After crawling successfully and validating that documents were indexed by FAST, you were still unable to search from SharePoint, and receive this error:

The search request was unable to connect to the search service

In ULS we are getting

03-28-2011 11:25:05.71 w3wp.exe (0x0B90) 0x1D08 SharePoint Server Search Query g4hx Monitorable FASTSearchRuntime::SendRequest--Search Service Not Found Exception when trying to search FAST backend: Microsoft.Office.Server.Search.Query.SearchServiceNotFoundException: The search request was unable to connect to the Search Service. ---> System.ServiceModel.FaultException`1[Microsoft.Office.Server.Search.Administration.SearchServiceApplicationFault]: The creator of this fault did not specify a Reason. (Fault Detail is equal to Microsoft.Office.Server.Search.Administration.SearchServiceApplicationFault).     --- End of inner exception stack trace ---     at Microsoft.Office.Server.Search.Administration.SearchServiceApplicationProxy.ThrowBackwardCompatibleException(FaultException`1 ex)     at

Cause:

The processes responsible for Claims-based authorization in FAST were not able to initialize successfully, with the following seen in the authorization-admin log:

AdminLibrary.dll:MakeRemoteRequestToWorker - Unable to complete request to sam worker node

AdminLibrary.dll:Resuscitate - Unable to get LastCommittedTransactionId

Resolution:

These communication errors were causing by a missing SPN either for the user or the machine name, which we correct with setspn on the domain controller:

setspn -A HTTP/fastmachine DOMAIN\fastuser

setspn -A HTTP/fastmachine.domain.extension DOMAIN\fastuser