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backendcritical PriorityID: pg17-logical-replication-failover-slot-wal-quota

Postgres 17 Logical Failover Slots with Mandatory WAL Quota Boundaries

Postgres 17 introduces native failover-capable logical replication slots (FAILOVER = true) that synchronize across physical streaming standbys. To prevent catastrophic primary disk exhaustion when subscribers fail or network partitions occur, every logical failover slot provisioning and health-check routine must enforce a non-negotiable max_slot_wal_keep_size boundary paired with reactive invalidation monitoring.

Prohibited Anti-Pattern: Unbounded Logical Failover Slots Retaining Indefinite WAL Logs Causing Primary Node Outages

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---
description: Postgres 17 Logical Failover Slots with Mandatory WAL Quota Boundaries - Postgres 17 introduces native failover-capable logical replication slots (FAILOVER = true) that synchronize across physical streaming standbys. To prevent catastrophic primary disk exhaustion when subscribers fail or network partitions occur, every logical failover slot provisioning and health-check routine must enforce a non-negotiable max_slot_wal_keep_size boundary paired with reactive invalidation monitoring.
globs: src/database/**/*.{ts,js,sql}, src/infra/**/*.{ts,js}, migrations/**/*.sql, src/services/replication/**/*.{ts,js}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/pg17-logical-replication-failover-slot-wal-quota"
---

# Postgres 17 Logical Failover Slots with Mandatory WAL Quota Boundaries

> **Severity**: CRITICAL | **Category**: backend
> **Anti-Pattern Warning**: Unbounded Logical Failover Slots Retaining Indefinite WAL Logs Causing Primary Node Outages

## System Constraints
You are a Principal Database Reliability Engineer and Distributed Systems Architect enforcing the Postgres 17 Logical Replication Failover Slot WAL-Quota Invariant. You must reject any logical replication provisioning, schema migration, or HA automation scripts that configure pg_create_logical_replication_slot with failover=true without setting, validating, and monitoring max_slot_wal_keep_size and pg_replication_slots.wal_status. Ensure every logical slot fails safe by prioritizing primary database write-availability over subscriber replication lag.

## Strict Guidelines
- Engineers MUST ALWAYS set failover=true and two_phase=true (when distributed transactions are used) in pg_create_logical_replication_slot on Postgres 17+ to allow smooth physical standby promotions.
- Engineers MUST ALWAYS enforce max_slot_wal_keep_size to a strictly bounded positive threshold (e.g., 64GB to 256GB) at cluster and database level to prevent subscriber lag from consuming 100% of the primary cluster's disk volume.
- Engineers MUST NOT allow logical replication slots to be provisioned without automated monitoring of pg_replication_slots.wal_status and pg_replication_slots.invalidated flags.
- Services MUST implement explicit handling for slot invalidation (wal_status = 'lost') and gracefully trigger an initial catch-up snapshot resync instead of expecting continuous WAL replay.
- Components NEVER execute replication slot creation queries without verifying that the upstream standby synchronization parameter sync_replication_slots is configured on physical standby nodes.

## Bad vs Good Implementations
### Postgres 17 Failover Slot Provisioning and Quota Enforcement

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Creating a logical replication slot without Postgres 17's failover=true breaks CDC consumers upon physical standby failover. Furthermore, relying on unverified cluster WAL settings without checking max_slot_wal_keep_size or handling slot invalidation creates a critical risk of primary disk full outages.
```typescript
import { Client } from 'pg';

export async function setupLogicalReplicationSlot(client: Client, slotName: string): Promise<void> {
  // VULNERABLE: Unbounded slot with failover=false or default settings
  // When failover happens, logical slots are lost. Furthermore, missing WAL limits allows runaway disk usage.
  await client.query(`
    SELECT pg_create_logical_replication_slot($1, 'pgoutput');
  `, [slotName]);
}
```

#### Production Standard (Correct)
> **Standard**: Enforces Postgres 17 failover compatibility (failover=true, two_phase=true) and checks that max_slot_wal_keep_size is strictly bounded before slot creation, preventing disk exhaustion during subscriber outages.
```typescript
import { Client } from 'pg';

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

export async function setupPg17FailoverSlot(
  client: Client,
  config: ReplicationSlotConfig
): Promise<{ slotName: string; lsn: string }> {
  const plugin = config.plugin ?? 'pgoutput';

  // 1. Verify global WAL quota guardrail is active (not set to -1 / infinite)
  const quotaRes = await client.query(
    `SHOW max_slot_wal_keep_size;`
  );
  const quotaSetting = quotaRes.rows[0].max_slot_wal_keep_size;
  if (quotaSetting === '-1' || quotaSetting === '0') {
    throw new Error(
      `Refusing to create failover slot ${config.slotName}: max_slot_wal_keep_size is unconstrained (${quotaSetting}). Risk of primary disk exhaustion.`
    );
  }

  // 2. Postgres 17 syntax: pg_create_logical_replication_slot(slot_name, plugin, temporary, two_phase, failover)
  const createSlotQuery = `
    SELECT slot_name, lsn
    FROM pg_create_logical_replication_slot(
      $1,
      $2,
      false,
      true,
      true
    );
  `;

  const result = await client.query(createSlotQuery, [config.slotName, plugin]);
  const { slot_name: createdSlot, lsn } = result.rows[0];

  return { slotName: createdSlot, lsn };
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

PostgreSQL Diagnostic MCP Server
Inspects replication slots, WAL lag, and replication cluster topology directly via safe read-only SQL queries.
Recommended Agent Skills (askills.dev)

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

Patroni / PG17 Slot Inspector
CLI automation skill to check synchronized slot status across primary and standby nodes.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Postgres 17 Logical Failover Slots with Mandatory WAL Quota Boundaries

Postgres 17 Logical Failover Slots with Mandatory WAL Quota Boundaries defines an industrial-grade behavioral standard for backend engineering. Postgres 17 introduces native failover-capable logical replication slots (FAILOVER = true) that synchronize across physical streaming standbys. To prevent catastrophic primary disk exhaustion when subscribers fail or network partitions occur, every logical failover slot provisioning and health-check routine must enforce a non-negotiable max_slot_wal_keep_size boundary paired with reactive invalidation monitoring. 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: Unbounded Logical Failover Slots Retaining Indefinite WAL Logs Causing Primary Node Outages 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 DriftUnbounded Logical Failover Slots Retaining Indefinite WAL Logs Causing Primary Node OutagesCritical
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 set failover=true and two_phase=true (when distributed transactions are used) in pg_create_logical_replication_slot on Postgres 17+ to allow smooth physical standby promotions.

Engineers MUST ALWAYS set failover=true and two_phase=true (when distributed transactions are used) in pg_create_logical_replication_slot on Postgres 17+ to allow smooth physical standby promotions.

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 ALWAYS enforce max_slot_wal_keep_size to a strictly bounded positive threshold (e.g., 64GB to 256GB) at cluster and database level to prevent subscriber lag from consuming 100% of the primary cluster's disk volume.

Engineers MUST ALWAYS enforce max_slot_wal_keep_size to a strictly bounded positive threshold (e.g., 64GB to 256GB) at cluster and database level to prevent subscriber lag from consuming 100% of the primary cluster's disk volume.

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 NOT allow logical replication slots to be provisioned without automated monitoring of pg_replication_slots.wal_status and pg_replication_slots.invalidated flags.

Engineers MUST NOT allow logical replication slots to be provisioned without automated monitoring of pg_replication_slots.wal_status and pg_replication_slots.invalidated flags.

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 implement explicit handling for slot invalidation (wal_status = 'lost') and gracefully trigger an initial catch-up snapshot resync instead of expecting continuous WAL replay.

Services MUST implement explicit handling for slot invalidation (wal_status = 'lost') and gracefully trigger an initial catch-up snapshot resync instead of expecting continuous WAL 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.
5

Components NEVER execute replication slot creation queries without verifying that the upstream standby synchronization parameter sync_replication_slots is configured on physical standby nodes.

Components NEVER execute replication slot creation queries without verifying that the upstream standby synchronization parameter sync_replication_slots is configured on physical 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.
Technical Questions & Answers

Frequently Asked Technical Questions

Common questions developers face when configuring AI coding assistants for this stack:

Q:Why is Postgres 17 Logical Failover Slots with Mandatory WAL Quota Boundaries critical for AI coding assistants?

Postgres 17 introduces native failover-capable logical replication slots (FAILOVER = true) that synchronize across physical streaming standbys. To prevent catastrophic primary disk exhaustion when subscribers fail or network partitions occur, every logical failover slot provisioning and health-check routine must enforce a non-negotiable max_slot_wal_keep_size boundary paired with reactive invalidation monitoring. 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 (Unbounded Logical Failover Slots Retaining Indefinite WAL Logs Causing Primary Node Outages) 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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