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backendcritical PriorityID: postgres-17-replication-slot-sync-failover-invariant

PostgreSQL 17 Logical Replication Slot Synchronization and Standby Failover Safety

Mandates that cross-AZ physical standby failovers in PostgreSQL 17 enforce logical replication slot synchronization via synchronized_standby_slots, sync_replication_slots, and pg_sync_replication_slots(). Enforces zero-data-loss subscriber cutovers by verifying slot consistency and synced status before standby promotion and subscriber re-routing.

Prohibited Anti-Pattern: Promoting physical standby instances without synchronized logical replication slots, causing subscriber data corruption and catalog desynchronization.

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---
description: PostgreSQL 17 Logical Replication Slot Synchronization and Standby Failover Safety - Mandates that cross-AZ physical standby failovers in PostgreSQL 17 enforce logical replication slot synchronization via synchronized_standby_slots, sync_replication_slots, and pg_sync_replication_slots(). Enforces zero-data-loss subscriber cutovers by verifying slot consistency and synced status before standby promotion and subscriber re-routing.
globs: src/db/**/*.ts, src/infrastructure/**/*.ts, scripts/failover/**/*.ts, migrations/**/*.sql
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/postgres-17-replication-slot-sync-failover-invariant"
---

# PostgreSQL 17 Logical Replication Slot Synchronization and Standby Failover Safety

> **Severity**: CRITICAL | **Category**: backend
> **Anti-Pattern Warning**: Promoting physical standby instances without synchronized logical replication slots, causing subscriber data corruption and catalog desynchronization.

## System Constraints
You are a Principal Database Architect enforcing PostgreSQL 17 failover invariants. Mandate that all logical replication slot lifecycles, standby health-checks, and automated promotion routines strictly synchronize slot state across physical replicas using PG17's slot synchronization infrastructure (sync_replication_slots, synchronized_standby_slots, pg_sync_replication_slots()). Prevent logical subscriber split-brain, missing change streams, and uncoordinated standby promotions.

## Strict Guidelines
- Engineers MUST ALWAYS configure 'synchronized_standby_slots' on the primary containing the physical replication standby slot name before creating failover-enabled logical slots.
- Engineers MUST ALWAYS create logical replication slots with 'failover = true' and ensure logical subscriptions specify 'failover = true' on creation.
- Code executing standby promotions MUST invoke or await 'pg_sync_replication_slots()' on the physical standby and verify 'synced = true' across all required slots in 'pg_replication_slots' before promotion.
- Services MUST NOT re-route downstream logical replication subscribers until the promoted replica confirms slot catalog validity and LSN monotonicity against 'pg_last_wal_replay_lsn()'.
- Components NEVER execute raw standby promotion ('pg_promote()') if 'pg_replication_slots' displays pending unsynchronized logical slots with active upstream consumers.

## Bad vs Good Implementations
### PostgreSQL 17 Automated Standby Promotion Guardrail

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Fails to enforce PostgreSQL 17 slot synchronization. Downstream logical replication consumers referencing slots not yet synchronized to this standby will experience slot invalidation or lost transactions upon cutover.
```typescript
import { Client } from 'pg';

export async function promoteStandbyToPrimary(standbyClient: Client): Promise<void> {
  // DANGEROUS: Blind promotion without syncing or verifying logical replication slots
  // Subscribers configured with failover will fail or miss WAL records
  console.warn('Initiating unvalidated standby promotion...');
  await standbyClient.query('SELECT pg_promote(wait => true, wait_seconds => 60)');
  console.log('Standby promoted successfully.');
}
```

#### Production Standard (Correct)
> **Standard**: Enforces Postgres 17 replication slot invariants by invoking pg_sync_replication_slots(), inspecting pg_replication_slots for synced=true and valid confirmed_flush_lsn, and halting promotion if slots are desynchronized.
```typescript
import { Client } from 'pg';

interface ReplicationSlotStatus {
  slot_name: string;
  synced: boolean;
  confirmed_flush_lsn: string | null;
}

export async function promoteStandbyToPrimary(standbyClient: Client): Promise<void> {
  // 1. Manually trigger slot synchronization cycle on standby before promotion
  await standbyClient.query('SELECT pg_sync_replication_slots()');

  // 2. Validate that all required failover-enabled slots are marked as synced
  const { rows: unsyncedSlots } = await standbyClient.query<ReplicationSlotStatus>(
    `SELECT slot_name, synced, confirmed_flush_lsn 
     FROM pg_replication_slots 
     WHERE failover = true AND (synced IS NOT TRUE OR confirmed_flush_lsn IS NULL);`
  );

  if (unsyncedSlots.length > 0) {
    const slotNames = unsyncedSlots.map((s) => s.slot_name).join(', ');
    throw new Error(
      `Cannot promote standby: logical slots not synchronized: [${slotNames}]. Failover aborted.`
    );
  }

  // 3. Execute safe promotion once slot invariants are guaranteed
  const { rows: promotionResult } = await standbyClient.query<{
    pg_promote: boolean;
  }>('SELECT pg_promote(wait => true, wait_seconds => 60)');

  if (!promotionResult[0]?.pg_promote) {
    throw new Error('pg_promote timed out or returned false');
  }
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

PostgreSQL Schema & Health MCP
Inspects replication slot health, LSN progression, and Postgres 17 parameter configurations in real time.
Recommended Agent Skills (askills.dev)

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

PostgreSQL Failover Health CLI
CLI utility to verify synchronized_standby_slots and failover slot statuses prior to cluster switchovers.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: PostgreSQL 17 Logical Replication Slot Synchronization and Standby Failover Safety

PostgreSQL 17 Logical Replication Slot Synchronization and Standby Failover Safety defines an industrial-grade behavioral standard for backend engineering. Mandates that cross-AZ physical standby failovers in PostgreSQL 17 enforce logical replication slot synchronization via synchronized_standby_slots, sync_replication_slots, and pg_sync_replication_slots(). Enforces zero-data-loss subscriber cutovers by verifying slot consistency and synced status before standby promotion and subscriber re-routing. 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: Promoting physical standby instances without synchronized logical replication slots, causing subscriber data corruption and catalog desynchronization. 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 DriftPromoting physical standby instances without synchronized logical replication slots, causing subscriber data corruption and catalog desynchronization.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 configure 'synchronized_standby_slots' on the primary containing the physical replication standby slot name before creating failover-enabled logical slots.

Engineers MUST ALWAYS configure 'synchronized_standby_slots' on the primary containing the physical replication standby slot name before creating failover-enabled logical slots.

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 ALWAYS create logical replication slots with 'failover = true' and ensure logical subscriptions specify 'failover = true' on creation.

Engineers MUST ALWAYS create logical replication slots with 'failover = true' and ensure logical subscriptions specify 'failover = true' on creation.

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

Code executing standby promotions MUST invoke or await 'pg_sync_replication_slots()' on the physical standby and verify 'synced = true' across all required slots in 'pg_replication_slots' before promotion.

Code executing standby promotions MUST invoke or await 'pg_sync_replication_slots()' on the physical standby and verify 'synced = true' across all required slots in 'pg_replication_slots' before promotion.

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

Services MUST NOT re-route downstream logical replication subscribers until the promoted replica confirms slot catalog validity and LSN monotonicity against 'pg_last_wal_replay_lsn()'.

Services MUST NOT re-route downstream logical replication subscribers until the promoted replica confirms slot catalog validity and LSN monotonicity against 'pg_last_wal_replay_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.
5

Components NEVER execute raw standby promotion ('pg_promote()') if 'pg_replication_slots' displays pending unsynchronized logical slots with active upstream consumers.

Components NEVER execute raw standby promotion ('pg_promote()') if 'pg_replication_slots' displays pending unsynchronized logical slots with active upstream consumers.

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 Logical Replication Slot Synchronization and Standby Failover Safety critical for AI coding assistants?

Mandates that cross-AZ physical standby failovers in PostgreSQL 17 enforce logical replication slot synchronization via synchronized_standby_slots, sync_replication_slots, and pg_sync_replication_slots(). Enforces zero-data-loss subscriber cutovers by verifying slot consistency and synced status before standby promotion and subscriber re-routing. 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 (Promoting physical standby instances without synchronized logical replication slots, causing subscriber data corruption and catalog desynchronization.) 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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