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backendcritical PriorityID: pg17-failover-slot-synchronization-standard

PostgreSQL 17 Failover Slot Synchronization High-Availability Standard

Enforces seamless logical CDC replication persistence across physical replica failovers using PostgreSQL 17 logical failover slots and synchronized standby slots. Ensures upstream applications create replication slots with `failover = true`, mandates primary `synchronized_standby_slots` alignment to prevent slot advance beyond standby LSN boundaries, and configures standbys with `sync_replication_slots = true` to prevent data loss or duplicate streaming upon promotion.

Prohibited Anti-Pattern: Unsynchronized Logical Slots and Uncoordinated LSN Advance Leading to CDC Split-Brain or Slot Eviction Upon Standby Promotion

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---
description: PostgreSQL 17 Failover Slot Synchronization High-Availability Standard - Enforces seamless logical CDC replication persistence across physical replica failovers using PostgreSQL 17 logical failover slots and synchronized standby slots. Ensures upstream applications create replication slots with `failover = true`, mandates primary `synchronized_standby_slots` alignment to prevent slot advance beyond standby LSN boundaries, and configures standbys with `sync_replication_slots = true` to prevent data loss or duplicate streaming upon promotion.
globs: src/database/**/*.{ts,js,sql}, src/infrastructure/**/*.{ts,js,tf,yaml}, src/cdc/**/*.{ts,js}, src/replication/**/*.{ts,js}, docker-compose*.{yml,yaml}, infra/**/*.tf
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/pg17-failover-slot-synchronization-standard"
---

# PostgreSQL 17 Failover Slot Synchronization High-Availability Standard

> **Severity**: CRITICAL | **Category**: backend
> **Anti-Pattern Warning**: Unsynchronized Logical Slots and Uncoordinated LSN Advance Leading to CDC Split-Brain or Slot Eviction Upon Standby Promotion

## System Constraints
You are a Principal Database Architect and Infrastructure Guardrail enforcing PostgreSQL 17 Logical Replication Slot Synchronization standards. Mandate that all logical replication slots (CDC consumers, Outbox listeners, Debezium, pgoutput) created against PG 17 clusters set failover = true. Ensure primary cluster parameters mandate synchronized_standby_slots with the standby application_names, and physical standbys enable sync_replication_slots. Reject any slot setup or connection pooling abstraction that fails to handle failover slot persistence and non-advance boundary checks.

## Strict Guidelines
- Engineers MUST ALWAYS create logical replication slots with `failover = true` on PostgreSQL 17+ primaries (e.g., `SELECT pg_create_logical_replication_slot('slot_name', 'pgoutput', false, false, true);` or via replication connection option `CREATE_REPLICATION_SLOT ... (FAILOVER true)`).
- Engineers MUST configure `synchronized_standby_slots` on the primary instance containing the `application_name` of all physical standbys that participate in automated failover to prevent logical consumers from consuming WAL ahead of physical replication replay.
- Engineers MUST set `sync_replication_slots = true` in postgresql.conf on all physical standbys to enable the background slot synchronization worker (`slotsync worker`).
- Engineers MUST configure `hot_standby_feedback = on` across all physical standbys to avoid catalog bloat and prevent `xmin` horizon eviction of tuples needed by downstream logical slots.
- Services MUST verify slot synchronization status prior to cutover by checking `pg_replication_slots.sync_state` (`synced` or `in_sync`) and verifying `pg_replication_slots.failover = true` on all participating standby nodes.
- Engineers MUST NOT allow logical replication clients (e.g., Debezium, custom CDC pipelines) to consume logical changes directly from physical standbys without verifying that the standby has completed synchronization and is designated as the promoted primary, unless using PG17 cascading logical decoding directly supported by active standby slots.

## Bad vs Good Implementations
### PostgreSQL 17 Client-Side Failover Slot Creation & Sync Verification

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Creating a logical replication slot without explicitly passing `failover: true` (the fifth parameter in `pg_create_logical_replication_slot` or replication command option) causes the slot to remain non-failover capable. Physical replicas will not synchronize this slot, resulting in data loss or mandatory pipeline rebuild when a physical replica is promoted.
```typescript
import { Client } from 'pg';

export async function setupCdcReplicationSlot(client: Client, slotName: string): Promise<void> {
  // Anti-Pattern: Standard slot creation without failover=true or standby synchronization check
  // When the primary fails over to a physical replica, this slot disappears, causing full resync or permanent CDC halt
  await client.query(`
    SELECT pg_create_logical_replication_slot($1, 'pgoutput');
  `, [slotName]);
}
```

#### Production Standard (Correct)
> **Standard**: Validates cluster settings for `synchronized_standby_slots`, correctly invokes `pg_create_logical_replication_slot` with the `failover` boolean parameter set to `true`, and asserts metadata verification against `pg_replication_slots`.
```typescript
import { Client } from 'pg';

interface SlotMetadata {
  slot_name: string;
  plugin: string;
  failover: boolean;
  synced: boolean;
}

export async function setupFailoverSyncCdcSlot(client: Client, slotName: string): Promise<void> {
  // Invariant 1: Ensure PG 17+ and synchronized standby settings are present
  const configCheck = await client.query<{ synchronized_standby_slots: string }>(
    "SELECT current_setting('synchronized_standby_slots', true) AS synchronized_standby_slots;"
  );

  if (!configCheck.rows[0]?.synchronized_standby_slots) {
    throw new Error(
      'FATAL: synchronized_standby_slots must be configured on primary to prevent replication divergence.'
    );
  }

  // Invariant 2: Create logical slot with failover = true (PG 17 syntax)
  // 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
    );
  `, [slotName]);

  // Invariant 3: Verify slot configuration and failover eligibility
  const verification = await client.query<SlotMetadata>(`
    SELECT slot_name, plugin, failover, synced
    FROM pg_replication_slots
    WHERE slot_name = $1;
  `, [slotName]);

  const slot = verification.rows[0];
  if (!slot || !slot.failover) {
    throw new Error(`Replication slot ${slotName} failed to initialize with failover=true.`);
  }
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

PostgreSQL Context MCP Server
Inspects running PostgreSQL 17 clusters, validates pg_replication_slots configuration, and monitors replication sync states.
Recommended Agent Skills (askills.dev)

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

PostgreSQL HA & Replication Auditor
Validates PostgreSQL 17 cluster configurations including synchronized_standby_slots and sync_replication_slots.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: PostgreSQL 17 Failover Slot Synchronization High-Availability Standard

PostgreSQL 17 Failover Slot Synchronization High-Availability Standard defines an industrial-grade behavioral standard for backend engineering. Enforces seamless logical CDC replication persistence across physical replica failovers using PostgreSQL 17 logical failover slots and synchronized standby slots. Ensures upstream applications create replication slots with `failover = true`, mandates primary `synchronized_standby_slots` alignment to prevent slot advance beyond standby LSN boundaries, and configures standbys with `sync_replication_slots = true` to prevent data loss or duplicate streaming upon promotion. 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 Uncoordinated LSN Advance Leading to CDC Split-Brain or Slot Eviction Upon Standby 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 DriftUnsynchronized Logical Slots and Uncoordinated LSN Advance Leading to CDC Split-Brain or Slot Eviction Upon Standby PromotionCritical
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 (e.g., `SELECT pg_create_logical_replication_slot('slot_name', 'pgoutput', false, false, true)

Engineers MUST ALWAYS create logical replication slots with `failover = true` on PostgreSQL 17+ primaries (e.g., `SELECT pg_create_logical_replication_slot('slot_name', 'pgoutput', false, false, true);` or via replication connection option `CREATE_REPLICATION_SLOT ... (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 configure `synchronized_standby_slots` on the primary instance containing the `application_name` of all physical standbys that participate in automated failover to prevent logical consumers from consuming WAL ahead of physical replication replay.

Engineers MUST configure `synchronized_standby_slots` on the primary instance containing the `application_name` of all physical standbys that participate in automated failover to prevent logical consumers from consuming WAL ahead of physical replication replay.

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 set `sync_replication_slots = true` in postgresql.conf on all physical standbys to enable the background slot synchronization worker (`slotsync worker`).

Engineers MUST set `sync_replication_slots = true` in postgresql.conf on all physical standbys to enable the background slot synchronization worker (`slotsync 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 configure `hot_standby_feedback = on` across all physical standbys to avoid catalog bloat and prevent `xmin` horizon eviction of tuples needed by downstream logical slots.

Engineers MUST configure `hot_standby_feedback = on` across all physical standbys to avoid catalog bloat and prevent `xmin` horizon eviction of tuples needed by downstream 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.
5

Services MUST verify slot synchronization status prior to cutover by checking `pg_replication_slots.sync_state` (`synced` or `in_sync`) and verifying `pg_replication_slots.failover = true` on all participating standby nodes.

Services MUST verify slot synchronization status prior to cutover by checking `pg_replication_slots.sync_state` (`synced` or `in_sync`) and verifying `pg_replication_slots.failover = true` on all participating standby nodes.

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

Engineers MUST NOT allow logical replication clients (e.g., Debezium, custom CDC pipelines) to consume logical changes directly from physical standbys without verifying that the standby has completed synchronization and is designated as the promoted primary, unless using PG17 cascading logical decoding directly supported by active standby slots.

Engineers MUST NOT allow logical replication clients (e.g., Debezium, custom CDC pipelines) to consume logical changes directly from physical standbys without verifying that the standby has completed synchronization and is designated as the promoted primary, unless using PG17 cascading logical decoding directly supported by active 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.
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 High-Availability Standard critical for AI coding assistants?

Enforces seamless logical CDC replication persistence across physical replica failovers using PostgreSQL 17 logical failover slots and synchronized standby slots. Ensures upstream applications create replication slots with `failover = true`, mandates primary `synchronized_standby_slots` alignment to prevent slot advance beyond standby LSN boundaries, and configures standbys with `sync_replication_slots = true` to prevent data loss or duplicate streaming upon promotion. 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 Uncoordinated LSN Advance Leading to CDC Split-Brain or Slot Eviction Upon Standby 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

Citation & Open Source Attribution Guidelines

This specification is licensed under Creative Commons Attribution 4.0 International (CC BY 4.0). You are completely free to adapt and share this rule in your workspace. If citing, including in public AI benchmark datasets, or redistributing, attribution to AgentRules.org is required.

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