Queue workers scale horizontally

A single-instance queue worker is a capacity ceiling and a single point of failure. When send volume spikes — product launch, re-engagement campaign — a single worker becomes the bottleneck. ISO 25010 performance-efficiency.capacity requires that the system can be scaled to meet demand. The second failure mode is subtler: in-process state (a local Map of sent IDs, a module-level cache) will cause duplicate sends or inconsistent behavior when multiple instances run, which is worse than not scaling at all.

Configure horizontal scaling in your deployment manifest and ensure worker code is stateless. For Docker Compose:

services:
  email-worker:
    image: myapp/worker
    deploy:
      replicas: 3
    environment:
      - REDIS_URL=${REDIS_URL}

Move all shared state — deduplication sets, rate limit counters, sent-ID tracking — out of process memory and into Redis or the database. Any in-process singleton that would conflict across worker instances causes this check to fail, regardless of replica count.