Ticket queue design

Core shape

A ticket queue is a fixed-capacity FIFO with three priority lanes — high, normal, low — drained by a pool of workers. Each ticket carries a TTL; expired tickets are silently dropped on dequeue. A circuit breaker halts enqueue when the queue reaches 80% capacity, returning HTTP 503.

Data model

ticket {
  id:        uuid v7
  lane:      high | normal | low
  payload:   jsonb
  created:   timestamp
  expires:   timestamp
  attempts:  uint8 (max 3)
}

Worker loop

1. Poll high lane → normal → low (strict priority).
2. Skip expired tickets; increment dead-letter counter.
3. Process payload; on failure, increment attempts.
4. If attempts < 3, re-enqueue with exponential backoff.
5. If attempts ≥ 3, move to dead-letter store.

Circuit breaker

A sliding-window counter tracks enqueue rate. When the window exceeds 80% of capacity for 3 consecutive sampling intervals, the breaker opens. Enqueue returns 503 with a Retry-After header. The breaker half-opens after 30s, allowing one probe request. Success closes it; failure re-opens.

Storage backends

Observability

• queue.depth — gauge per lane
• queue.enqueue.rate — counter
• queue.expired — counter
• queue.dead_letter — counter
• circuit.breaker.state — gauge (0 closed, 1 open, 2 half-open)