2.20.6. Error 503 Service Temporarily Unavailable

Each hosting account on the server is allocated a number of worker processes that process user requests. Requests go to the server and queue up. Light requests are processed quickly, and heavy problem requests are processed slowly, slowing down the progress of the queue. When the queue reaches a certain length, the server stops accepting new requests, returning the error “503 Service Temporarily Unavailable" (service is temporarily unavailable).

The most common causes of the problem and possible solutions:

• Large number of heavy or damaged components CMS:
  • Check all the components and plugins of your CMSdisabling them one by one, and find the most difficult or damaged ones, which slow down the loading of the site. If possible, discard such components or look for a faster analogue. Also uninstall any unnecessary components that you do not use or that are not required.
  • The best way to run the mailing list script is cron during the least load on the server, for example at night. In this case, one should take into account the restrictions established Terms of Service hosting regarding the number of letters per day and the time of the PHP script.
• Lots of slow MySQL queries:
  • 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 all else fails, it may be worth changing the engine to a more optimal one.
• Large number of requests to the web server:
  • The loaded resource is broken / contains links to too many files (images, style sheets, 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 (for example, a chat). The number of requests depends not only on the number of visitors, but also on their habit of opening multiple tabs in the browser.
  • Indexing bots that crawl website resources (search engines, Sape and others).
  • Using resource elements or scripts on other people's sites (links to pictures, informer scripts).

Also, the reason may be a lack of memory allocated to the site for OPcache.