Error 503 Service Unavailable

A lot of our applications were hosted on IIS6 Server and as a part of upgrading all our applications we decided to migrate all the existing applications which are running on IIS6 should be migrated to IIS7 or later versions. We started doing the stuff from the scratch and things started going ahead pretty nicely. But then later on post migrating our applications from IIS6 to IIS7 we started getting the error "Error 503 Service Unavailable" upon opening the website. Post some research we found that this happens due to a setting related to the application pool identity It happens due to Start Automatically property set to false.

Sometimes, this just happens when rapid fail back is enabled in the application pool. This feature of IIS 6.0 is available only when IIS is running in worker process isolation mode.When an application pool detects that too many worker processes assigned to it have become unhealthy in a given period of time, rapid-fail protection is initiated. The communication channel with the World Wide Web Publishing Service (WWW service) is dropped. The WWW service detects the loss of communication and initiates the appropriate actions, typically sending an error or warning to the event log and then restarting the worker process. The error can be suppressed by disabling the same.

HTTP 503 errors are returned more often in IIS 6.0 than in earlier versions of IIS. Typically, the HTTP 503-Service Unavailable error is returned directly from HTTP.sys when there is a problem with getting the request up to the worker process. However, this error can occur for a variety of reasons, as listed in Table 11.11   HTTP 503 Errors from HTTP.sys.

HTTP 503 Errors from HTTP.sys
Reason String	Text in HTTP.sys Error Log	Cause(s) of Error
Service unavailable.	N/A	• IIS cannot start any new worker processes because of limited system resources or because starting a new worker process would exceed the DemandStartThresholdproperty. • Bandwidth throttling is enabled, but the filter addition fails. • The control channel or internal configuration group for the URL is inactive. • The send for a request that was serviced from the cache failed (typically under low memory conditions).
Too many users.	ConnLimit	The connection limit for the site or server as a whole has been reached.
Application taken offline.	AppOffline	The application pool has been put into Rapid Fail Protection and has been disabled automatically.
Application request queue full.	QueueFull	The application pools request queue has been exceeded and the request cannot be queued.
Administrator has taken application offline.	Disabled	The administrator has stopped the application pool.
Application automatically shut down due to administrator policy.	AppShutdown	The application pool has been shut down because it exceeded its CPU usage limit.
Request timed out in app pool queue.	AppPoolTimer	The application pool is too busy to handle the request. The request has, therefore, timed out in the queue and has been returned with a 503 error.

So We have seen, by following these simple steps we can get rid of Error 503 Service Unavailable error.

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