The difference between a 502, 503 and 504 error

We all come across website errors at some point, and every website out there has inevitably experienced their share of 404 and 500 status codes, which are shown when a URL is not found either due to their typo or a broken link (404), and when there is a general server error (500).

There are many other less well known status codes in the 400 and 500 range: Those HTTP status codes beginning with 4 are errors on the client side, and codes beginning with 5 are errors on the side of the website server.

Some of the common 500 error messages that websites may encounter are 502, 503, and 504, which are all related to the website server being unable to fill a request that should be valid. These codes provide both the user and the website owner with additional information as to why a page is not loading. Websites can utilize logs and alerts to keep track of how often these errors occur and be notified when errors are occurring so that they can be resolved if needed.

502 Bad Gateway

A 502 error means that a website server that is serving as a reverse proxy for the website origin server (for example, a CDN PoP) did not receive a valid response from the origin server. This may be because the origin server is experiencing issues, there is an invalid or incorrect DNS name, or because a firewall on the origin server has blocked the reverse proxy server request.

This may also occur when requests at the origin server are taking several minutes to complete and a caching tool such as Varnish Cache has been instructed to timeout after a set number of seconds. Varnish Cache has a default timeout of 60 seconds, which section.io recommends keeping for security and alerting reasons.

503 Service Unavailable

The 503 service unavailable message means that the website origin server is not available and is usually a temporary state. This error could be triggered because something running on the website server side has crashed or your site is purposefully down for maintenance. This error is also commonly served when a site has more traffic than it can handle - a problem with poor website scalability. The famous Twitter “Fail Whale” which was often triggered in the early days of the social media site is one example of a 503 triggered due to excess traffic.

To avoid serving 503 errors due to an overload of visitors, we recommend using a Content Delivery Network and caching tool so that more of your requests can be served from the CDN and cache.

fail whale

504 Gateway Timeout

Similar to the 502 error, the 504 Gateway Timeout error occurs if the server that is acting as a proxy for the website origin server did not receive a response from the website origin server within a set time period. This may indicate an issue with the DNS host or hosting company, or with the connection or configuration between the reverse proxy servers and the website origin server.

Troubleshooting 500 errors

502 and 504 errors are related to a bad gateway, meaning that while the reverse proxy server is operational, something it needs to collect from the origin server is not working, or the connection between the reverse proxy server and the origin server is broken. To troubleshoot this issue, websites should check that their origin server and all the servers it needs to access are running correctly, and then check the configuration between the origin server and reverse proxy server (such as the CDN PoP or locally installed Varnish Cache server). You should also check that your firewall is not accidentally blocking legitimate traffic, your DNS name is set up correctly, and your origin hosting provider is not down.

503 errors are often expected when your website is going through downtime in order to make updates or changes. They may also be triggered by a large influx of traffic to your website. If your website does not scale well, we suggest exploring utilizing a CDN such as section.io’s and/or a caching reverse proxy such as Varnish Cache. To learn more please contact us or sign up for a trial account to explore our features free for 2 weeks.

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