CODEWITHSUNDEEP
c#wcfservice
spyter
spyter

Reputation: 727

Does a self hosted wcf service have state?

I have a self hosted WCF service that I am using in a silverlight application. I am trying to store a list of user guids in an IDictionary object. Each time a user hits the service, it updates the users datetime so I can keep track of which users have active "sessions". The problem is, every time I am hitting the service, the list is empty. It appears to be dropping the values on each soap request?

Can you store information in a self hosted service that will be available across multiple service requests?

Thanks in advance!

Upvotes: 1

Views: 1784

Answers (4)

Phil Patterson
Phil Patterson

Reputation: 1262

If you need to store things in between requests you will need to create either a static dictionary with the appropriate locking to store these requests as they come in, or store this info in a database (or other external store) and check to see if it exists there in each method call. The reason for this is that the service class is instantiated on every client request.

Since you are already updating the users datetime when a user hits the service it would be better do a lookup to see if this is an active user or not by comparing to the datetime field. This has the advantage of being accurate on every call (the dictionary could get out of sync with the db if the service is restarted). Databases already have mechanisms in place to deal with concurrency, so rather than rolling your own locking solution around a singleton object you can push the complexity to the data store.

If the second solution is not fast enough (and you have profiled the app and determined it's the bottleneck), then the other option is to use some kind of cache solution in front of the db so that data can first be checked in memory before going to the db. This cache object would need to be static like the dictionary and has the same pitfalls around locking as any other multi-threaded application.

EDIT: If this hosted WCF service is being used as session storage for the users of the silverlight application and the data is not being stored in an external data store, then you better be sure that tracking if they are active is not mission critical. This data cannot be guaranteed to be correct as described.

Based on the accepted answer if your service faults and needs to be rebooted (since this is self hosted it is advised that you monitor the faulted event) you have to dispose of the service host and instantiate a new one. The only way the Guid data can be kept is if it is rebound to the service in between restarts (assuming the host app itself isn't restarted which is a different issue).

private Dictionary<Guid,string> _session;

Service service = new Service(_session);
_serviceHost = new ServiceHost(service, GetUriMethodInHostApp());

Better would be to store this externally and do a lookup as @marc_s suggests. Then this complexity goes away.

Upvotes: 2

landoncz
landoncz

Reputation: 2017

You need to change the InstanceContextMode. You can do so by adding the following compiler directive to your WCF class:

[ServiceBehavior(InstanceContextMode = InstanceContextMode.Single)]

This will run the WCF service as a singleton of sorts. See more on WCF Instance Context Mode

And then you should construct your service host with your singleton object. Here's code from a working example where I'm doing something similar:

            private ServiceHost serviceHost;

            if (serviceHost != null)
                serviceHost.Close();

            if (log.IsInfoEnabled)
                log.Info("Starting WCF service host for endpoint: " + ConfiguredWCFEndpoint);

            // Create our service instance, and add create a new service host from it
            ServiceLayer.TagWCFService service = new ServiceLayer.TagWCFService(ApplicationName,
                ApplicationDescription,
                SiteId,
                ConfiguredUpdateRateMilliseconds);

            serviceHost = new ServiceHost(service, new Uri(ConfiguredWCFEndpoint));

            // Open the ServiceHostBase to create listeners and start listening for messages.
            serviceHost.Open();

As others have politely noted, this can have "consequences" if you're not familiar with how it works or if it's not a good fit for your particular application.

Upvotes: 1

Stefan P.
Stefan P.

Reputation: 9519

If you don't what to involve locking and thread-safe specific code, you can use a NoSQL database to store your session data, something like MongoDB or RavenDB

Like @marc_s, I think that using the Singleton mode is a risky thing, you have to be very careful in making your own thread-safe session mechanism.

Upvotes: 1

Science_Fiction
Science_Fiction

Reputation: 3433

It's on a per instance basis. I.e session-less by default.

Have a look at this

When a service contract sets the System.ServiceModel.ServiceContractAttribute.SessionMode property to System.ServiceModel.SessionMode.Required, that contract is saying that all calls (that is, the underlying message exchanges that support the calls) must be part of the same conversation.

Upvotes: 2

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