How do you choose between SKIP LOCKED, Redis Streams, and SQS for an async job queue?

SKIP LOCKED on a PostgreSQL jobs table is simplest when you already use Postgres and job volume is moderate — it provides transactional exactly-once claiming with no extra infrastructure. Redis Streams suit high-throughput, low-latency workloads where persistence requirements are relaxed and you want consumer groups with built-in acknowledgment. SQS is ideal for cloud-native architectures needing managed visibility timeouts, DLQ integration, and near-infinite scale without operational overhead.

What is the worker claim and heartbeat lifecycle in an async job system?

A worker atomically claims a job by setting its status to 'processing' and recording worker_id and claimed_at. It then sends periodic heartbeat updates (e.g., every 30 seconds) to a heartbeat column or Redis key. A separate watchdog process scans for jobs whose heartbeat has expired beyond a timeout threshold and resets them to 'pending' for reclaim, handling crashed workers automatically.

How should an async job worker handle graceful shutdown?

On SIGTERM the worker stops polling for new jobs and sets a draining flag. It allows in-flight jobs to complete up to a max_drain_seconds deadline. After the deadline, any still-running jobs are released back to the queue (status reset, claimed_at cleared) so another worker can pick them up. The process then exits cleanly, ensuring no jobs are silently dropped.

What observability signals matter most for an async job system?

Track queue depth (pending job count) and age of oldest job as leading indicators of processing lag. Instrument job duration histograms, success/failure rates, and retry counts per job type. Emit structured log events for claim, complete, fail, and dead-letter transitions. Alert on queue depth exceeding a threshold for N minutes and on dead-letter growth rate exceeding baseline.

Async Job System Low-Level Design: Queue Selection, Worker Lifecycle, and Observability

⏱ 5 min read

What Is an Async Job System?

An async job system decouples work submission from work execution. A caller enqueues a job and receives an acknowledgment immediately; one or more workers pick up that job and execute it in the background. This pattern is fundamental to web services that need to offload slow operations — image processing, email delivery, report generation, third-party API calls — without blocking the request thread. The design must address queue selection, worker lifecycle, job state management, and operational visibility.

Requirements

Functional Requirements

Callers submit jobs with a type, payload, and optional scheduling parameters (delay, priority).
Workers claim jobs, execute them, and report outcomes back to the system.
Failed jobs are retried with configurable backoff up to a maximum attempt count.
Callers can query job status and retrieve results or error details.
Workers support graceful shutdown: finish the current job before exiting.

Non-Functional Requirements

At-least-once delivery: a job must never be silently dropped.
Job throughput must scale horizontally by adding workers without system changes.
Observability: p95 job latency, queue depth, and error rate must be monitorable.

Data Model

Job Record

job_id — UUID v4, primary key.
job_type — string identifier mapping to a registered handler.
payload — JSON blob, max 64 KB; large inputs stored in object storage with a reference.
status — ENUM: PENDING, RUNNING, COMPLETE, FAILED, DEAD.
attempt_count, max_attempts.
visible_at — future timestamp for delayed/retried jobs; workers only claim jobs where visible_at <= NOW().
locked_until — heartbeat-based visibility timeout; prevents two workers from executing the same job.
result, error_message, created_at, updated_at.

Queue Selection

The choice of backing queue depends on throughput, ordering, and durability requirements:

Database-backed queue (PostgreSQL SKIP LOCKED): simplest operational model, ACID durability, suits up to ~1000 jobs/sec. Use SELECT ... FOR UPDATE SKIP LOCKED to atomically claim jobs.
Redis (BRPOPLPUSH / streams): sub-millisecond enqueue latency, suitable for high-frequency lightweight jobs. Use Redis Streams with consumer groups for at-least-once delivery.
Message broker (SQS, RabbitMQ, Kafka): purpose-built for async messaging, provides visibility timeout, dead-letter queues, and fan-out natively. Best for cross-service job dispatch.

Worker Lifecycle

Startup

Register the worker instance (ID, hostname, supported job types) in a workers table. Begin polling the queue for claimable jobs matching registered types.

Job Claim

Atomically set job status to RUNNING, record worker_id, and set locked_until = NOW() + visibility_timeout. Only one worker can claim a given job due to the atomic update.

Heartbeat

While executing, the worker periodically extends locked_until. If the worker crashes, the lock expires and a supervisor or another worker can reclaim the job after the timeout elapses.

Completion

On success: set status to COMPLETE, write result. On retriable failure: increment attempt_count, set visible_at to NOW() + backoff, reset status to PENDING. On permanent failure (max attempts reached): set status to DEAD and write to dead-letter log.

Graceful Shutdown

On SIGTERM, stop claiming new jobs. Finish the current job. Deregister from the workers table. Exit cleanly. Kubernetes readiness probes should fail immediately on SIGTERM receipt to stop new traffic.

API Design

POST /jobs — enqueue a job; returns job_id and estimated start time.
GET /jobs/{id} — returns status, attempt_count, result or error.
DELETE /jobs/{id} — cancel a PENDING job.
GET /jobs?type=X&status=FAILED — filtered listing for operators.
POST /jobs/{id}/retry — manually re-enqueue a DEAD job.

Observability

Emit counters: jobs enqueued, jobs completed, jobs failed, jobs dead-lettered — labelled by job_type.
Emit histograms: job execution duration, queue wait time (time from PENDING to RUNNING).
Alert on queue depth exceeding a threshold for more than N minutes, indicating worker starvation.
Structured logs per job execution with job_id, type, attempt, duration, and outcome enable per-job audit trails.

Scalability Considerations

Scale workers horizontally; the queue acts as a natural load balancer. Use priority queues or separate queues per job type to prevent low-priority bulk jobs from starving latency-sensitive ones. Auto-scale worker pools based on queue depth metrics via KEDA or a similar operator. For very high throughput, shard the job table by job_type and use connection pooling (PgBouncer) to prevent database saturation from polling workers.

Summary

An async job system requires careful attention to atomicity in job claiming, heartbeat-based lock extension for crash recovery, and structured retry with backoff. Queue selection should match the operational complexity budget and throughput target. Rich observability — counters, histograms, and depth alerts — makes the system maintainable as job volume grows.