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ulak

Transactional Outbox Pattern for PostgreSQL

ulak is a PostgreSQL extension that writes messages to a transactional outbox table atomically with your business data, then delivers them reliably via background workers to HTTP, Apache Kafka, MQTT, Redis Streams, AMQP (RabbitMQ), and NATS (JetStream). Workers use FOR UPDATE SKIP LOCKED with modulo partitioning for zero-contention parallel processing. The extension ships with a built-in circuit breaker, exponential backoff retry, dead letter queue with redrive, role-based access control, SSRF protection, and pub/sub fan-out -- giving your application exactly-once semantics for writes and at-least-once delivery guarantees.

Quick Example

-- Create an HTTP endpoint
SELECT ulak.create_endpoint('my-webhook', 'http',
  '{"url": "https://httpbin.org/post", "method": "POST"}'::jsonb);

-- Send a message atomically with your business transaction
BEGIN;
  INSERT INTO orders (id, total) VALUES (1, 99.99);
  SELECT ulak.send('my-webhook', '{"order_id": 1, "total": 99.99}'::jsonb);
COMMIT;
-- The message is now queued and will be delivered by a background worker

Key Features

• 6 Protocols -- HTTP (built-in), Apache Kafka, MQTT, Redis Streams, AMQP (RabbitMQ), NATS (JetStream). Each protocol is an optional compile-time module except HTTP which is always included.

• Multi-Worker -- 1 to 32 parallel background workers with modulo partitioning (id % N). Each worker processes a disjoint slice of the queue using FOR UPDATE SKIP LOCKED, eliminating row contention entirely.

• Reliability -- Per-endpoint circuit breaker (closed/open/half-open state machine), configurable retry with exponential, linear, or fixed backoff, and a dead letter queue with single-message, per-endpoint, and bulk redrive.

• Security -- Three RBAC roles (ulak_admin, ulak_application, ulak_monitor), SSRF protection that blocks private/loopback addresses by default, TLS/mTLS certificate verification, OAuth2 bearer tokens, AWS Signature V4, and webhook HMAC signing.

• Pub/Sub -- Event types with JSONB containment filters and multi-endpoint fan-out. Publish once, deliver to every matching subscriber.

• Message Control -- Priority levels 0-10 (higher first), ordering keys for per-key FIFO delivery, idempotency keys with payload hash conflict detection, TTL with automatic expiration, and scheduled delivery with future timestamps.

• Operational -- Backpressure via configurable max_queue_size (rejects new messages when the queue is full), monthly archive partitions with automatic rotation, health checks, worker status monitoring, endpoint health dashboard, and optional response body capture.

• CloudEvents -- Binary and structured mode support with configurable source attribute, enabling interoperability with CloudEvents-aware systems.

Supported PostgreSQL Versions

Version	Status
PostgreSQL 14	Supported
PostgreSQL 15	Supported
PostgreSQL 16	Supported
PostgreSQL 17	Supported
PostgreSQL 18	Supported (default)

Documentation

Getting Started

Page	Description
Quick Start	Prerequisites, installation, Docker setup, and your first message

Architecture

Page	Description
System Architecture	Worker model, queue design, dispatcher factory, and data flow

Protocols

Page	Description
HTTP	Webhook delivery, batching, TLS, OAuth2, AWS SigV4, HMAC signing
Apache Kafka	Topic publishing, acks, compression, delivery guarantees
MQTT	Broker connections, QoS levels, topic publishing
Redis Streams	Stream appends, XADD, maxlen trimming
AMQP / RabbitMQ	Exchange routing, publisher confirms, TLS
NATS / JetStream	Subject publishing, JetStream acknowledgments, reconnection

Features

Page	Description
Pub/Sub Events	Event types, subscriptions, JSONB filters, fan-out publishing
Message Features	Priority, ordering keys, idempotency, TTL, scheduled delivery

Operations

Page	Description
Reliability	Circuit breaker, retry backoff strategies, DLQ management, redrive
Monitoring	Health checks, worker status, endpoint health, metrics, DLQ summary
Security	RBAC roles, SSRF protection, TLS configuration, authentication

Reference

Page	Description
Configuration (57 GUCs)	All ulak.* GUC parameters with defaults, ranges, and reload semantics
SQL API (40 Functions)	Complete function reference: send, publish, monitor, DLQ, archive

Development

Page	Description
Building & Testing	Build flags, regression tests, code formatting, CI pipeline

License

ulak is licensed under the Apache License 2.0. You may use, modify, and distribute the project in commercial and non-commercial settings.