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backendcritical PriorityID: postgres-17-synchronized-failover-slots-guardrail

Postgres 17 Synchronized Logical Replication Failover Slots Guardrail

Enforces Postgres 17 failover-ready logical replication slots by mandating the failover flag on physical primaries, configuring standby sync slots, and binding logical slots to physical replication standby slots via synchronized slot machinery. Guarantees zero change-data-capture message loss, prevents WAL bloat, and avoids downstream subscriber divergence during HA promotion.

Prohibited Anti-Pattern: Creating unsynchronized logical slots without failover-enablement or standby synchronization in Postgres 17 topologies

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---
description: Postgres 17 Synchronized Logical Replication Failover Slots Guardrail - Enforces Postgres 17 failover-ready logical replication slots by mandating the failover flag on physical primaries, configuring standby sync slots, and binding logical slots to physical replication standby slots via synchronized slot machinery. Guarantees zero change-data-capture message loss, prevents WAL bloat, and avoids downstream subscriber divergence during HA promotion.
globs: src/infra/**/*.ts, src/database/**/*.ts, src/services/**/*.ts, infra/**/*.tf, infra/**/*.sql
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/postgres-17-synchronized-failover-slots-guardrail"
---

# Postgres 17 Synchronized Logical Replication Failover Slots Guardrail

> **Severity**: CRITICAL | **Category**: backend
> **Anti-Pattern Warning**: Creating unsynchronized logical slots without failover-enablement or standby synchronization in Postgres 17 topologies

## System Constraints
You are a Principal Database Reliability Engineer enforcing strict adherence to Postgres 17 synchronized logical failover slots. Mandate that every logical replication slot provisioned for CDC or streaming pipelines sets `failover = true` on the primary, is tracked in `synchronized_standby_slots` on the primary, and uses `sync_replication_slots = on` with a configured `primary_slot_name` on physical standby instances. Reject manual slot re-creation post-failover and forbid subscribing directly to unmanaged slots.

## Strict Guidelines
- Engineers MUST ALWAYS provision logical slots using `pg_create_logical_replication_slot(slot_name, plugin, false, false, failover => true)` or SQL equivalent `CREATE_REPLICATION_SLOT ... LOGICAL ... (FAILOVER)` in Postgres 17+.
- Engineers MUST NOT rely on external slot-copy daemon scripts or manual LSN synchronization when Postgres 17 native failover slot replication is available.
- Primary database configurations MUST include the physical standby's replication slot name within `synchronized_standby_slots` before advancing the logical slot confirmation LSN beyond the standby's received LSN.
- Physical standby instances MUST enable `sync_replication_slots = on` and configure valid `primary_conninfo` with `primary_slot_name` to safely poll and mirror logical slots via `pg_sync_replication_slots()`.
- Subscribers and CDC drivers MUST verify that target slots maintain `failover = true` and `synced = false` on the active primary, and handle transient disconnects during standby promotion without recreating subscriber state.

## Bad vs Good Implementations
### Configuring Postgres 17 Logical Replication Slots with Failover Synchronization

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Legacy `pg_create_logical_replication_slot` without `failover => true` does not synchronize with Postgres 17 physical replicas. If the primary crashes, the promoted replica lacks this logical slot, forcing downstream CDC engines to rebuild state from scratch and leading to silent message divergence.
```typescript
import { Client } from 'pg';

export async function initCdcSubscriber(client: Client, slotName: string): Promise<void> {
  // Anti-pattern: Creates legacy non-failover slot; slot does not survive standby promotion
  await client.query(`
    SELECT pg_create_logical_replication_slot(
      $1,
      'pgoutput'
    );
  `, [slotName]);
}
```

#### Production Standard (Correct)
> **Standard**: Specifying `failover => true` activates Postgres 17 native logical failover synchronization. Together with primary `synchronized_standby_slots` and standby `sync_replication_slots = on`, this guarantees that standby instances continuously mirror the confirmed LSN, enabling lossless subscriber reconnection immediately after failover.
```typescript
import { Client } from 'pg';

export async function initCdcSubscriber(client: Client, slotName: string): Promise<void> {
  // Invariant: Enforce Postgres 17 failover-enabled logical slot
  await client.query(`
    SELECT pg_create_logical_replication_slot(
      slot_name => $1,
      plugin => 'pgoutput',
      temporary => false,
      twophase => true,
      failover => true
    );
  `, [slotName]);

  // Verify slot status and confirm failover readiness
  const res = await client.query(`
    SELECT slot_name, plugin, failover, synced
    FROM pg_replication_slots
    WHERE slot_name = $1;
  `, [slotName]);

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

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

Postgres Context Protocol Server
Inspects live replication slot states, confirmed flush LSN offsets, and catalog configurations on PostgreSQL clusters.
Recommended Agent Skills (askills.dev)

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

PostgreSQL Failover Slot Auditor CLI
Automated CLI utility to audit Postgres 17 synchronized_standby_slots and verify replication slot health across HA topologies.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Postgres 17 Synchronized Logical Replication Failover Slots Guardrail

Postgres 17 Synchronized Logical Replication Failover Slots Guardrail defines an industrial-grade behavioral standard for backend engineering. Enforces Postgres 17 failover-ready logical replication slots by mandating the failover flag on physical primaries, configuring standby sync slots, and binding logical slots to physical replication standby slots via synchronized slot machinery. Guarantees zero change-data-capture message loss, prevents WAL bloat, and avoids downstream subscriber divergence during HA 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 unsynchronized logical slots without failover-enablement or standby synchronization in Postgres 17 topologies 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 unsynchronized logical slots without failover-enablement or standby synchronization in Postgres 17 topologiesCritical
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 slots using `pg_create_logical_replication_slot(slot_name, plugin, false, false, failover => true)` or SQL equivalent `CREATE_REPLICATION_SLOT ... LOGICAL ... (FAILOVER)` in Postgres 17+.

Engineers MUST ALWAYS provision logical slots using `pg_create_logical_replication_slot(slot_name, plugin, false, false, failover => true)` or SQL equivalent `CREATE_REPLICATION_SLOT ... LOGICAL ... (FAILOVER)` in Postgres 17+.

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 NOT rely on external slot-copy daemon scripts or manual LSN synchronization when Postgres 17 native failover slot replication is available.

Engineers MUST NOT rely on external slot-copy daemon scripts or manual LSN synchronization when Postgres 17 native failover slot replication is available.

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 database configurations MUST include the physical standby's replication slot name within `synchronized_standby_slots` before advancing the logical slot confirmation LSN beyond the standby's received LSN.

Primary database configurations MUST include the physical standby's replication slot name within `synchronized_standby_slots` before advancing the logical slot confirmation LSN beyond the standby's received 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.
4

Physical standby instances MUST enable `sync_replication_slots = on` and configure valid `primary_conninfo` with `primary_slot_name` to safely poll and mirror logical slots via `pg_sync_replication_slots()`.

Physical standby instances MUST enable `sync_replication_slots = on` and configure valid `primary_conninfo` with `primary_slot_name` to safely poll and mirror logical slots via `pg_sync_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.
5

Subscribers and CDC drivers MUST verify that target slots maintain `failover = true` and `synced = false` on the active primary, and handle transient disconnects during standby promotion without recreating subscriber state.

Subscribers and CDC drivers MUST verify that target slots maintain `failover = true` and `synced = false` on the active primary, and handle transient disconnects during standby promotion without recreating subscriber state.

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 Synchronized Logical Replication Failover Slots Guardrail critical for AI coding assistants?

Enforces Postgres 17 failover-ready logical replication slots by mandating the failover flag on physical primaries, configuring standby sync slots, and binding logical slots to physical replication standby slots via synchronized slot machinery. Guarantees zero change-data-capture message loss, prevents WAL bloat, and avoids downstream subscriber divergence during HA 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 unsynchronized logical slots without failover-enablement or standby synchronization in Postgres 17 topologies) 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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