What possibilities can cause Service Unavailable 503 error

Question

we have a asp net MVC application deployed to a server  and but when there is too many request to the server  the client will just get a  503 service unavailable  error  But if I deployed the application to another server   lower hardware configuration    everything worked fine  even more requests it can handle well  My question is what possible configuration can cause the previous server to just throw out a 503 error    which means the requests didn t reach our application

User · Answer

We recently encountered this error  root cause turned out to be an expired SSL cert on the IIS server   The Load Balancer  infront of our web tier  found the SSL expired  and instead of handling the HTTP traffic over to one of the IIS servers  started showing this error   So basically IIS unable to server requests  for a totally different reason

User · Answer

Primarily what that means is that there are too many concurrent requests and further that they exceed the default 1000 queued requests  That is there are 1000 or more queued requests to your website   This could happen  assuming there are no faults in your app  if there are long running tasks and as a result the Request queue is backed up   Depending on how the application pool has been set up you may see this kind of thing  Typically  the app pool s Process Model has an item called Maximum Worker Processes  By default this is 1  If you set it to more than 1  typically up to a max of the number of cores on the hardware  you may not see this happen   Just to note that unless the site is extremely busy you should not see this  If you do  it s really pointing to long running tasks

User · Answer

Your web pages are served by an application pool  If you disable stop the application pool  and anyone tries to browse the application  you will get a Service Unavailable  It can happen due to multiple reasons      Your application may have crashed  check the event viewer and see if you can find event logs in your Application System log  Your application may be crashing very frequently  If an app pool crashes for 5 times in 5 minutes  check your application pool settings for rapid fail   your application pool is disabled by IIS and you will end up getting this message    In either case  the issue is that your worker process is failing and you should troubleshoot it from crash perspective      What is a Crash  technically     in ASP NET and what to do if it happens

User · Answer

If the server doesn t have enough memory also will cause this problem  This is my personal experience with Godaddy VPS

[asp.net-mvc] What possibilities can cause "Service Unavailable 503" error?

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