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backendcritical PriorityID: postgres-17-logical-replication-standby-slot-sync

PostgreSQL 17 Failover Slot Synchronization & Physical Standby Synchronization

Postgres 17 introduces native logical slot failover support via synchronized replication slots. Downstream CDC/logical consumers encounter stream corruption, missing WAL offsets, or duplicate message explosions during a physical switchover if logical replication slots are not created with 'failover = true', synchronized via 'sync_replication_slots = true', and guarded via 'standby_slot_names' on the primary.

Prohibited Anti-Pattern: Creating unsynced logical slots without standby_slot_names and failover flags leading to broken downstream CDC on primary promotion.

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---
description: PostgreSQL 17 Failover Slot Synchronization & Physical Standby Synchronization - Postgres 17 introduces native logical slot failover support via synchronized replication slots. Downstream CDC/logical consumers encounter stream corruption, missing WAL offsets, or duplicate message explosions during a physical switchover if logical replication slots are not created with 'failover = true', synchronized via 'sync_replication_slots = true', and guarded via 'standby_slot_names' on the primary.
globs: src/database/**/*.{ts,js}, src/infra/**/*.{ts,js}, infrastructure/**/*.{tf,ts}, scripts/migrations/**/*.{sql,ts,js}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/postgres-17-logical-replication-standby-slot-sync"
---

# PostgreSQL 17 Failover Slot Synchronization & Physical Standby Synchronization

> **Severity**: CRITICAL | **Category**: backend
> **Anti-Pattern Warning**: Creating unsynced logical slots without standby_slot_names and failover flags leading to broken downstream CDC on primary promotion.

## System Constraints
You are a Principal Database Architect and Infrastructure Reliability Engineer enforcing PostgreSQL 17 logical replication failover slot standards. When provisioning logical slots or managing replication topology, you MUST ensure zero message loss and continuous CDC operation across physical HA switchovers. Validate that logical slots use Postgres 17 failover capabilities, require sync_replication_slots on standbys, and enforce standby_slot_names on the primary to protect WAL from advancing past logical subscribers.

## Strict Guidelines
- Engineers MUST ALWAYS provision logical replication slots with 'failover = true' in PostgreSQL 17+ topologies to mark them eligible for synchronization.
- Engineers MUST configure 'sync_replication_slots = true' on all physical standby nodes via postgresql.conf or patroni/helm chart configurations.
- Primary nodes MUST configure 'standby_slot_names' to include the physical replication slots of all failover target standbys, preventing the primary from discarding WAL that standbys have not yet received.
- Database connection pooling and client switchover logic MUST verify 'pg_replication_slots.synced = true' on the standby prior to reconnecting logical replication downstream consumers after promotion.
- Engineers MUST NOT manually drop or force-update synced logical slots on physical standbys directly; slot lifecycle MUST be driven strictly by the primary publisher.

## Bad vs Good Implementations
### PostgreSQL 17 Logical Replication Slot Provisioning & Verification

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Creating a logical slot without setting failover=true leaves the slot unsynchronized across physical standbys. If the primary crashes, the promoted standby will lack this slot or its LSN positions, permanently severing downstream logical replication streams.
```typescript
import { Client } from 'pg';

export async function setupConsumerSlot(client: Client, slotName: string): Promise<void> {
  // Anti-pattern: Postgres 16 or unmanaged slot creation without failover=true or standby guards
  await client.query(`SELECT pg_create_logical_replication_slot($1, 'pgoutput');`, [slotName]);
}
```

#### Production Standard (Correct)
> **Standard**: Validates engine compatibility, ensures the primary has registered physical standby slots to prevent early WAL pruning, and specifies 'failover => true' to allow physical standbys to mirror the logical slot state natively.
```typescript
import { Client } from 'pg';

export async function setupConsumerSlot(client: Client, slotName: string): Promise<void> {
  // Invariant: Verify Postgres version is >= 17 before using failover parameters
  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('Automated failover slot synchronization requires PostgreSQL 17.0+');
  }

  // Invariant: Primary must enforce standby_slot_names to avoid WAL discarding
  const standbyCheck = await client.query<{ setting: string }>('SHOW standby_slot_names;');
  if (!standbyCheck.rows[0].setting || standbyCheck.rows[0].setting.trim() === '') {
    throw new Error('Safety check failed: primary requires non-empty standby_slot_names to prevent WAL loss');
  }

  // Invariant: Postgres 17 pg_create_logical_replication_slot with failover = true
  await client.query(
    `SELECT pg_create_logical_replication_slot(
       slot_name => $1,
       plugin => 'pgoutput',
       temporary => false,
       twophase => false,
       failover => true
     );`,
    [slotName]
  );
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

PostgreSQL MCP Server
Enables inspection of pg_replication_slots, pg_stat_replication, and cluster parameters in development and CI environments.
Recommended Agent Skills (askills.dev)

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

Patroni HA Orchestrator CLI
Automates HA switchovers and orchestrates standby_slot_names alongside Postgres 17 slot synchronization.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: PostgreSQL 17 Failover Slot Synchronization & Physical Standby Synchronization

PostgreSQL 17 Failover Slot Synchronization & Physical Standby Synchronization defines an industrial-grade behavioral standard for backend engineering. Postgres 17 introduces native logical slot failover support via synchronized replication slots. Downstream CDC/logical consumers encounter stream corruption, missing WAL offsets, or duplicate message explosions during a physical switchover if logical replication slots are not created with 'failover = true', synchronized via 'sync_replication_slots = true', and guarded via 'standby_slot_names' on the primary. 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 unsynced logical slots without standby_slot_names and failover flags leading to broken downstream CDC on primary promotion. 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 unsynced logical slots without standby_slot_names and failover flags leading to broken downstream CDC on primary promotion.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 provision logical replication slots with 'failover = true' in PostgreSQL 17+ topologies to mark them eligible for synchronization.

Engineers MUST ALWAYS provision logical replication slots with 'failover = true' in PostgreSQL 17+ topologies to mark them eligible for synchronization.

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 'sync_replication_slots = true' on all physical standby nodes via postgresql.conf or patroni/helm chart configurations.

Engineers MUST configure 'sync_replication_slots = true' on all physical standby nodes via postgresql.conf or patroni/helm chart configurations.

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

Primary nodes MUST configure 'standby_slot_names' to include the physical replication slots of all failover target standbys, preventing the primary from discarding WAL that standbys have not yet received.

Primary nodes MUST configure 'standby_slot_names' to include the physical replication slots of all failover target standbys, preventing the primary from discarding WAL that standbys have 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.
4

Database connection pooling and client switchover logic MUST verify 'pg_replication_slots.synced = true' on the standby prior to reconnecting logical replication downstream consumers after promotion.

Database connection pooling and client switchover logic MUST verify 'pg_replication_slots.synced = true' on the standby prior to reconnecting logical replication downstream consumers after 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.
5

Engineers MUST NOT manually drop or force-update synced logical slots on physical standbys directly

Engineers MUST NOT manually drop or force-update synced logical slots on physical standbys directly; slot lifecycle MUST be driven strictly by the primary publisher.

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 Failover Slot Synchronization & Physical Standby Synchronization critical for AI coding assistants?

Postgres 17 introduces native logical slot failover support via synchronized replication slots. Downstream CDC/logical consumers encounter stream corruption, missing WAL offsets, or duplicate message explosions during a physical switchover if logical replication slots are not created with 'failover = true', synchronized via 'sync_replication_slots = true', and guarded via 'standby_slot_names' on the primary. 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 unsynced logical slots without standby_slot_names and failover flags leading to broken downstream CDC on primary promotion.) 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.

Open Specification · CC BY 4.0·Free for Personal, Team & Commercial In-Repo Use

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