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

PostgreSQL 17 Logical Replication Slot Failover Synchrony & Zero-Downtime Migration Standard

Enforces failover-enabled logical replication slots and synchronized standby slot topologies introduced in PostgreSQL 17. Guarantees downstream subscribers (e.g., Debezium, audit consumers, microservice outboxes) survive physical primary failover without silent message drops, LSN regression, or full table rebuilds.

Prohibited Anti-Pattern: Unsynchronized Logical Slots and Unconfigured Standby Slot Promotion Leading to LSN Drift and Consumer Resync Loops

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---
description: PostgreSQL 17 Logical Replication Slot Failover Synchrony & Zero-Downtime Migration Standard - Enforces failover-enabled logical replication slots and synchronized standby slot topologies introduced in PostgreSQL 17. Guarantees downstream subscribers (e.g., Debezium, audit consumers, microservice outboxes) survive physical primary failover without silent message drops, LSN regression, or full table rebuilds.
globs: src/**/*.{ts,js,sql}, infra/**/*.{ts,js,tf}, migrations/**/*.{sql,ts}, k8s/**/*.yaml
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/postgres-17-logical-replication-slot-failover"
---

# PostgreSQL 17 Logical Replication Slot Failover Synchrony & Zero-Downtime Migration Standard

> **Severity**: CRITICAL | **Category**: backend
> **Anti-Pattern Warning**: Unsynchronized Logical Slots and Unconfigured Standby Slot Promotion Leading to LSN Drift and Consumer Resync Loops

## System Constraints
You are a Principal Software Architect enforcing PostgreSQL 17 Logical Replication Slot Failover Synchrony. Mandate failover-enabled replication slots, synchronized_standby_slots configuration, and verification of slot retention to guarantee downstream consumers remain uninterrupted across primary failovers.

## Strict Guidelines
- Engineers MUST ALWAYS provision logical replication slots with `failover = true` on PostgreSQL 17+ via `pg_create_logical_replication_slot('slot_name', 'pgoutput', false, false, true)` or `CREATE SUBSCRIPTION ... WITH (failover = true)`.
- Engineers MUST ALWAYS configure `synchronized_standby_slots` in primary database configuration (`postgresql.conf` or DCS) containing the `application_name` of all critical physical standby replicas.
- Engineers MUST ensure hot standby instances enable `sync_replication_slots = on` to continuously sync slot LSN progression and snapshot metadata from the primary.
- Engineers MUST NOT allow logical replication slot consumers to bypass physical standby WAL flush confirmation, preventing downstream consumers from consuming WAL that a newly promoted standby has not yet received.
- Services connecting to logical replication slots MUST verify that the target slot has `failover = true` and `synced = false` on the active primary, or `synced = true` on hot standbys, prior to starting CDC stream consumption.

## Bad vs Good Implementations
### Configuring PostgreSQL 17 Logical Replication Slot and Subscriber with Failover Synchrony

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Creates a logical replication slot without enabling PG17 failover (`failover = true`) and without validating `synchronized_standby_slots`. Physical failover will destroy the subscriber's state, leading to missed transactions or fatal WAL retention errors.
```typescript
import { Client } from 'pg';

export async function setupOutboxSlot(client: Client, slotName: string): Promise<void> {
  // Anti-pattern: Pre-PG17 standard slot creation lacking failover argument
  // When primary physical failover occurs, the promoted standby has no record
  // of this slot, causing downstream consumer crash and requiring full table re-sync.
  await client.query(
    `SELECT pg_create_logical_replication_slot($1, 'pgoutput');`,
    [slotName]
  );

  await client.query(`
    CREATE SUBSCRIPTION outbox_sub
    CONNECTION 'host=pg-primary dbname=app user=rep_user'
    PUBLICATION outbox_pub;
  `);
}
```

#### Production Standard (Correct)
> **Standard**: Validates server compatibility, enforces `synchronized_standby_slots` presence to prevent WAL desynchronization, creates a failover-safe slot (`failover = true`), and binds the subscription with explicit failover handling for seamless standby promotions.
```typescript
import { Client } from 'pg';

interface ReplicationSlotConfig {
  slotName: string;
  publicationName: string;
  primaryConnString: string;
  expectedStandbyNames: string[];
}

export async function setupResilientOutboxSlot(
  client: Client,
  config: ReplicationSlotConfig
): Promise<void> {
  // 1. Verify PostgreSQL 17+ server version
  const versionRes = await client.query('SHOW server_version_num;');
  const serverVersion = parseInt(versionRes.rows[0].server_version_num, 10);
  if (serverVersion < 170000) {
    throw new Error('PostgreSQL 17.0+ is strictly required for logical slot failover synchrony.');
  }

  // 2. Validate synchronized_standby_slots is active to prevent logical advance beyond standbys
  const syncSlotsRes = await client.query('SHOW synchronized_standby_slots;');
  const configuredSyncStandbys: string = syncSlotsRes.rows[0].synchronized_standby_slots || '';
  
  const hasMissingStandbys = config.expectedStandbyNames.some(
    standby => !configuredSyncStandbys.includes(standby)
  );
  if (hasMissingStandbys) {
    throw new Error(`Primary lacks required synchronized_standby_slots configuration: ${configuredSyncStandbys}`);
  }

  // 3. Create slot with failover enabled (5th argument: failover boolean in PG17)
  // 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);`,
    [config.slotName]
  );

  // 4. Create subscription explicitly requiring failover synchrony
  await client.query(`
    CREATE SUBSCRIPTION outbox_sub
    CONNECTION '${config.primaryConnString}'
    PUBLICATION ${config.publicationName}
    WITH (
      create_slot = false,
      slot_name = '${config.slotName}',
      failover = true
    );
  `);
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

PostgreSQL MCP Inspector
Inspect replication slots, lag metrics, and PostgreSQL 17 cluster configurations directly within AI workflows.
Recommended Agent Skills (askills.dev)

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

Patroni HA Failover Validator
Validates high-availability DCS configurations and slot synchronization states across primary and standby PostgreSQL clusters.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: PostgreSQL 17 Logical Replication Slot Failover Synchrony & Zero-Downtime Migration Standard

PostgreSQL 17 Logical Replication Slot Failover Synchrony & Zero-Downtime Migration Standard 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 downstream subscribers (e.g., Debezium, audit consumers, microservice outboxes) survive physical primary failover without silent message drops, LSN regression, or full table rebuilds. 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 and Unconfigured Standby Slot Promotion Leading to LSN Drift and Consumer Resync Loops 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 and Unconfigured Standby Slot Promotion Leading to LSN Drift and Consumer Resync LoopsCritical
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 provision logical replication slots with `failover = true` on PostgreSQL 17+ via `pg_create_logical_replication_slot('slot_name', 'pgoutput', false, false, true)` or `CREATE SUBSCRIPTION ... WITH (failover = true)`.

Engineers MUST ALWAYS provision logical replication slots with `failover = true` on PostgreSQL 17+ via `pg_create_logical_replication_slot('slot_name', 'pgoutput', false, false, true)` or `CREATE SUBSCRIPTION ... WITH (failover = true)`.

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 configure `synchronized_standby_slots` in primary database configuration (`postgresql.conf` or DCS) containing the `application_name` of all critical physical standby replicas.

Engineers MUST ALWAYS configure `synchronized_standby_slots` in primary database configuration (`postgresql.conf` or DCS) containing the `application_name` of all critical physical standby replicas.

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

Engineers MUST ensure hot standby instances enable `sync_replication_slots = on` to continuously sync slot LSN progression and snapshot metadata from the primary.

Engineers MUST ensure hot standby instances enable `sync_replication_slots = on` to continuously sync slot LSN progression and snapshot metadata from the primary.

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

Engineers MUST NOT allow logical replication slot consumers to bypass physical standby WAL flush confirmation, preventing downstream consumers from consuming WAL that a newly promoted standby has not yet received.

Engineers MUST NOT allow logical replication slot consumers to bypass physical standby WAL flush confirmation, preventing downstream consumers from consuming WAL that a newly promoted standby has not yet received.

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

Services connecting to logical replication slots MUST verify that the target slot has `failover = true` and `synced = false` on the active primary, or `synced = true` on hot standbys, prior to starting CDC stream consumption.

Services connecting to logical replication slots MUST verify that the target slot has `failover = true` and `synced = false` on the active primary, or `synced = true` on hot standbys, prior to starting CDC stream consumption.

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 Failover Synchrony & Zero-Downtime Migration Standard critical for AI coding assistants?

Enforces failover-enabled logical replication slots and synchronized standby slot topologies introduced in PostgreSQL 17. Guarantees downstream subscribers (e.g., Debezium, audit consumers, microservice outboxes) survive physical primary failover without silent message drops, LSN regression, or full table rebuilds. 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 and Unconfigured Standby Slot Promotion Leading to LSN Drift and Consumer Resync Loops) 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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