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backendcritical PriorityID: pg17-logical-replication-slot-synchronization-guardrail

PostgreSQL 17 Logical Replication Slot Synchronization Guardrail

Enforces failover-ready logical replication slots on PostgreSQL 17 primary instances and validates slot synchronization across physical streaming standbys prior to promotion. Guarantees zero data loss and prevents subscriber divergence or invalidated slots during planned failovers or ungraceful primary promotions.

Prohibited Anti-Pattern: Promoting Physical Standbys or Reconnecting CDC Subscribers Without Validating PG17 Failover Slot Synchronization State

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---
description: PostgreSQL 17 Logical Replication Slot Synchronization Guardrail - Enforces failover-ready logical replication slots on PostgreSQL 17 primary instances and validates slot synchronization across physical streaming standbys prior to promotion. Guarantees zero data loss and prevents subscriber divergence or invalidated slots during planned failovers or ungraceful primary promotions.
globs: src/database/**/*.{ts,js,py,go}, src/infra/**/*.{ts,js,py,go}, scripts/**/*.{sh,ts,py}, deploy/**/*.{yaml,yml,sql}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/pg17-logical-replication-slot-synchronization-guardrail"
---

# PostgreSQL 17 Logical Replication Slot Synchronization Guardrail

> **Severity**: CRITICAL | **Category**: backend
> **Anti-Pattern Warning**: Promoting Physical Standbys or Reconnecting CDC Subscribers Without Validating PG17 Failover Slot Synchronization State

## System Constraints
You are a Principal Database Reliability Engineer enforcing PostgreSQL 17 Logical Replication Slot Synchronization Guardrails. You mandate that any logical replication slot supporting CDC pipelines, downstream subscriber databases, or audit outboxes utilizes failover = true, registers standby_slot_names on the primary, and verifies synchronized_standby_slots on standbys before failover/promotion workflows execute.

## Strict Guidelines
- Engineers MUST ALWAYS provision logical replication slots with 'failover: true' in PostgreSQL 17 (via pg_create_logical_replication_slot) when slots are intended to persist across standby promotion.
- Engineers MUST configure 'standby_slot_names' on the PG17 primary to include the physical streaming replication slot names of candidate failover standbys to guarantee the primary does not prune WAL before standbys consume it.
- Engineers MUST ensure candidate standbys enable 'sync_replication_slots = on' (or run pg_sync_replication_slots()) to continuously pull logical slot states from the primary.
- Promotion and orchestrator scripts MUST verify that 'pg_replication_slots.synced = true' and confirmed_flush_lsn matches the primary before executing 'pg_ctl promote' or Patroni switchovers.
- Services MUST NOT re-point downstream logical subscribers or Debezium/Kafka CDC connectors to a newly promoted standby until confirming the standby slot's confirmed_flush_lsn is strictly monotonic and non-regressive.
- Database migration scripts NEVER drop or recreate failover-enabled logical slots during runtime without explicitly draining downstream consumer groups and confirming WAL continuity.

## Bad vs Good Implementations
### PostgreSQL 17 Failover Slot Creation & Promotion Validation Automation

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Creating logical slots without specifying failover=true in PG17 leaves the slot un-synced on physical standbys. Promoting the standby without asserting pg_replication_slots.synced and WAL catch-up results in slot loss or missing CDC events.
```typescript
import { Client } from 'pg';

// ANTI-PATTERN: Default slot creation without failover=true or standby synchronization checks
export async function promoteStandbyAndReconnectCDC(primary: Client, standby: Client, slotName: string): Promise<void> {
  // Creates legacy slot without PG17 failover capability
  await primary.query(`SELECT pg_create_logical_replication_slot($1, 'pgoutput');`, [slotName]);

  // Unsafe promotion: does not verify if standby has caught up or synced the logical slot
  await standby.query(`SELECT pg_promote();`);

  // Subscriber reconnects to a promoted node where slotName either does not exist or has stale LSN,
  // causing catastrophic replication divergence or ERROR: replication slot does not exist.
}
```

#### Production Standard (Correct)
> **Standard**: Explicitly provisions PostgreSQL 17 logical slots with failover => true, validates slot reflection on physical standbys via pg_replication_slots.synced, and guarantees no CDC state is lost upon promotion.
```typescript
import { Client } from 'pg';

interface ReplicationSlotStatus {
  slot_name: string;
  synced: boolean;
  confirmed_flush_lsn: string;
}

export async function provisionAndVerifyPG17FailoverSlot(
  primary: Client,
  standby: Client,
  slotName: string,
  plugin = 'pgoutput'
): Promise<void> {
  // 1. Create slot with PG17 failover flag enabled and two-phase disabled or enabled as required
  await primary.query(
    `SELECT pg_create_logical_replication_slot(
      slot_name => $1,
      plugin => $2,
      temporary => false,
      two_phase => false,
      failover => true
    );`,
    [slotName, plugin]
  );

  // 2. Validate sync status on physical standby before any promotion workflow
  const res = await standby.query<ReplicationSlotStatus>(
    `SELECT slot_name, synced, confirmed_flush_lsn
     FROM pg_replication_slots
     WHERE slot_name = $1 AND failover = true;`,
    [slotName]
  );

  if (res.rowCount === 0) {
    throw new Error(`Logical slot ${slotName} does not exist on standby. Check sync_replication_slots configuration.`);
  }

  const slot = res.rows[0];
  if (!slot.synced) {
    // Force sync if pg_sync_replication_slots() is available and standby slot is lagging
    await standby.query(`SELECT pg_sync_replication_slots();`);
  }
}
```
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 topology directly within agent workspaces.
Recommended Agent Skills (askills.dev)

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

Patroni HA Failover Validator
Automated verification of standby_slot_names and Patroni logical slot synchronization hooks for zero-data-loss failover.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: PostgreSQL 17 Logical Replication Slot Synchronization Guardrail

PostgreSQL 17 Logical Replication Slot Synchronization Guardrail defines an industrial-grade behavioral standard for backend engineering. Enforces failover-ready logical replication slots on PostgreSQL 17 primary instances and validates slot synchronization across physical streaming standbys prior to promotion. Guarantees zero data loss and prevents subscriber divergence or invalidated slots during planned failovers or ungraceful primary promotions. 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: Promoting Physical Standbys or Reconnecting CDC Subscribers Without Validating PG17 Failover Slot Synchronization State 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 DriftPromoting Physical Standbys or Reconnecting CDC Subscribers Without Validating PG17 Failover Slot Synchronization StateCritical
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

Engineers MUST ALWAYS provision logical replication slots with 'failover: true' in PostgreSQL 17 (via pg_create_logical_replication_slot) when slots are intended to persist across 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 'standby_slot_names' on the PG17 primary to include the physical streaming replication slot names of candidate failover standbys to guarantee the primary does not prune WAL before standbys consume it.

Engineers MUST configure 'standby_slot_names' on the PG17 primary to include the physical streaming replication slot names of candidate failover standbys to guarantee the primary does not prune WAL before standbys consume it.

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 candidate standbys enable 'sync_replication_slots = on' (or run pg_sync_replication_slots()) to continuously pull logical slot states from the primary.

Engineers MUST ensure candidate standbys enable 'sync_replication_slots = on' (or run pg_sync_replication_slots()) to continuously pull logical slot states 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

Promotion and orchestrator scripts MUST verify that 'pg_replication_slots.synced = true' and confirmed_flush_lsn matches the primary before executing 'pg_ctl promote' or Patroni switchovers.

Promotion and orchestrator scripts MUST verify that 'pg_replication_slots.synced = true' and confirmed_flush_lsn matches the primary before executing 'pg_ctl promote' or Patroni switchovers.

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 NOT re-point downstream logical subscribers or Debezium/Kafka CDC connectors to a newly promoted standby until confirming the standby slot's confirmed_flush_lsn is strictly monotonic and non-regressive.

Services MUST NOT re-point downstream logical subscribers or Debezium/Kafka CDC connectors to a newly promoted standby until confirming the standby slot's confirmed_flush_lsn is strictly monotonic and non-regressive.

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

Database migration scripts NEVER drop or recreate failover-enabled logical slots during runtime without explicitly draining downstream consumer groups and confirming WAL continuity.

Database migration scripts NEVER drop or recreate failover-enabled logical slots during runtime without explicitly draining downstream consumer groups and confirming WAL continuity.

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 Synchronization Guardrail critical for AI coding assistants?

Enforces failover-ready logical replication slots on PostgreSQL 17 primary instances and validates slot synchronization across physical streaming standbys prior to promotion. Guarantees zero data loss and prevents subscriber divergence or invalidated slots during planned failovers or ungraceful primary promotions. 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 (Promoting Physical Standbys or Reconnecting CDC Subscribers Without Validating PG17 Failover Slot Synchronization State) 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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