss, split-brain LSN divergence, or massive duplicate event replays. Without explicit behavioral boundaries, AI coding assistants (Cursor, Claude Code, Windsurf, Codex) rely on training data averages, which frequently produce deprecated, inefficient, or vulnerable code patterns.

Q:How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (Unsynchronized logical slots on HA clusters causing catastrophic event loss or missing outbox CDC feeds during primary failover.) and prescribes verified positive examples. Grounding the LLM with negative and positive pairs eliminates guesswork.

Q:Which AI coding environments are supported?

This specification is multi-platform compatible: Cursor (.cursor/rules/*.mdc & .cursorrules), Anthropic Claude Code (CLAUDE.md), Windsurf (.windsurfrules), Codex / Cline (AGENTS.md), and GitHub Copilot (.github/copilot-instructions.md).

Q:Can I combine this rule with other technology stacks?

Yes. Use the Interactive Stack Builder or download an Agent Pack to merge this rule with other frontend, backend, database, security, and DevOps rules without syntax collision.

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