Apache: 503 Service Unavailable sometimes without any server load

I have an apache server v2. Sometimes I get 503 error without any server load at all this error appears randomly not at any specific time or when using specific services. How to find the cause or trace it? I've checked error logs and last modified date is yesterday though the error appeared today multiple times. Thanks and regards.

Solution 1:

If you have a dedicated VPS, these are some recommended troubleshooting steps:

1. Check your mysql logs to see if there are any slow running queries that are hogging resources; if you run a CMS- are there any particular tasks being executed by the site at the time of the 503 errors (backups, image or media compression, etc)? Also, you may want to truncate any log tables in the database. If the database is excessively large and being served on the webserver, then this could contribute to the 503 errors.
2. Check if you have any cron jobs that are occurring at the time of the 503 errors- including backup imaging, syncing of files between servers.
3. If your web application itself keeps logs, check for any php errors or warnings that have been repeatedly occurring and try to eliminate the most frequent ones to help streamline the logs moving forward. Sometimes old PHP applications may call variables/values without properly checking if the value is set or even exists.
4. If you run through the above steps and still encounter the issue, it may be worth monitoring CPU/RAM utilization on your server to further diagnose the issue (determine if its hardware specific or hosting provider maintenance specific).

If you have a shared VPS, then you will want to run through the suggestions in steps 1-3 above and possibly moving to a dedicated VPS or increasing your hosting package for more performance.