The 503 error often appears intermittently. If you want to monitor the website and receive information about service availability by email or through other channels, consider using the add-on service EWM ⧉.
If you are dealing with frequent occurrences of Error 503 on a hosting with WordPress, also visit the article Webhosting – WordPress.
In this article you will learn:
Error 503: Service temporarily unavailable
You can recognize Error 503 by the large Service Temporarily Unavailable message 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 may encounter Error 503 Service Temporarily Unavailable on any website that uses PHP or similar technologies to function. The server is running at full speed, yet it still does not respond to new requests. The problem is usually caused by reaching the limits of certain web hosting parameters.
Resolving Error 503
Every Webhosting NoLimit ⧉ has up to 25 PHP processes reserved on our server (7 for LowCost ⧉, 30 for Extra ⧉) that can run at the same time. This means that at any given moment, the processor can handle at most the specified number of PHP requests. Requests that the processor cannot process in time because they take too long are placed in a queue and wait until they are processed. If they do not get a free PHP process within the given time (called a timeout), they return Error 503.
Under normal circumstances, 25 PHP processes are enough – if your pages are properly optimized, you will not encounter this problem.
Error 503 is most often caused by:
- an ongoing attack on the webserver,
- poor optimization of the website or database,
- insufficient service performance,
- a stuck PHP process.
Attack on the webserver
A common cause of Error 503 are attacks, or other forms of flooding your website with requests that exhaust processes and block real users from accessing the website. In this situation, the error usually appears intermittently and lasts from minutes to tens of minutes.
You can detect this problem by enabling logging according to the guide Webhosting – Logging, for both Accesslog and Errorlog.
- In Accesslog you will find a list of visits to your website, including specific addresses. Nonsensical addresses or parameters (in the URL after the ? character) are a common sign of an attack.
- In Errorlog you will find the times when Error 503 occurred. If these times match spikes in Accesslog traffic caused by suspicious activity, the error was most likely caused by an attack.
WEDOS offers effective protection against attacks – the service WEDOS Protection ⧉. If you observe frequent occurrences of Error 503 due to attacks, consider deploying this protection.
Website and database optimization
If the website is not optimized, even normal traffic or inefficient work with external resources or the database can often lead to overload. The error appears intermittently, either during traffic peaks or during automatic maintenance; it lasts from minutes to tens of minutes.
Common causes of Error 503 and their solutions include:
- Incorrectly optimized PHP application: The application does not send its requests to the processor efficiently. If you use a content management system, find out whether it supports so-called caching. If so, enable it. This significantly shortens script execution time, reduces the number of SQL queries to the database, and speeds up your pages. More information can be found in the article Webhosting – Cache and proxy.
- Too frequent or too long background processes: Various maintenance scripts, especially those that upload something somewhere or download something from somewhere, can easily occupy a large number of processes for a long time, leaving none available for regular users. For regular maintenance processes, we recommend setting them up in Webhosting CRON. Limit backups and other process-intensive tasks to off-peak times.
- Large number of records in the database: The application records visitor accesses or similarly extensive data files in the database. Consider which data you really need, and adapt either the application or the hosting accordingly.
- Unsuitable database configuration: The application has improperly configured database tables. PHP then waits a very long time for the results of SQL queries.
- PHP application connects to an unavailable external resource: The application downloads external data, but it is not possible to connect to the target server, it does not respond, or it is overloaded. If you encounter this problem often, consider changing the external data source.
- A script called by cron runs too long: If you have a large number of crons, or if crons run for a long time, they can block the PHP processes available for your website. Think about which crons the application actually needs, how often, and how you can optimize them.
- Downloading files via a PHP script: The application facilitates downloading large files using a PHP script, thus occupying a PHP process for the entire duration of the download. It is therefore better to solve file downloads by linking or redirecting directly to the file URL.
- Large number of files in an unsuitable directory structure: The application contains more than thousands of items in one directory. Organize the files into suitable directories.
So Error 503 may not be related at all to current traffic or changes on the website. You will often encounter loading data from a slow or unavailable external source, or a cron job that spends a long time loading something, updating, performing maintenance, and so on. A larger number of complex requests may also occur by chance, usually searches in extensive databases.
Increasing service performance
If you cannot find a way to optimize the website, you can adapt the service performance to its demands. Consider these options:
- Changing the Webhosting plan: You can upgrade the Webhosting plan from LowCost to NoLimit, or from NoLimit to Extra. You can find a comparison of plan performance at this link ⧉. Instructions for changing the plan can be found in the article Services – Change of plan or billing period.
- Changing the service: If your application needs more power than shared web hosting can provide, but you do not want to lose administration in the customer portal, consider moving the application to the WMS ⧉ service.
- Separating aliases and subdomains. Aliases and subdomains of Webhosting share service performance. If traffic on one or more aliases or subdomains causes Error 503, consider setting up a separate hosting for the given domain or subdomain.
Stuck PHP process
Rarely, Error 503 can also be caused by a stuck PHP process. If you observe Error 503 continuously for several tens of minutes to hours and even clearing the cache does not help (for example with the keyboard shortcut Ctrl+Shift+R), restart PHP on your hosting.
During PHP version changes, PHP errors may occur due to website incompatibility with the temporarily selected PHP version.
Proceed with these steps:
- Log in to the customer administration ⧉.
- In the top menu, select Hosting Services Webhosting.
- Select the webhosting on which you want to restart PHP.
- In the left menu, select PHP Configuration.
- Change the PHP version to another one, preferably as close as possible (e.g. PHP 7.4 7.3, or 8.0 8.1)
- Click the Edit button.
- Wait half an hour for the PHP version change to take effect. Then, in the same way, switch back to the original version.
If error 503 persists even after such a restart, it is probably caused by something other than a stuck process.
Frequently asked questions
Is Error 503 specific to WEDOS servers?
You can encounter error 503 on any web server. When looking for suitable hosting for your application, first familiarize yourself with its requirements and then adapt your request accordingly. In addition to available space for files and the database, pay attention to the RAM and CPU parameters. Keep in mind that performance goes hand in hand with the price of the service.
Does 503 mean I am being blocked by WEDOS.protection?
No. The protection displays a fallback page when the website does not respond in time. If you are unsure, write to us ⧉ with the date, time, and Req-ID from the error page. We will look up the specific request and the reason for its failure.
Will increasing the plan fix the error?
Increasing the plan will help partly, because a higher plan has more PHP processes. However, it will not eliminate the cause – if one task runs for thirty seconds, it occupies a process even on a higher plan, and if several such tasks occur together, there will be no processes left for normal operation. We recommend doing both.
How should I resolve Error 503 on WordPress?
With WordPress, it is often about overly demanding background processes. More information and the resolution procedure can be found in the article Webhosting – WordPress.
I shut down the entire website, but I still see Error 503, what is the reason?
Try clearing the browser cache (for example with the keyboard shortcut Ctrl+Shift+R). If that does not help, try restarting PHP according to the procedure in the chapter Stuck PHP process.