Error 503 is nearly always caused by the application that you installed on your instance, or its configuration.

To understand how this error occurs, and to fix it, it is necessary to understand how our PaaS infrastructure works (the following explanation is simplified for demonstration purposes).

Here is a diagram that represents our Simple Hosting offer in action:

First, there is a Varnish cache server, which exists (in part) in order to relieve the instances by keeping elements in its cache for a limited duration. When you request a page that is found on a Simple Hosting instance, it is the cache server that first receives the request, and in turn it transmits the request to the web server installed on the instance.

The web server that receives the request then searches for the content on the instance, and sends it to the cache server which in turn sends the visitor to the requested page. In the event of HTML files, the page is sent directly, since the content is not dynamic. If you use a language for generating your pages (ex. php, Python, node.js, etc.) then it is generated by the interpreter of the language before being transmitted to the web server, which in turn sends the request.

This is precisely when the 503 error occurs; it happens when not everything works correctly at this point (the cache server that sent the request to the instance is waiting for a response: the page that the visitor wanted to see. If the instance does not reply, for whatever reason, this will display the 503 error).

There are many reasons for this, and so it is necessary to analyze what is happening on the instance in order to understand what happened. Here are some of the most common reasons:

• The instance is too small: If the instance is too small with regards to its needs (ex. too many visitors, plugins that are very large or take up too many processes or resources).
• Timeouts: When a script takes longer than 180 seconds to execute, it stops. This is normal and allows avoiding the consumption of resources in the event that the script contains a bug.
• The instance's disk space is full: The data disk of the instance is 100% full. In this case, the services present on the instance will be unable to work correctly and the requests will not be able to be sent to the Varnish cache server.

These are the most common reasons that we are contacted by customers who encounter an Error 503, though the list is not exhaustive. The errors may originate from an incident on our end, a bug of the language interpreter, one of the plugins you installed on your CMS, etc.

Therefore, we recommend that you use the cache server made available to you in order to maximize the power of Simple Hosting. If you don't feel confident in configuring the cache system yourself, there are plugins that exist for most CMS that will assist you in its installation and configuration.