Cron job failures create chaos for your users and your team. These 4 problems emerged as the most likely causes of preventable job failures:
1. System resources have been depleted
Without careful cleanup and log rotation, system resources like disk space can be consumed slowly over months or years before leading to job failures. By carefully cleaning up any files your cron job creates and using a tool like logrotate to automatically prune log files you can prevent disk space related failures.
Other resource problems, especially when running cron jobs in a virtual machine, are limits to the number of open file descriptors, threads, and memory usage. You can view limits on your system by running ulimit -a as the cron user.
2. Recent infrastructure updates were made
When developers are making changes to database connections or upstream APIs it's easy to overlook cron jobs, especially when they're running on a separate host. It can help if cron jobs are deployed like the rest of your app and are included in post-release verification steps.
3. Your dataset has grown too large
Cron jobs are often used for batch processing, event sourcing and other data intensive tasks that can reveal the constraints of your stack. Jobs may work fine until your data size grows to a point where a bottleneck is reached.
Optimizing database queries and API calls is the right place to start, and depending on the job, running it more frequently may be effective by reducing the data processed each time it runs.
4. Cron job invocations begin to overlap
Cron will run your command at the scheduled time even if the last invocation is still running, or even if the last 10 are. If your job invocations are close to overlapping, consider spacing them out further or adding a tool like flock to ensure only 1 instance is running.
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