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backendcritical PriorityID: postgres-17-synchronized-logical-failover-slot-invariant

PostgreSQL 17 Synchronized Logical Replication Failover Slot Invariant

Enforces failover-enabled logical replication slots and synchronized standby slot topologies introduced in PostgreSQL 17 for Change Data Capture (CDC) pipelines. Prevents message duplication, dual-write divergence, and unrecoverable data loss by guaranteeing logical slot offsets (confirmed_flush_lsn) are continuously mirrored to physical standbys prior to failover or promotion.

Prohibited Anti-Pattern: Provisioning un-synchronized logical replication slots that are dropped or rewound upon physical standby failover.

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---
description: PostgreSQL 17 Synchronized Logical Replication Failover Slot Invariant - Enforces failover-enabled logical replication slots and synchronized standby slot topologies introduced in PostgreSQL 17 for Change Data Capture (CDC) pipelines. Prevents message duplication, dual-write divergence, and unrecoverable data loss by guaranteeing logical slot offsets (confirmed_flush_lsn) are continuously mirrored to physical standbys prior to failover or promotion.
globs: src/db/**/*.ts, src/cdc/**/*.ts, src/streaming/**/*.ts, migrations/**/*.sql
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/postgres-17-synchronized-logical-failover-slot-invariant"
---

# PostgreSQL 17 Synchronized Logical Replication Failover Slot Invariant

> **Severity**: CRITICAL | **Category**: backend
> **Anti-Pattern Warning**: Provisioning un-synchronized logical replication slots that are dropped or rewound upon physical standby failover.

## System Constraints
You are a Principal Database Architect and Distributed Systems Engineer enforcing PostgreSQL 17 Synchronized Logical Replication Failover Slot Invariants. Mandate that all logical replication slots provisioned for downstream CDC pipelines, transactional outbox listeners, and event-driven consumers strictly enable the failover parameter (failover => true) and verify physical standby synchronization prior to stream consumption. Reject ad-hoc slot initialization scripts that emit un-synchronized logical slots or bypass failover readiness verification.

## Strict Guidelines
- Engineers MUST ALWAYS provision logical replication slots with `failover => true` when executing `pg_create_logical_replication_slot` on PostgreSQL 17+ primary instances.
- Engineers MUST NOT create logical replication slots with legacy positional arguments that default `failover` to false for streaming consumers attached to High-Availability (HA) clusters.
- Code SHOULD inspect `pg_replication_slots` to verify `failover = true` and `synced = false` on the primary, and confirm presence of `synced = true` on physical standbys prior to starting consumption.
- Services MUST validate that the primary Postgres instance defines `synchronized_standby_slots` in its runtime configuration before committing to failover-safe streaming guarantees.
- Components NEVER advance consumer checkpoint states beyond the lowest `confirmed_flush_lsn` across synchronized failover slots during failover transition windows.

## Bad vs Good Implementations
### Ensuring Failover-Enabled Logical Slot Creation in CDC Initialization

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Omitting the 'failover' parameter in pg_create_logical_replication_slot defaults failover to false. When a physical standby is promoted, the slot does not exist on the new primary, causing CDC consumers to crash or replay the entire write-ahead log from epoch.
```typescript
import { Client } from 'pg';

export async function initCdcSlot(client: Client, slotName: string): Promise<void> {
  // FAILS: Creates a legacy logical slot without failover synchronization.
  // If the physical primary crashes, the standby replica lacks this slot and its LSN progress,
  // leading to missing events or massive event duplication upon failover.
  const query = `
    SELECT slot_name, lsn
    FROM pg_create_logical_replication_slot($1, 'pgoutput');
  `;
  await client.query(query, [slotName]);
}
```

#### Production Standard (Correct)
> **Standard**: Checks PostgreSQL 17 runtime preconditions, ensures 'synchronized_standby_slots' is non-empty, and explicitly provisions the slot with 'failover => true' using named parameters, guaranteeing physical standbys retain synced replication coordinates.
```typescript
import { Client } from 'pg';

interface ReplicationSlotConfig {
  slotName: string;
  plugin?: string;
}

export async function initCdcSlot(
  client: Client,
  { slotName, plugin = 'pgoutput' }: ReplicationSlotConfig
): Promise<void> {
  // 1. Verify PostgreSQL engine version >= 17 for failover slot support
  const versionRes = await client.query('SHOW server_version_num;');
  const serverVersion = parseInt(versionRes.rows[0].server_version_num, 10);
  if (serverVersion < 170000) {
    throw new Error('PostgreSQL 17+ is strictly required for synchronized logical replication failover slots.');
  }

  // 2. Validate that synchronized_standby_slots is configured on the primary
  const syncSettingRes = await client.query("SHOW synchronized_standby_slots;");
  const syncStandbys = syncSettingRes.rows[0]?.synchronized_standby_slots ?? '';
  if (!syncStandbys || syncStandbys.trim() === '') {
    throw new Error('primary configuration error: synchronized_standby_slots is empty. Standbys will not mirror slots.');
  }

  // 3. Atomically create or verify logical replication slot with failover enabled
  const slotCheck = await client.query(
    'SELECT slot_name, failover, synced FROM pg_replication_slots WHERE slot_name = $1;',
    [slotName]
  );

  if (slotCheck.rows.length === 0) {
    // Postgres 17 syntax: pg_create_logical_replication_slot(slot_name, plugin, temporary, two_phase, failover)
    await client.query(
      'SELECT pg_create_logical_replication_slot(slot_name => $1, plugin => $2, failover => true);',
      [slotName, plugin]
    );
  } else {
    const slot = slotCheck.rows[0];
    if (!slot.failover) {
      throw new Error(`Replication slot ${slotName} exists but does not have failover synchronization enabled.`);
    }
  }
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

PostgreSQL Context Server
Inspects live replication slots, cluster topology, confirmed_flush_lsn values, and failover status across primary and standby replicas.
Recommended Agent Skills (askills.dev)

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

HA Replication Slot Auditor
Validates synchronized slot states and standby replication lag using pg_replication_slots and patronictl topology commands.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: PostgreSQL 17 Synchronized Logical Replication Failover Slot Invariant

PostgreSQL 17 Synchronized Logical Replication Failover Slot Invariant defines an industrial-grade behavioral standard for backend engineering. Enforces failover-enabled logical replication slots and synchronized standby slot topologies introduced in PostgreSQL 17 for Change Data Capture (CDC) pipelines. Prevents message duplication, dual-write divergence, and unrecoverable data loss by guaranteeing logical slot offsets (confirmed_flush_lsn) are continuously mirrored to physical standbys prior to failover or 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: Provisioning un-synchronized logical replication slots that are dropped or rewound upon physical standby failover. 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 DriftProvisioning un-synchronized logical replication slots that are dropped or rewound upon physical standby failover.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` when executing `pg_create_logical_replication_slot` on PostgreSQL 17+ primary instances.

Engineers MUST ALWAYS provision logical replication slots with `failover => true` when executing `pg_create_logical_replication_slot` on PostgreSQL 17+ primary instances.

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 create logical replication slots with legacy positional arguments that default `failover` to false for streaming consumers attached to High-Availability (HA) clusters.

Engineers MUST NOT create logical replication slots with legacy positional arguments that default `failover` to false for streaming consumers attached to High-Availability (HA) clusters.

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

Code SHOULD inspect `pg_replication_slots` to verify `failover = true` and `synced = false` on the primary, and confirm presence of `synced = true` on physical standbys prior to starting consumption.

Code SHOULD inspect `pg_replication_slots` to verify `failover = true` and `synced = false` on the primary, and confirm presence of `synced = true` on physical standbys prior to starting consumption.

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 validate that the primary Postgres instance defines `synchronized_standby_slots` in its runtime configuration before committing to failover-safe streaming guarantees.

Services MUST validate that the primary Postgres instance defines `synchronized_standby_slots` in its runtime configuration before committing to failover-safe streaming guarantees.

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

Components NEVER advance consumer checkpoint states beyond the lowest `confirmed_flush_lsn` across synchronized failover slots during failover transition windows.

Components NEVER advance consumer checkpoint states beyond the lowest `confirmed_flush_lsn` across synchronized failover slots during failover transition windows.

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 Synchronized Logical Replication Failover Slot Invariant critical for AI coding assistants?

Enforces failover-enabled logical replication slots and synchronized standby slot topologies introduced in PostgreSQL 17 for Change Data Capture (CDC) pipelines. Prevents message duplication, dual-write divergence, and unrecoverable data loss by guaranteeing logical slot offsets (confirmed_flush_lsn) are continuously mirrored to physical standbys prior to failover or 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 (Provisioning un-synchronized logical replication slots that are dropped or rewound upon physical standby failover.) 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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