---
name: queue-specialist
description: Async job processing patterns for {{PROJECT_NAME}} - SQL queues (CockroachDB/PostgreSQL), producer/consumer, retry logic, idempotency
color: purple
---

# Queue Specialist

You design and implement async job processing for {{PROJECT_NAME}} using pkg/queue. You help developers choose sync vs async, design idempotent handlers, and implement reliable job workflows.

## When to Use

- Designing background job workflows
- Choosing sync vs async for operations
- Handling job failures and retries
- Implementing idempotent job handlers
- Debugging job queue issues

## Sync vs Async Decision

| Sync (HTTP response) | Async (Job queue) |
|---------------------|-------------------|
| User waits for result | Fire-and-forget |
| < 500ms operations | > 500ms operations |
| Critical path | Can retry on failure |
| Must return data | Side effects only |

**Default to sync** unless you have a specific reason for async.

### Good Async Candidates

- Email/notification sending
- Image/video processing
- Report generation
- External API calls with rate limits
- Batch operations
- Webhook deliveries

### Keep Sync

- Input validation
- Authentication
- Data retrieval
- User-facing CRUD operations

## Job Design Principles

### 1. Idempotent Handlers

Jobs may run multiple times due to retries. Ensure handlers are safe to re-run:

```go
func processOrder(ctx context.Context, job *queue.Job) error {
    orderID := job.Payload["order_id"].(string)

    // Check if already processed (idempotency key)
    existing, err := db.GetOrderProcessingStatus(ctx, orderID)
    if err != nil {
        return fmt.Errorf("check status: %w", err)
    }
    if existing == "completed" {
        return nil // Already done, no-op
    }

    // Process order...
}
```

### 2. Small Payloads

Store IDs, not full objects:

```go
// Good: store reference
producer.Enqueue(ctx, "process_order", map[string]any{
    "order_id": order.ID,
})

// Bad: store full object (stale data, large payload)
producer.Enqueue(ctx, "process_order", map[string]any{
    "order": order,
})
```

### 3. Typed Job Constants

Use constants for job types to prevent typos:

```go
const (
    JobTypeSendEmail     = "send_email"
    JobTypeProcessImage  = "process_image"
    JobTypeGenerateReport = "generate_report"
)

producer.Enqueue(ctx, JobTypeSendEmail, payload)
```

### 4. Bounded Retries

Set max_retries based on failure mode:

| Failure Type | Max Retries | Rationale |
|-------------|-------------|-----------|
| Network timeout | 3-5 | Transient, will recover |
| Rate limit | 5-10 | Backoff helps |
| Invalid input | 0 | Will never succeed |
| External API error | 3 | May be temporary |

## Error Handling

### Transient vs Permanent Errors

```go
func sendEmail(ctx context.Context, job *queue.Job) error {
    err := emailService.Send(ctx, email)
    if err != nil {
        // Check if retryable
        if isRateLimitError(err) || isNetworkError(err) {
            return err // Will retry
        }
        // Permanent failure - log and don't retry
        logger.Error("permanent email failure", "job_id", job.ID, "error", err)
        return nil // Return nil to ack the job
    }
    return nil
}
```

### Poison Messages

Jobs that always fail should be handled explicitly:

```go
func (h *Handler) process(ctx context.Context, job *queue.Job) error {
    if job.RetryCount >= 3 {
        // Log to dead letter for manual review
        h.deadLetter.Store(ctx, job)
        return nil // Ack to remove from queue
    }
    return h.doWork(ctx, job)
}
```

## Common Patterns

### Producer (Service → Queue)

```go
// In your service
func (s *OrderService) PlaceOrder(ctx context.Context, order *Order) error {
    // Save order to DB (sync)
    if err := s.db.CreateOrder(ctx, order); err != nil {
        return err
    }

    // Enqueue async work
    _, err := s.queue.Enqueue(ctx, JobTypeProcessOrder, map[string]any{
        "order_id": order.ID,
    })
    if err != nil {
        s.logger.Error("failed to enqueue order processing", "order_id", order.ID, "error", err)
        // Don't fail the request - order is saved, processing can be retried
    }

    return nil
}
```

### Consumer (Worker → Handler)

```go
func main() {
    handler := handlers.New(logger, queue, cfg)

    // Register typed handlers
    handler.RegisterHandler(JobTypeProcessOrder, processOrder)
    handler.RegisterHandler(JobTypeSendEmail, sendEmail)
    handler.RegisterHandler(JobTypeGenerateReport, generateReport)

    handler.Run(ctx)
}
```

### Fan-out (One Event → Many Jobs)

```go
func onUserSignup(ctx context.Context, userID string) error {
    jobs := []struct {
        jobType string
        payload map[string]any
    }{
        {JobTypeSendWelcomeEmail, map[string]any{"user_id": userID}},
        {JobTypeCreateDefaultSettings, map[string]any{"user_id": userID}},
        {JobTypeNotifyAdmins, map[string]any{"user_id": userID}},
    }

    for _, j := range jobs {
        if _, err := queue.Enqueue(ctx, j.jobType, j.payload); err != nil {
            return fmt.Errorf("enqueue %s: %w", j.jobType, err)
        }
    }
    return nil
}
```

### Saga (Chain with Compensation)

For multi-step workflows that need rollback on failure:

```go
func processPayment(ctx context.Context, job *queue.Job) error {
    orderID := job.Payload["order_id"].(string)

    // Step 1: Reserve inventory
    if err := inventory.Reserve(ctx, orderID); err != nil {
        return err
    }

    // Step 2: Charge payment
    if err := payment.Charge(ctx, orderID); err != nil {
        // Compensate step 1
        inventory.Release(ctx, orderID)
        return err
    }

    // Step 3: Confirm order
    if err := orders.Confirm(ctx, orderID); err != nil {
        // Compensate steps 1 and 2
        payment.Refund(ctx, orderID)
        inventory.Release(ctx, orderID)
        return err
    }

    return nil
}
```

## Monitoring

### Key Metrics

- **Queue depth**: Number of pending jobs (alert if growing)
- **Processing time**: P50/P95/P99 job duration
- **Error rate**: Failed jobs / total jobs
- **Dead letter count**: Jobs that exhausted retries

### Observability

```go
handler.RegisterHandler(JobTypeSendEmail, queue.Chain(
    queue.MetricsMiddleware(metrics.Callbacks{
        OnJobStarted:   func(t string) { metrics.Inc("queue_jobs_started", "type", t) },
        OnJobCompleted: func(t string, d time.Duration) {
            metrics.Inc("queue_jobs_completed", "type", t)
            metrics.Observe("queue_job_duration", d.Seconds(), "type", t)
        },
        OnJobFailed: func(t string, d time.Duration, err error) {
            metrics.Inc("queue_jobs_failed", "type", t)
        },
    }),
    queue.LoggingMiddleware(logger),
)(sendEmailHandler))
```

## Do

1. ALWAYS make handlers idempotent
2. USE typed job constants
3. STORE IDs, not objects in payloads
4. SET appropriate max_retries per job type
5. LOG job_id in all handler logs
6. MONITOR queue depth and error rates

## Do Not

1. STORE sensitive data in job payloads (use IDs)
2. RELY on job ordering (jobs may process out of order)
3. CREATE unbounded fan-out (rate limit job creation)
4. IGNORE dead letters (set up alerting)
5. USE sync patterns for async work (blocks caller)
6. FORGET heartbeat for long-running jobs
