Distributed Cron Jobs

Regular cron runs on one server. What happens when you have 10 servers? Each runs the job 10 times. That's a problem.

The Problem

Single server cron: ✅ Job runs once at 2am
10-server fleet:    ❌ Job runs 10 times at 2am — duplicate emails, double charges!

Solutions

1. Dedicated Scheduler Node

• One designated machine runs all cron jobs
• ✅ Simple
• ❌ Single point of failure — if this node dies, no cron jobs run

2. Leader Election + Cron

• Cluster elects a leader (using etcd/ZooKeeper)
• Only the leader runs cron jobs
• If leader dies → new leader elected → cron resumes
• ✅ No SPOF | ❌ Leader election complexity

3. Distributed Lock Before Job Execution

def run_job():
    lock = redis.set("cron:daily-email", "locked", nx=True, ex=300)  # 5min TTL
    if not lock:
        return  # another node is running it
    try:
        send_daily_emails()
    finally:
        redis.delete("cron:daily-email")

• ✅ Simple, works with existing Redis
• ❌ Lock expiry must be > job duration, or use heartbeat extension

4. Dedicated Scheduling Services

• AWS EventBridge Scheduler — cron without servers
• Temporal.io — workflow orchestration with scheduling
• Airflow — DAG-based job scheduling
• Sidekiq-Cron (Ruby), Bull (Node.js)

Exactly-Once Execution

For critical jobs (billing, emails):

1. Use idempotency keys in the job itself
2. Record execution in DB before running: INSERT INTO job_runs (job_id, run_date) ON CONFLICT DO NOTHING → if 0 rows inserted, skip
3. Use a distributed lock with heartbeat

Observability for Cron

• Alert if a job DIDN'T run (missed heartbeat)
• Track job duration — sudden increase = something is wrong
• Log job result (success/failure, records processed)
• Tool: Healthchecks.io — cron monitoring via ping