Exceptions to be Thrown by BlobStore Implementations

I recently got a question on what Exceptions a BlobStore implementation should throw and this blog post answers that question. BlobStore implementations should throw either BlobStoreException or other exceptions.

BlobStoreException is meant to be used for conditions that RBS client library expects and knows how to handle (either now or in the future). For this reason, there is a defined list of cases where BlobStoreException should be thrown. This is the list of values of the BlobStoreExceptionCode enum. If a condition falls in this list, then a BlobStoreException must be thrown. If a condition does not fall into this list, then a different exception must be thrown.

So the next question is: which exception should a BlobStore implementation throw? .NET Framework guidelines say that as far as possible, we should throw system exceptions. This helps with easier handling of the error, possibly by higher level applications or by admins. In the case of RBS, the client library wraps any non-BlobStoreException Exceptions in another exception (RemoteBlobStoreException with ExceptionCode = BlobStoreUnhandledException) and sets the InnerException to the original exception thrown by the BlobStore. So applications will need to catch and handle this new exception. If LogLevel is configured to Error or higher, the log will also contain details of the exception as well.

In general, it would be very rare to have a condition not covered by the list in BlobStoreExceptionCode. One example where it is not covered is present in the sample provider. The sample FileStoreLibrary throws non-BlobStoreExceptions such as ArgumentNullException. In this case, if this exception gets thrown, it indicates a bug in RBS client code, and it should not be hidden by a BlobStoreException.

If other components/dlls called by a blob store provider can throw exceptions such as OutOfMemoryException, the BlobStore implementation should be careful about letting it bubble up directly. Often times the correct thing to do is to throw a BlobStoreException with BlobStoreExceptionCode set to OperationFailedAuthoritative. Remember, the list of conditions indicated by BlobStoreExceptionCode are the ones that RBS client library knows how to handle, and it always takes precedence.

- Pradeep.