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.
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.
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N/A
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Too many users.
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ConnLimit
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The connection limit for the site or server as a whole has been reached.
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Application taken offline.
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AppOffline
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The application pool has been put into Rapid Fail Protection and has been disabled automatically.
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Application request queue full.
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QueueFull
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The application pools request queue has been exceeded and the request cannot be queued.
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Administrator has taken application offline.
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Disabled
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The administrator has stopped the application pool.
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Application automatically shut down due to administrator policy.
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AppShutdown
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The application pool has been shut down because it exceeded its CPU usage limit.
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Request timed out in app pool queue.
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AppPoolTimer
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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.
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So We have seen, by following these simple steps we can get rid of Error 503 Service Unavailable error.