2.4.3.6. Error 503 Service Temporarily Unavailable

Every hosting account on the server is allocated a number of worker processes that process user requests. Requests receive on the server and become queued. Lite requests are processed quickly, while heavy and problematic requests are processed slowly, thus slowing down the progress of the queue. When the queue length reaches a certain value, the server stops accepting new requests and returns the error "503 Service Temporarily Unavailable".

The most common causes of the problem and possible solutions:

  • Many heavy or corrupted CMS components:
    • Check all components and plugins of your CMS, disabling them one by one, and find the most heavy or corrupted ones that slow down site loading. If possible, abandon such components or look for a faster analog. Also uninstall all unnecessary components that you do not use or the use of which is not necessary.
    • It is best to run the mailing script via cron at the time of lowest server load, e.g. at night. In this case, you should take into account the limitations set by services hosting rules regarding the number of emails per day and the PHP script run time.
  • Many slow queries to MySQL:
    • Install caching components in the engine that could reduce the number of SQL queries.
    • Optimize SQL queries.
    • Index the database tables by the columns used in the selection.
    • If nothing helps, it may be worth changing to a more optimal engine.
  • Large number of requests to the web server:
    • Downloaded resource is split/links to too many files (images, stylesheets, JS scripts) that are loaded through separate requests. Try to combine resources into a single file where possible.
    • The site has an element that periodically sends AJAX requests to the server (e.g. chat). The number of requests depends not only on the number of visitors, but also on their habit of opening several tabs in a browser.
    • Indexer bots that scan resources of sites (search engines, Sape and others).
    • Use of resource elements or scripts on other people's sites (links to images, informer scripts).
  • Lack of RAM:
    • You can track RAM usage on server resource consumption charts.
    • The error occurs when all the RAM of the hosting account is already used and the scripts try to allocate more, but can't.
    • Reasons may be insufficient optimization of site scripts, too many requests, etc.
  • Reaching the limit on the number of concurrent processes:
    • You can track the number of processes on server resource consumption charts.
    • The error occurs when there are too many processes running simultaneously in the hosting account.
    • The reasons may be suboptimal operation of the site itself, a large number of requests when scripts are not fast enough (for example, they do unnecessary work, process too much data, or in the process access third-party servers that cause delays), etc.

It can also be caused by insufficient memory allocated to the site for OPcache.

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