[haertig]

All 5xx errors, including your 504 Gateway Timeout error, are server errors. You and your web browser are the client - there is nothing you can do on your end to fix a 504 error. It must be fixed on the website that you are trying to access.

Where I have seen these errors is when you have a server that is trying to do something on your behalf. It is basically a middleman. Also known as a "gateway".

Since you said this was from Overdrive, I expect that you were trying to go to a specific library, e.g. "my_library.overdrive.com". So your web browser accessed the main overdrive.com webserver, that was then acting on your behalf to contact the my_library webserver. overdrive.com is only going to wait so long for my_library to respond, and if it doesn't get a response it will throw a 504 error. If overdrive.com knew that my_library was down, most likely you'd get a 502 Bad Gateway error. But if overdrive.com didn't know that my_library was down (or very slow to respond), it would send its request out to my_library. Receiving no response in a timely manner, overdrive.com would eventually timeout and give you the 504 error (the timeout length is configured on overdrive.com)

Many, but not all, 504 errors are transient. Because they can be caused if my_library is overloaded. Wait a few moments, try again, and it will succeed (after the load on my_library has dropped). But some 504's can last a long time - say, my_library's response depends on it being able to query its database, and that database is down.

Since you said that multiple libraries were giving you the 504, it is entirely possible that overdrive.com contacts my_library_1 and my_library_2, but then these my_libraries go back and contact overdrive.com as part of their algorithm to fulfill your request. So in that case the problem is actually on overdrive.com, it spreads its problem to the my_libraries, then it sees the reflection of its own problem when it goes to contact the my_libraries.

So basically, you don't know anything about what caused this (unless you are an engineer at overdrive.com with access to their logfiles). The best you can do is "try again later and hope it works".

overdrive.com is no doubt large enough that they have automated server monitoring that contacts appropriate tech and engineering staff when errors like these pop up (this server monitoring should be looking at logfiles, constantly searching for errors). If they don't have that type of monitoring in place, they are terribly remiss. This all used to be what I did before retiring. Network management, network/server security, and network/sever/application automated monitoring and problem resolution.