Recently Udemy faces both errors of Bad gateway and Server Unavailable errors. If someone is not familiar with Udemy than here the meta about Udemy;
Udemy is an online learning platform aimed at professional adults and students, developed in May 2010. As of Jan 2020, the platform has more than 50 million students and 57,000 instructors teaching courses in over 65 languages. There have been over 295 million course enrollments. Wikipedia
Both the errors were 502 and 503 respectfully.
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 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.
Udemy team send notifications of new arrival courses and many others offer to their users also they must share about their server down and maintenance issues that user will manage their time vice versa.