When does HTTP Error 503 (Service Unavailable) occur?
An HTTP 503 error always occurs when a server can’t deliver the requested resources at the time the client requests. Following are roughly three possible reasons for this 503 error:
1. The server is being subjected to maintenance, such as bringing in updates, securing databases, or creating backups and is, therefore, the server would not connected to the internet during these processes.
2. The server is overloaded, meaning that is it receiving more requests than it can handle. This is why it responds with the error message. There are many reasons for an overload to occur: often an unexpected increase in traffic is the cause, but also when a web project continues to grow it can cause an overload if resources aren’t upgraded at the same time. Other possible reasons are malware/spam attacks as well as web applications or the content management system being incorrectly programmed. If your site gets error on shared hosting, then you can think of upgrading to VSP servers
3. In rare cases, an incorrect DNS server configuration on the client side (computer or router) may result in an HTTP 503 error message. The selected DNS server itself might temporarily have problems, which then results in the HTTP request showing a ‘Service Unavailable’ message.