Queue depth and age monitoring

A queue that is silently backing up means recipients are waiting hours for time-sensitive transactional email — password resets, order confirmations, MFA codes. Without polling queue depth and job age on a schedule, a backlog caused by an ESP outage or a misconfigured concurrency setting is only discovered when users complain. ISO 25010 reliability requires that fault conditions are observable before they become user-facing failures. Job age is the earlier signal: a large depth with young jobs is a spike; a small depth with ancient jobs is a stuck worker.

Add a periodic health poller in your queue worker bootstrap file (e.g., src/workers/email.worker.ts or src/lib/queue/monitor.ts), running on a setInterval of no more than 5 minutes:

const counts = await queue.getJobCounts('wait', 'active', 'delayed')
const oldest = await queue.getJobs(['wait'], 0, 1, true) // oldest first
const ageMs = oldest[0] ? Date.now() - oldest[0].timestamp : 0
metrics.gauge('email_queue_depth', counts.wait + counts.delayed)
metrics.gauge('email_queue_oldest_job_age_seconds', Math.floor(ageMs / 1000))

Both depth and age are required — emitting only one of the two is not sufficient for this check to pass.