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

Postgres 17 Failover Slot Synchronization and Zero-Divergence CDC Invariant

Enforces failover-safe logical replication slot synchronization introduced in PostgreSQL 17 for transactional outbox CDC consumers. Mandates `failover = true` on primary logical slots, `sync_replication_slots = true` on physical standbys, and explicit standby validation in `synchronized_standby_slots` to guarantee zero-event divergence and prevent data loss during physical standby promotion.

Prohibited Anti-Pattern: Creating unmanaged logical replication slots without failover sync or primary standby acknowledgment checks.

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---
description: Postgres 17 Failover Slot Synchronization and Zero-Divergence CDC Invariant - Enforces failover-safe logical replication slot synchronization introduced in PostgreSQL 17 for transactional outbox CDC consumers. Mandates `failover = true` on primary logical slots, `sync_replication_slots = true` on physical standbys, and explicit standby validation in `synchronized_standby_slots` to guarantee zero-event divergence and prevent data loss during physical standby promotion.
globs: src/infra/db/**/*.{ts,js,sql}, src/cdc/**/*.{ts,js}, src/outbox/**/*.{ts,js}, migrations/**/*.{sql,ts,js}, docker/postgres/**/*.{sh,sql,conf}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/pg17-logical-replication-slot-sync-invariant"
---

# Postgres 17 Failover Slot Synchronization and Zero-Divergence CDC Invariant

> **Severity**: CRITICAL | **Category**: backend
> **Anti-Pattern Warning**: Creating unmanaged logical replication slots without failover sync or primary standby acknowledgment checks.

## System Constraints
You are a Principal Database Reliability Engineer enforcing strict PostgreSQL 17 logical replication invariants for transactional outbox pipelines. You must mandate that every logical replication slot intended to survive physical failover is initialized with `failover: true`, that the primary instance configures `synchronized_standby_slots` with physical replica application names, and that CDC connectors verify slot synchronization status prior to advancing restart/confirmed flush LSNs. Reject any slot configuration that permits CDC consumers to progress past un-replicated physical standby boundaries.

## Strict Guidelines
- Engineers MUST ALWAYS provision logical replication slots with `failover = true` in PostgreSQL 17+ (`pg_create_logical_replication_slot(slot_name, plugin, false, false, true)`).
- Engineers MUST configure `synchronized_standby_slots` on the primary instance containing all critical physical standbys before consuming downstream events.
- Engineers MUST configure `sync_replication_slots = true` on physical standby nodes to enable automated slots sync daemon execution.
- Services MUST NOT read from or advance slots if `pg_replication_slots.sync_state` indicates an unsynchronized or invalid failover state on newly promoted primaries.
- CDC consumers MUST monitor and enforce that `confirmed_flush_lsn` is strictly behind or equal to the standby's received LSN prior to node cutovers.
- Engineers MUST verify that `hot_standby_feedback = on` is configured on physical replicas to prevent WAL cleanup of rows still required by synchronized logical decoding slots.

## Bad vs Good Implementations
### Postgres 17 Logical Replication Slot Initialization and Failover Validation

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Creating a slot without Postgres 17's `failover` argument (parameter 5) means the slot is not marked for synchronization. Upon promotion of a physical standby, the slot will be absent, breaking the CDC pipeline and potentially causing duplicate or dropped events during manual reconstruction.
```typescript
import { Client } from 'pg';

export async function initOutboxSlot(client: Client, slotName: string): Promise<void> {
  // Anti-Pattern: Defaults to PostgreSQL 16- style slot without failover parameter
  // When physical standby promotes, this slot will vanish, causing CDC pipeline failure or massive data duplication
  await client.query(`
    SELECT pg_create_logical_replication_slot($1, 'pgoutput');
  `, [slotName]);

  // No check to verify if physical standbys acknowledge WAL prior to consumption
  console.log(`Slot ${slotName} created without failover sync guardrails.`);
}
```

#### Production Standard (Correct)
> **Standard**: Ensures the slot is explicitly created with Postgres 17 `failover = true` and validates that `synchronized_standby_slots` is actively preventing WAL progress beyond what physical standby nodes have applied, preventing CDC divergence on failover.
```typescript
import { Client } from 'pg';

export async function initOutboxSlot(client: Client, slotName: string, standbyAppName: string): Promise<void> {
  // Invariant: Verify synchronized_standby_slots includes the required standby to prevent divergence
  const syncCheck = await client.query(`
    SELECT setting FROM pg_settings WHERE name = 'synchronized_standby_slots';
  `);
  const configuredStandbys = syncCheck.rows[0]?.setting?.split(',').map((s: string) => s.trim()) ?? [];
  
  if (!configuredStandbys.includes(standbyAppName)) {
    throw new Error(
      `Primary misconfiguration: 'synchronized_standby_slots' must include '${standbyAppName}'. Configured: ${syncCheck.rows[0]?.setting}`
    );
  }

  // Invariant: PG17 explicit failover slot creation (slot_name, plugin, temporary, two_phase, failover)
  await client.query(`
    SELECT pg_create_logical_replication_slot(
      $1,
      'pgoutput',
      false,
      false,
      true -- PG17 failover = true
    );
  `, [slotName]);

  // Invariant: Validate slot attributes post-creation
  const slotResult = await client.query(`
    SELECT slot_name, failover, synced 
    FROM pg_replication_slots 
    WHERE slot_name = $1;
  `, [slotName]);

  const slot = slotResult.rows[0];
  if (!slot || slot.failover !== true) {
    throw new Error(`Invariant violation: Slot '${slotName}' was not created with failover=true.`);
  }
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

PostgreSQL Schema & Slot Inspector MCP
Inspects database replication slots, replication lag, and synchronized standbys settings directly from PostgreSQL instances.
Recommended Agent Skills (askills.dev)

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

Postgres Slot Sync Auditor CLI
CLI utility to verify logical slot sync status, failover flags, and standby LSN synchronization across PostgreSQL clusters.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Postgres 17 Failover Slot Synchronization and Zero-Divergence CDC Invariant

Postgres 17 Failover Slot Synchronization and Zero-Divergence CDC Invariant defines an industrial-grade behavioral standard for backend engineering. Enforces failover-safe logical replication slot synchronization introduced in PostgreSQL 17 for transactional outbox CDC consumers. Mandates `failover = true` on primary logical slots, `sync_replication_slots = true` on physical standbys, and explicit standby validation in `synchronized_standby_slots` to guarantee zero-event divergence and prevent data loss during physical standby 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: Creating unmanaged logical replication slots without failover sync or primary standby acknowledgment checks. 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 unmanaged logical replication slots without failover sync or primary standby acknowledgment checks.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+ (`pg_create_logical_replication_slot(slot_name, plugin, false, false, true)`).

Engineers MUST ALWAYS provision logical replication slots with `failover = true` in PostgreSQL 17+ (`pg_create_logical_replication_slot(slot_name, plugin, false, false, 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 all critical physical standbys before consuming downstream events.

Engineers MUST configure `synchronized_standby_slots` on the primary instance containing all critical physical standbys before consuming downstream events.

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 configure `sync_replication_slots = true` on physical standby nodes to enable automated slots sync daemon execution.

Engineers MUST configure `sync_replication_slots = true` on physical standby nodes to enable automated slots sync daemon execution.

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 NOT read from or advance slots if `pg_replication_slots.sync_state` indicates an unsynchronized or invalid failover state on newly promoted primaries.

Services MUST NOT read from or advance slots if `pg_replication_slots.sync_state` indicates an unsynchronized or invalid failover state on newly promoted primaries.

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

CDC consumers MUST monitor and enforce that `confirmed_flush_lsn` is strictly behind or equal to the standby's received LSN prior to node cutovers.

CDC consumers MUST monitor and enforce that `confirmed_flush_lsn` is strictly behind or equal to the standby's received LSN prior to node cutovers.

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 verify that `hot_standby_feedback = on` is configured on physical replicas to prevent WAL cleanup of rows still required by synchronized logical decoding slots.

Engineers MUST verify that `hot_standby_feedback = on` is configured on physical replicas to prevent WAL cleanup of rows still required by synchronized logical decoding 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 Postgres 17 Failover Slot Synchronization and Zero-Divergence CDC Invariant critical for AI coding assistants?

Enforces failover-safe logical replication slot synchronization introduced in PostgreSQL 17 for transactional outbox CDC consumers. Mandates `failover = true` on primary logical slots, `sync_replication_slots = true` on physical standbys, and explicit standby validation in `synchronized_standby_slots` to guarantee zero-event divergence and prevent data loss during physical standby 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 (Creating unmanaged logical replication slots without failover sync or primary standby acknowledgment checks.) 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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