Looking at some explanations, the most common reasons for 503 errors are technical issues, server maintenance, or even Denial of Service attacks. Basically, the web browser is sending a request to the server, but the request isn't reaching the server for one reason or another.
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When met with an HTTP error 503, the first question is usually “why?”, coupled with the hope that determining the cause of the issue will help us solve it quickly and painlessly. Unfortunately, this is not always the case.
Similar to other HTTP errors, determining the root cause of the Error 503 is much harder than one would expect – especially without technical IT expertise. However, there are a few possible scenarios that can be considered as plausible causes for an HTTP error 503.
In the majority of cases, the 503 is triggered when the website in question is no longer able to connect with its supported server, meaning that any information requested or issued by your browser is simply hitting a wall. This usually happens when the server experiences a technical issue, is undergoing maintenance works, or is facing some sort of malicious disruption, such as a denial of service (DDoS) attack. Regardless of the cause, this information will likely not be readily available.
Frequent technical issues, such as those occurring multiple times a month, can be especially disruptive and excessive downtime can cause serious financial difficulty – especially if it heavily depends on online traffic or orders made through e-commerce. Perhaps the best example of this is Amazon's Prime Day disruption in 2018, which should serve as a blueprint for businesses in how not to handle an outage.
Hence, if your website is displaying an HTTP error 503 more often than its own home page, it might be worth switching hosting providers.
Although an HTTP error 503 can diminish the number of customers visiting the site, it can also be caused by an inundation of visitors, such as during seasonal sales. In these cases, the server remains connected, yet is incapable of supporting the avalanche of requests from many different users.
Alternatively, this can also be caused by malicious traffic instead of eager customers, such as in the case of distributed denial of service (DDoS) attacks. Misconfigured web apps may also cause a 503 error to appear, such as a plugin conflict caused by WordPress, while regular 503 errors could suggest an issue with the domain name system (DNS), whether that's an incorrect server configuration or an issue with the DNS server itself.
However, if the 503 error is a result of a problem found on the server's side, then unfortunately there isn't a lot you can do yourself to remedy it. This is where the IT administrator for the site should troubleshoot the issue to find a solution to the fault that users are reporting if they are encountering HTTP 503 errors. If you find yourself in that position and discover that updates need to be applied to a site, it's recommended to schedule them when your site's traffic is likely to be lowest, so your users don't regularly come across errors.
Alternatively, if recurring HTTP 503 errors are regularly caused by traffic spikes, it's best to use this as a sign that you might want to increase your web server resources investment. In addition to this, a surge of traffic could be the result of a denial of service (DoS) attack, in which case it might be a clever idea to approach your hosting provider to ask about the possible mitigations they can offer you to prevent attacks in the future.
Further investing in security protections or increasing the frequency of patch management could also serve to prevent any subsequent incidents from occurring. Several providers already include DDoS protection as part of their default packages, which may restrict the number of users that are allowed to access a site at any one time.
Finally, should the HTTP 503 error be a result of a programming bug, you'll need to undergo further investigation to pinpoint the issue and rectify it permanently.