Will my WCF service be scaleable using a singleton?

My ASP .Net C# web application allows its users to send files from their account on my server to any remote server using FTP. I have implemented a WCF service to do this. The service instantiates a class for each user that spawns a worker thread which performs the FTP operations on the server. The client sends a command to the service, the service finds the worker thread assigned to the client and starts the FTP commands. The client then polls the service every two seconds to get the status of the FTP operation. When the client sends the "disconnect" command, the class and the worker thread doing the FTP operations is destroyed.

The FTP worker thread needed to persist between the client's queries because the FTP processing can take a long time. So, I needed a way for the client to always get the same instance of the FTP class between calls to the service. I implemented this service as a开发者_如何学C singleton, thus:

[ServiceBehavior(InstanceContextMode = InstanceContextMode.Single)]
public class UserFtpService : IUserFtpService
{
    private SortedDictionary<string, UserFTPConnection> _clients = new SortedDictionary<string, UserFTPConnection>();

    ...
}

Where "UserFTPConnection" is the class containing the worker thread and the user's account name is used for the indexing in the dictionary.

The question I have is this: In the books I have read about WCF, the singleton instance is called "the enemy of scalability." And I can see why this is so. Is there a better way to make sure the client gets the same instance of UserFTPConnection between queries to the WCF service other than using a singleton?

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Actually here your first problem is synchronizing the access to this static object. Dictionary<TKey, TValue> is not thread safe so you must ensure that only one thread is accessing it at the same time. So you should wrap every access to this dictionary in a lock, assuming of course you have methods that are writing and others that are reading. If you are only going to be reading you don't need to synchronize. As far as singleton being the enemy of scalability, that's really an exaggerated statement and pretty meaningless without a specific scenario. It would really depend on the exact scenario and implementation. In your example you've only shown a dictionary => so all we can say is that you need to ensure that no thread is reading from this dictionary while other is writing and that no thread is writing to this dictionary while other thread is reading.

For example in .NET 4.0 you could use the ConcurrentDictionary<TKey, TValue> class which is thread safe in situations like this.

One thing's for sure though: while the singleton pattern might or might not be an enemy of scalability depending on the specific implementation, the singleton pattern is the arch-enemy of unit testability in isolation.

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If you are going to use a singleton, I'd recommend also setting ConcurrencyMode to ConcurrencyMode.Multiple. For example...

[ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Multiple, InstanceContextMode = InstanceContextMode.Single)]
public class UserFtpService : IUserFtpService
{
}

If you don't do this, your WCF service will be a singleton but only allow one thread to access at a time, which would certainly effect performance. Of course you will need to ensure thread safety of collections (as in previously mentioned answer).