Win2k8R2 / IIS 7.5 - users getting 503 response, no 503 error reported in logs

Solution 1:

You've mentioned "IIS Log File" (singular), but there are always two logs you need to evaluate:

• W3SVCnn logs (C:\Inetpub\Logs) from the website worker process, and
• HTTPERR logs (C:\Windows\System32\Logfiles\HTTPERR) from HTTP.SYS, which routes requests to worker processes and provides a kernel-mode queue designed to buffer clients from worker process changes (like recycling)

503s are far more likely to appear in HTTPERR logs, along with a reason for failure, because they're more likely to reflect a failure of HTTP.SYS to communicate with the worker process (or a queue overflow, which amounts to a similar thing).

See also http://support.microsoft.com/kb/820729 - not sure why there's a "Fix it" when the article describes what the logging does and (at the bottom) the reasons it might log for a failure.

A bit more

There are two queues in most app frameworks - the HTTP.SYS request queue (in kernel mode) and a user-mode request queue. If a user-mode framework (like ASP.Net) queues requests internally, then the failure of a worker process will result in a 503 (or at best a 500) for all requests queued within it, causing HTTP.SYS to regard those requests as abandoned and unsalvageable.

If your app is failing with unhandled exceptions, you need to fix that - there's nothing the IIS architecture can do to insulate {batches of in-flight requests} from {the app lunching itself} and you will get some form of error communicated to the client - Recycling ensures there's a new worker process ready to serve the second try.