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backendcritical PriorityID: postgres-17-failover-replication-slot-sync-standard

PostgreSQL 17 Failover Logical Replication Slot Synchronization Invariant

Enforces PostgreSQL 17 native logical replication slot synchronization across physical High Availability (HA) replicas. Prevents event loss, catastrophic replay storms, and corrupted downstream stream offsets during replica promotion by requiring failover-enabled slots and synchronized standby barriers.

Prohibited Anti-Pattern: Unflagged Logical Slot Creation and Missing Standby Synchronization Barriers

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---
description: PostgreSQL 17 Failover Logical Replication Slot Synchronization Invariant - Enforces PostgreSQL 17 native logical replication slot synchronization across physical High Availability (HA) replicas. Prevents event loss, catastrophic replay storms, and corrupted downstream stream offsets during replica promotion by requiring failover-enabled slots and synchronized standby barriers.
globs: src/**/*.{ts,tsx,js,jsx}, migrations/**/*.{sql,ts}, infra/**/*.{tf,yaml,yml}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/postgres-17-failover-replication-slot-sync-standard"
---

# PostgreSQL 17 Failover Logical Replication Slot Synchronization Invariant

> **Severity**: CRITICAL | **Category**: backend
> **Anti-Pattern Warning**: Unflagged Logical Slot Creation and Missing Standby Synchronization Barriers

## System Constraints
You are a Principal Software Architect enforcing the Postgres 17 Failover Replication Slot Synchronization Standard. In Postgres 17+, logical decoding slots can and must persist across physical standby promotions to maintain continuous CDC and transactional outbox streams. You must reject any slot creation or migration that omits the 'failover' parameter, lacks physical standby slot synchronization barriers in 'synchronized_standby_slots', or fails to enforce 'sync_replication_slots' on standbys.

## Strict Guidelines
- Engineers MUST ALWAYS set 'failover => true' when invoking 'pg_create_logical_replication_slot' directly or configure 'failover: true' in CDC orchestrators (e.g., Debezium 2.7+, custom PG clients) targeting PostgreSQL 17+.
- Engineers MUST ALWAYS verify that the primary PostgreSQL cluster configuration explicitly includes physical standby slot names in 'synchronized_standby_slots' before enabling downstream consumers to prevent the primary from recycling WAL segments needed by the standby.
- Standby database configurations MUST enable 'sync_replication_slots = on' to ensure slot positions (restart_lsn and confirmed_flush_lsn) are continuously mirrored from the primary to the physical replica.
- Downstream stream ingestion services MUST NEVER attempt to consume or advance logical slots directly against unpromoted physical standbys where slots are marked 'sync_state = sync'.
- Application bootstrap code and automated migrations MUST validate server version (>= 170000) and assert slot failover eligibility via 'pg_replication_slots' metadata checks ('failover = true' and 'synced = false' on primary).

## Bad vs Good Implementations
### Provisioning Logical Replication Slots with Postgres 17 Failover Guarantees

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Creating a logical replication slot without `failover => true` causes the slot to disappear or permanently lag on standby promotion in PostgreSQL 17, breaking CDC pipelines or replaying entire WAL history from transaction zero.
```typescript
import { Client } from 'pg';

export async function initCdcSlot(client: Client, slotName: string): Promise<void> {
  // ANTI-PATTERN: Pre-PG17 syntax or omitting failover flag.
  // Standby promotion will not preserve this slot, causing catastrophic CDC data loss or re-streaming duplicate storms.
  const query = `SELECT pg_create_logical_replication_slot($1, 'pgoutput');`;
  await client.query(query, [slotName]);
}
```

#### Production Standard (Correct)
> **Standard**: Validates PostgreSQL 17 runtime, verifies the physical HA standby barrier in `synchronized_standby_slots`, and provisions the logical slot with `failover => true` to guarantee seamless handover on node promotion.
```typescript
import { Client } from 'pg';

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

export async function initCdcSlot(
  client: Client,
  { slotName, plugin = 'pgoutput', standbySlotName }: ReplicationSlotConfig
): Promise<void> {
  // 1. Assert PostgreSQL 17+ runtime capability
  const versionRes = await client.query('SHOW server_version_num;');
  const versionNum = parseInt(versionRes.rows[0].server_version_num, 10);
  if (versionNum < 170000) {
    throw new Error(`Postgres 17+ required for native failover slots. Detected: ${versionNum}`);
  }

  // 2. Validate synchronization barrier configuration on primary
  const syncSlotsRes = await client.query('SHOW synchronized_standby_slots;');
  const syncSlots: string = syncSlotsRes.rows[0].synchronized_standby_slots || '';
  const hasStandbyConfigured = syncSlots.split(',').map((s: string) => s.trim()).includes(standbySlotName);

  if (!hasStandbyConfigured) {
    throw new Error(
      `Physical standby '${standbySlotName}' is missing from 'synchronized_standby_slots'. ` +
      `Refusing to create failover slot to prevent uncoordinated WAL eviction.`
    );
  }

  // 3. Create logical replication slot with native failover enabled
  // 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,
      false,
      true -- Postgres 17 native failover enabled
    );
  `;
  await client.query(createSlotQuery, [slotName, plugin]);
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

Postgres MCP Server
Inspects database catalogs, replication slot states, and server variables directly within AI agent contexts.
Recommended Agent Skills (askills.dev)

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

Postgres HA Failover Orchestrator
Validates cluster replication topology, slot synchronization health, and Patroni failover readiness.
Architectural Specification & Invariant Rationale

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

PostgreSQL 17 Failover Logical Replication Slot Synchronization Invariant defines an industrial-grade behavioral standard for backend engineering. Enforces PostgreSQL 17 native logical replication slot synchronization across physical High Availability (HA) replicas. Prevents event loss, catastrophic replay storms, and corrupted downstream stream offsets during replica promotion by requiring failover-enabled slots and synchronized standby barriers. 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: Unflagged Logical Slot Creation and Missing Standby Synchronization Barriers 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 DriftUnflagged Logical Slot Creation and Missing Standby Synchronization BarriersCritical
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' when invoking 'pg_create_logical_replication_slot' directly or configure 'failover

Engineers MUST ALWAYS set 'failover => true' when invoking 'pg_create_logical_replication_slot' directly or configure 'failover: true' in CDC orchestrators (e.g., Debezium 2.7+, custom PG clients) targeting PostgreSQL 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 ALWAYS verify that the primary PostgreSQL cluster configuration explicitly includes physical standby slot names in 'synchronized_standby_slots' before enabling downstream consumers to prevent the primary from recycling WAL segments needed by the standby.

Engineers MUST ALWAYS verify that the primary PostgreSQL cluster configuration explicitly includes physical standby slot names in 'synchronized_standby_slots' before enabling downstream consumers to prevent the primary from recycling WAL segments needed by the standby.

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

Standby database configurations MUST enable 'sync_replication_slots = on' to ensure slot positions (restart_lsn and confirmed_flush_lsn) are continuously mirrored from the primary to the physical replica.

Standby database configurations MUST enable 'sync_replication_slots = on' to ensure slot positions (restart_lsn and confirmed_flush_lsn) are continuously mirrored from the primary to the physical replica.

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

Downstream stream ingestion services MUST NEVER attempt to consume or advance logical slots directly against unpromoted physical standbys where slots are marked 'sync_state = sync'.

Downstream stream ingestion services MUST NEVER attempt to consume or advance logical slots directly against unpromoted physical standbys where slots are marked 'sync_state = sync'.

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

Application bootstrap code and automated migrations MUST validate server version (>= 170000) and assert slot failover eligibility via 'pg_replication_slots' metadata checks ('failover = true' and 'synced = false' on primary).

Application bootstrap code and automated migrations MUST validate server version (>= 170000) and assert slot failover eligibility via 'pg_replication_slots' metadata checks ('failover = true' and 'synced = false' on 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.
Technical Questions & Answers

Frequently Asked Technical Questions

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

Q:Why is PostgreSQL 17 Failover Logical Replication Slot Synchronization Invariant critical for AI coding assistants?

Enforces PostgreSQL 17 native logical replication slot synchronization across physical High Availability (HA) replicas. Prevents event loss, catastrophic replay storms, and corrupted downstream stream offsets during replica promotion by requiring failover-enabled slots and synchronized standby barriers. 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 (Unflagged Logical Slot Creation and Missing Standby Synchronization Barriers) 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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