Webhosting - Error 503 Service Temporarily Unavailable

In this article you will read:

• How to recognize Error 503
• How to solve Error 503
• Frequently Asked Questions

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Error 503: Service temporarily unavailable

You can tell a 503 error by the large Service Temporarily Unavailable sign at the top of the page. Under the heading you will find an English description of the error and a link to this article.

You can encounter Error 503 Service Temporarily Unavailable on any website that uses PHP or similar technologies. The server is running at full speed, but still does not respond to new requests. The cause of the problem is usually that the limits of some web hosting parameters have been reached.

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Solution Error 503

Each Webhosting NoLimit ⧉ has up to 25 PHP processes reserved on our server (7 for LowCost ⧉, 30 for Extra &box;) that can run simultaneously. This means that the processor can process at most a given number of PHP requests at one time. Requests that the processor cannot handle due to time constraints are queued and wait to be processed. If a free PHP process does not arrive within a given time (called a timeout), it returns Error 503.

Normally, 25 PHP processes are enough - if you have a properly optimized site, you won't run into this problem. 

Error 503 most often causes:

• ongoing attack on the webserver,
• poor site or database optimization,
• inadequate performance of the service,
• stuck PHP process.

Attack on webserver

A common cause of Error 503 is attacks or other forms of overwhelming your site with requests that exhaust processes and block real users from accessing the site. In this situation, the error usually occurs suddenly, lasting units to tens of minutes.

You can catch this problem by deploying logging according to the Webhosting - Logging instructions, both Accesslog and Errorlog.

• In Accesslog you will find a list of accesses to your website, including specific addresses. Nonsensical addresses or parameters (in the address after the ? character) are a common sign of an attack.
• In the Errorlog you can find the times during which Error 503 occurred. If these times coincide with Accesslog access spikes caused by suspicious activity, the error was probably caused by an attack.

Web and database optimization

If the website is not optimized, even normal traffic or inefficient work with external resources or databases can often lead to overload. Errors occur suddenly, either during peak traffic times or during automated maintenance; they last for units to tens of minutes.

Common causes of Error 503 and their solutions include:

• Improperly optimized PHP application: the application does not send its requests to the processor efficiently. If you are using a content management system, see if it allows caching. If so, turn it on. This will significantly reduce script execution time, reduce the number of SQL queries to the database and speed up your site. For more information, see the article Web hosting - Cache and proxy.
• Large number of records in the database: the application records visitor accesses or similarly large data sets in the database. Consider which data you really need and adjust either the application or the hosting to suit.
• Inappropriate database configuration: The application has improperly configureddatabase tables. PHP then waits a very long time for the results of SQL queries.
• The PHP application connects to an unavailable external resource: The application is downloading external data, but the target server cannot be reached, is not responding, or is overloaded. If you encounter this problem frequently, consider changing the source of the external data.
• The script called by cron is running too long: If you have a large number of crons, or if the crons run for a long time, they can block PHP processes available to your site. Think about which crons your application actually needs, how often, and how you can optimize them.
• File downloads mediated by a PHP script: The application mediates the download of large files using a PHP script, thus occupying the PHP process throughout the download. Therefore, it is better to handle file downloads by linking or redirecting directly to the URL of the file.
• A large number of files in an inappropriate directory structure: The application contains more than an order of thousands of items in a single directory. Sort the files into appropriate directories.

Error 503 may not be related to the current traffic or changes on the site at all. Often you will encounter loading some data from a slow or unavailable external source, or running a cron that takes a long time to load something, update, perform maintenance, etc. Also, a large number of complex requests may randomly come together, usually searches of large databases.

Increase in service performance

If you can't find a way to optimize the site, you can adjust the performance of the service to its requirements. Consider the following options:

• Change of Webhosting tariff: You can increase the Webhosting tariff from LowCost to NoLimit or from NoLimit to Extra. You can find a comparison of the performance of the tariffs at this link ⧉. For instructions on how to change your tariff, please see the article Webhosting - Change tariff.
• Change of service: if your application is not satisfied with the performance of shared web hosting, but you do not want to lose the administration in the customer administration, consider moving your application to the WMS ⧉ service.
• Separation of aliases and subdomains. Aliases and subdomains of a web host share the performance of the service. If traffic on one or more aliases or subdomains is causing Error 503, consider setting up separate hosting for that domain or subdomain.

Stuck PHP process

Rarely, Error 503 can also be caused by a PHP process getting stuck. If you observe Error 503 consistently for several tens of minutes or hours and clearing the cache (for example, using the Ctrl+Shift+R keyboard shortcut) does not help, restart PHP on your hosting.

Follow these steps:

1. Log in to the customer administration ⧉.
2. From the top menu, select Hosting Services Webhosting.
3. Select the web host where you want to restart PHP.
4. Select PHP Configuration from the left menu.
5. Change to another version of PHP, preferably as close as possible (e.g. PHP 7.4 7.3, or 8.0 8.1)
6. Click the Edit button.
7. Wait half an hour for the PHP version change to take place. Then revert the original version in the same way.

If error 503 persists after such a restart, it is probably caused by something other than a stuck process.

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Frequently Asked Questions