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backendcritical PriorityID: pg17-logical-replication-failover-slot-sync

PostgreSQL 17 Logical Replication Failover Slot Synchronization and Zero-Lag Subscriber Migration

PostgreSQL 17 introduces native logical replication failover slot synchronization (`sync_replication_slots` and `failover = true`). When orchestrating high-availability database clusters, downstream subscribers must never point to logical replication slots that are unmanaged by physical standbys, nor fail over without verifying `synchronized_standby_slots` on the primary and slot confirmation on the promoted node to prevent data loss or duplicate stream processing.

Prohibited Anti-Pattern: Creating un-synchronized logical slots or performing subscriber cutover without validating slot LSN advancement across physical replicas

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---
description: PostgreSQL 17 Logical Replication Failover Slot Synchronization and Zero-Lag Subscriber Migration - PostgreSQL 17 introduces native logical replication failover slot synchronization (`sync_replication_slots` and `failover = true`). When orchestrating high-availability database clusters, downstream subscribers must never point to logical replication slots that are unmanaged by physical standbys, nor fail over without verifying `synchronized_standby_slots` on the primary and slot confirmation on the promoted node to prevent data loss or duplicate stream processing.
globs: src/db/**/*.{ts,js,sql}, src/infra/**/*.{ts,js}, src/services/**/*.{ts,js}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/pg17-logical-replication-failover-slot-sync"
---

# PostgreSQL 17 Logical Replication Failover Slot Synchronization and Zero-Lag Subscriber Migration

> **Severity**: CRITICAL | **Category**: backend
> **Anti-Pattern Warning**: Creating un-synchronized logical slots or performing subscriber cutover without validating slot LSN advancement across physical replicas

## System Constraints
You are a Principal Database Reliability Engineer enforcing PostgreSQL 17 Logical Replication Failover Slot Synchronization invariants. Mandate that all logical replication slots intended for multi-node High Availability (HA) topologies enable native failover synchronization via `failover = true` on the primary and configure `synchronized_standby_slots` to ensure physical standby nodes advance before subscribers consume data. Prevent split-brain stream reprocessing or missing data during standby promotion.

## Strict Guidelines
- Engineers MUST ALWAYS create logical replication slots with `failover = true` on PostgreSQL 17+ primaries when subscribers must survive physical standby promotion.
- Engineers MUST configure `synchronized_standby_slots` on the primary cluster to include the application's physical standbys, ensuring the primary blocks logical slot progression until the physical replica confirms WAL receipt.
- Standby orchestrators MUST enable `sync_replication_slots = true` in `postgresql.conf` on physical standby instances to enable the background slot sync worker.
- Engineers MUST NOT execute `ALTER SUBSCRIPTION ... CONNECTION ...` cutover to a newly promoted standby until verifying via `pg_replication_slots` that the synchronized slot has `synced = true` or `active = false` and its `confirmed_flush_lsn` matches or exceeds the subscriber's current LSN.
- Applications MUST handle transient failover errors with idempotency keys and LSN checkpoints to prevent duplicate event ingestion during subscriber reconnection.
- Automation scripts MUST NEVER perform uncoordinated forced failovers that orphan downstream logical replication slots.

## Bad vs Good Implementations
### Configuring PG17 Failover Slot and Verified Subscriber Cutover

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Failing to inspect `sync_replication_slots` status or `confirmed_flush_lsn` on the new standby before switching subscriptions causes missing replication events or catastrophic `ERROR: logical decoding slot is already active` / `ERROR: replication slot does not exist` panics, stalling replication indefinitely.
```typescript
import { Client } from 'pg';

export async function switchSubscriberToNewPrimary(
  subscriberClient: Client,
  newPrimaryHost: string,
  subscriptionName: string
): Promise<void> {
  // Anti-pattern: Blindly repointing subscriber to promoted standby without verifying slot sync
  // Logical slots might be missing, un-synced, or lagging behind the subscriber's confirmed LSN
  const connStr = `host=${newPrimaryHost} port=5432 dbname=appdb user=repl_user password=secret`;
  await subscriberClient.query(
    `ALTER SUBSCRIPTION ${subscriptionName} CONNECTION '${connStr}'`
  );
  await subscriberClient.query(`ALTER SUBSCRIPTION ${subscriptionName} ENABLE`);
}
```

#### Production Standard (Correct)
> **Standard**: Validates that the target PostgreSQL 17 node has the replication slot with `failover = true` ready, verifies inactivity, and safely sequences the subscriber disable/re-point/enable workflow without stream corruption.
```typescript
import { Client } from 'pg';

interface SlotStatus {
  slot_name: string;
  synced: boolean;
  confirmed_flush_lsn: string;
  active: boolean;
}

export async function switchSubscriberToNewPrimary(
  subscriberClient: Client,
  newPrimaryClient: Client,
  newPrimaryHost: string,
  subscriptionName: string,
  slotName: string
): Promise<void> {
  // Step 1: Ensure the slot exists and was synchronized on the new primary (PG17)
  const slotRes = await newPrimaryClient.query<SlotStatus>(
    `SELECT slot_name, synced, confirmed_flush_lsn, active 
     FROM pg_replication_slots 
     WHERE slot_name = $1 AND failover = true`,
    [slotName]
  );

  if (slotRes.rows.length === 0) {
    throw new Error(`Failover slot ${slotName} not found on promoted target node.`);
  }

  const slot = slotRes.rows[0];
  if (slot.active) {
    throw new Error(`Slot ${slotName} is unexpectedly active on target node before subscription switch.`);
  }

  // Step 2: Disable subscriber before mutating connection to prevent half-state sync
  await subscriberClient.query(`ALTER SUBSCRIPTION ${subscriptionName} DISABLE`);

  // Step 3: Repoint connection to the newly promoted primary
  const connStr = `host=${newPrimaryHost} port=5432 dbname=appdb user=repl_user password=secret`;
  await subscriberClient.query(
    `ALTER SUBSCRIPTION ${subscriptionName} CONNECTION '${connStr}'`
  );

  // Step 4: Re-enable subscription against the synchronized slot
  await subscriberClient.query(`ALTER SUBSCRIPTION ${subscriptionName} ENABLE`);
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

PostgreSQL Diagnostic MCP Server
Inspects replication slot lag, physical WAL standby synchronization states, and PostgreSQL 17 catalogs.
Recommended Agent Skills (askills.dev)

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

PG Cluster Orchestration Tool
Automated validation for PG17 failover slot health and standby promotion validation.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: PostgreSQL 17 Logical Replication Failover Slot Synchronization and Zero-Lag Subscriber Migration

PostgreSQL 17 Logical Replication Failover Slot Synchronization and Zero-Lag Subscriber Migration defines an industrial-grade behavioral standard for backend engineering. PostgreSQL 17 introduces native logical replication failover slot synchronization (`sync_replication_slots` and `failover = true`). When orchestrating high-availability database clusters, downstream subscribers must never point to logical replication slots that are unmanaged by physical standbys, nor fail over without verifying `synchronized_standby_slots` on the primary and slot confirmation on the promoted node to prevent data loss or duplicate stream processing. 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: Creating un-synchronized logical slots or performing subscriber cutover without validating slot LSN advancement across physical replicas 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 DriftCreating un-synchronized logical slots or performing subscriber cutover without validating slot LSN advancement across physical replicasCritical
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` on PostgreSQL 17+ primaries when subscribers must survive physical standby promotion.

Engineers MUST ALWAYS create logical replication slots with `failover = true` on PostgreSQL 17+ primaries when subscribers must survive physical standby 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.
2

Engineers MUST configure `synchronized_standby_slots` on the primary cluster to include the application's physical standbys, ensuring the primary blocks logical slot progression until the physical replica confirms WAL receipt.

Engineers MUST configure `synchronized_standby_slots` on the primary cluster to include the application's physical standbys, ensuring the primary blocks logical slot progression until the physical replica confirms WAL receipt.

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 orchestrators MUST enable `sync_replication_slots = true` in `postgresql.conf` on physical standby instances to enable the background slot sync worker.

Standby orchestrators MUST enable `sync_replication_slots = true` in `postgresql.conf` on physical standby instances to enable the background slot sync 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

Engineers MUST NOT execute `ALTER SUBSCRIPTION ... CONNECTION ...` cutover to a newly promoted standby until verifying via `pg_replication_slots` that the synchronized slot has `synced = true` or `active = false` and its `confirmed_flush_lsn` matches or exceeds the subscriber's current LSN.

Engineers MUST NOT execute `ALTER SUBSCRIPTION ... CONNECTION ...` cutover to a newly promoted standby until verifying via `pg_replication_slots` that the synchronized slot has `synced = true` or `active = false` and its `confirmed_flush_lsn` matches or exceeds the subscriber's current 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

Applications MUST handle transient failover errors with idempotency keys and LSN checkpoints to prevent duplicate event ingestion during subscriber reconnection.

Applications MUST handle transient failover errors with idempotency keys and LSN checkpoints to prevent duplicate event ingestion during subscriber reconnection.

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

Automation scripts MUST NEVER perform uncoordinated forced failovers that orphan downstream logical replication slots.

Automation scripts MUST NEVER perform uncoordinated forced failovers that orphan downstream logical replication 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.
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 Failover Slot Synchronization and Zero-Lag Subscriber Migration critical for AI coding assistants?

PostgreSQL 17 introduces native logical replication failover slot synchronization (`sync_replication_slots` and `failover = true`). When orchestrating high-availability database clusters, downstream subscribers must never point to logical replication slots that are unmanaged by physical standbys, nor fail over without verifying `synchronized_standby_slots` on the primary and slot confirmation on the promoted node to prevent data loss or duplicate stream processing. 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 (Creating un-synchronized logical slots or performing subscriber cutover without validating slot LSN advancement across physical replicas) 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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