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backendcritical PriorityID: postgres-17-logical-replication-failover-slot-sync-postgres-17-logi-845

Postgres 17 Logical Replication Failover & Slot Synchronization Protocol

Enforces failover-enabled logical replication slots and synchronized standby slot topologies introduced in PostgreSQL 17. Guarantees that physical standbys lead or match logical subscriber LSNs, preventing data loss and unrecoverable downstream duplicates during HA switchovers.

Prohibited Anti-Pattern: Unsynchronized Logical Slots Advancing Past Physical Standbys During HA Switchovers

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---
description: Postgres 17 Logical Replication Failover & Slot Synchronization Protocol - Enforces failover-enabled logical replication slots and synchronized standby slot topologies introduced in PostgreSQL 17. Guarantees that physical standbys lead or match logical subscriber LSNs, preventing data loss and unrecoverable downstream duplicates during HA switchovers.
globs: src/**/*.{ts,js,py,go,sql}, migrations/**/*.sql, deploy/**/*.{tf,yaml,yml}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/postgres-17-logical-replication-failover-slot-sync-postgres-17-logi-845"
---

# Postgres 17 Logical Replication Failover & Slot Synchronization Protocol

> **Severity**: CRITICAL | **Category**: backend
> **Anti-Pattern Warning**: Unsynchronized Logical Slots Advancing Past Physical Standbys During HA Switchovers

## System Constraints
You are a Principal Database Reliability Engineer enforcing the Postgres 17 Logical Replication Failover Slot Sync Protocol. Mandate invariants that prevent data loss and duplicate consumption during High Availability (HA) switchovers by ensuring logical slots are synchronized with physical standbys.

## Strict Guidelines
- Engineers MUST ALWAYS create logical replication slots with `failover = true` (e.g., `pg_create_logical_replication_slot(slot_name, plugin, false, false, false, true)` or `CREATE SUBSCRIPTION ... WITH (failover = true)`) in Postgres 17+ environments.
- Engineers MUST configure `synchronized_standby_slots` on the primary cluster to explicitly include all physical failover standby slots before permitting logical replication consumers to advance.
- Code MUST NEVER advance logical subscriber consumption offsets beyond the confirmed flush LSN of the physical standbys registered in `synchronized_standby_slots`.
- Services MUST verify `sync_state` and `failover` booleans in `pg_replication_slots` during health checks and before executing failover or switchover orchestration.
- Standby sync workers or `pg_sync_replication_slots()` MUST be enabled and active on physical standbys to maintain slot metadata synchronization prior to promotion.

## Bad vs Good Implementations
### Configuring Postgres 17 Logical Subscription & Slot with Failover Sync Verification

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Omits 'failover = true' on PostgreSQL 17+, meaning the corresponding logical replication slot will not be mirrored to physical standbys. Upon HA switchover, the promoted standby will lack this slot, breaking replication or forcing a complete re-read.
```typescript
import { Client } from 'pg';

export async function setupLogicalConsumer(client: Client, subName: string, pubName: string, connStr: string): Promise<void> {
  // FAILS: Creates subscription with default failover = false.
  // When primary fails over, the logical replication slot is lost on promoted standby,
  // causing subscriber crash or complete re-sync resulting in duplicate events.
  await client.query(`
    CREATE SUBSCRIPTION ${subName}
    CONNECTION '${connStr}'
    PUBLICATION ${pubName};
  `);
}
```

#### Production Standard (Correct)
> **Standard**: Validates PostgreSQL 17 engine capabilities, asserts primary physical standby synchronization settings, and sets 'failover = true' with explicit verification of slot status in pg_replication_slots.
```typescript
import { Client } from 'pg';

export async function setupLogicalConsumer(client: Client, subName: string, pubName: string, connStr: string): Promise<void> {
  // Verify Postgres 17+ support and physical slot synchronization before creating failover subscription
  const versionRes = await client.query<{ server_version_num: string }>(
    'SHOW server_version_num;'
  );
  const versionNum = parseInt(versionRes.rows[0].server_version_num, 10);
  if (versionNum < 170000) {
    throw new Error('Postgres 17+ is required for failover slot synchronization');
  }

  // Verify synchronized_standby_slots contains active physical standbys
  const standbyCheck = await client.query<{ synchronized_standby_slots: string }>(
    'SHOW synchronized_standby_slots;'
  );
  if (!standbyCheck.rows[0].synchronized_standby_slots) {
    throw new Error('Primary must configure synchronized_standby_slots to prevent logical consumer drift');
  }

  // Enforce failover = true on PostgreSQL 17 subscription
  await client.query(`
    CREATE SUBSCRIPTION ${subName}
    CONNECTION '${connStr}'
    PUBLICATION ${pubName}
    WITH (failover = true, copy_data = true);
  `);

  // Validate slot state in pg_replication_slots
  const slotRes = await client.query<{
    slot_name: string;
    failover: boolean;
    sync_state: string;
  }>(
    `SELECT slot_name, failover, sync_state 
     FROM pg_replication_slots 
     WHERE slot_name = $1;`,
    [subName]
  );

  if (!slotRes.rows[0]?.failover) {
    throw new Error(`Replication slot ${subName} failed to initialize with failover enabled`);
  }
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

PostgreSQL Context Provider
Enables inspection of pg_replication_slots, cluster topology, and configuration parameters across primary and standby nodes.
Recommended Agent Skills (askills.dev)

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

Patroni HA Orchestrator CLI
Validates cluster failover slot synchronization and coordinates automated HA switchovers without slot drop.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Postgres 17 Logical Replication Failover & Slot Synchronization Protocol

Postgres 17 Logical Replication Failover & Slot Synchronization Protocol defines an industrial-grade behavioral standard for backend engineering. Enforces failover-enabled logical replication slots and synchronized standby slot topologies introduced in PostgreSQL 17. Guarantees that physical standbys lead or match logical subscriber LSNs, preventing data loss and unrecoverable downstream duplicates during HA switchovers. 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 Advancing Past Physical Standbys During HA Switchovers 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 Advancing Past Physical Standbys During HA SwitchoversCritical
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 create logical replication slots with `failover = true` (e.g., `pg_create_logical_replication_slot(slot_name, plugin, false, false, false, true)` or `CREATE SUBSCRIPTION ... WITH (failover = true)`) in Postgres 17+ environments.

Engineers MUST ALWAYS create logical replication slots with `failover = true` (e.g., `pg_create_logical_replication_slot(slot_name, plugin, false, false, false, true)` or `CREATE SUBSCRIPTION ... WITH (failover = true)`) in Postgres 17+ environments.

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 cluster to explicitly include all physical failover standby slots before permitting logical replication consumers to advance.

Engineers MUST configure `synchronized_standby_slots` on the primary cluster to explicitly include all physical failover standby slots before permitting logical replication consumers to advance.

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 MUST NEVER advance logical subscriber consumption offsets beyond the confirmed flush LSN of the physical standbys registered in `synchronized_standby_slots`.

Code MUST NEVER advance logical subscriber consumption offsets beyond the confirmed flush LSN of the physical standbys registered in `synchronized_standby_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.
4

Services MUST verify `sync_state` and `failover` booleans in `pg_replication_slots` during health checks and before executing failover or switchover orchestration.

Services MUST verify `sync_state` and `failover` booleans in `pg_replication_slots` during health checks and before executing failover or switchover orchestration.

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

Standby sync workers or `pg_sync_replication_slots()` MUST be enabled and active on physical standbys to maintain slot metadata synchronization prior to promotion.

Standby sync workers or `pg_sync_replication_slots()` MUST be enabled and active on physical standbys to maintain slot metadata synchronization prior to 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.
Technical Questions & Answers

Frequently Asked Technical Questions

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

Q:Why is Postgres 17 Logical Replication Failover & Slot Synchronization Protocol critical for AI coding assistants?

Enforces failover-enabled logical replication slots and synchronized standby slot topologies introduced in PostgreSQL 17. Guarantees that physical standbys lead or match logical subscriber LSNs, preventing data loss and unrecoverable downstream duplicates during HA switchovers. 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 Advancing Past Physical Standbys During HA Switchovers) 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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