WCF service calls to clients

Question

I am trying to create WCF service to handle client requests and modify a database. Beside my user clients i also have other interfaces, which can call my WCF service and modify the database.

My problem is, I wish to notify my clients about the database change occured because of my other interface.

So basicly: Clients are running, WCF service hosted by WAS/IIS, database changes and service should notify all of my connected clients about the change.

My user clients are simple WinForms applications and the service is referenced to them.

I would like to avoid initiating calls from the clients and waiting for the response, i'd rather like the service to make the call in the clients direction.

My question is that is there a way to implement this, or i should use something like a listener?

Answer 1

This can be achieved in WCF.

Implement what is termed the 'duplex communication pattern', which is basically two one-way contracts.

Here is an MSDN article for just that... How to: Create a Duplex Contract

"This topic shows the basic steps to create methods that use a duplex (two-way) contract. A duplex contract allows clients and servers to communicate with each other independently so that either can initiate calls to the other. The duplex contract is one of three message patterns available to Windows Communication Foundation (WCF) services. The other two message patterns are one-way and request-reply. A duplex contract consists of two one-way contracts between the client and the server and does not require that the method calls be correlated. Use this kind of contract when your service must query the client for more information or explicitly raise events on the client."

Encase they take down the page and I don't really like link only answers...

// Define a duplex service contract.
// A duplex contract consists of two interfaces.
// The primary interface is used to send messages from client to service.
// The callback interface is used to send messages from service back to client.
// ICalculatorDuplex allows one to perform multiple operations on a running result.
    // The result is sent back after each operation on the ICalculatorCallback interface.
[ServiceContract(Namespace = "http://Microsoft.ServiceModel.Samples", SessionMode=SessionMode.Required,
             CallbackContract=typeof(ICalculatorDuplexCallback))]
public interface ICalculatorDuplex
{
    [OperationContract(IsOneWay=true)]
    void Clear();
    [OperationContract(IsOneWay = true)]
    void AddTo(double n);
    [OperationContract(IsOneWay = true)]
    void SubtractFrom(double n);
    [OperationContract(IsOneWay = true)]
    void MultiplyBy(double n);
    [OperationContract(IsOneWay = true)]
    void DivideBy(double n);
}

// The callback interface is used to send messages from service back to client.
// The Equals operation will return the current result after each operation.
// The Equation opertion will return the complete equation after Clear() is called.
public interface ICalculatorDuplexCallback
{
    [OperationContract(IsOneWay = true)]
    void Equals(double result);
    [OperationContract(IsOneWay = true)]
    void Equation(string eqn);
}

// Service class which implements a duplex service contract.
// Use an InstanceContextMode of PerSession to store the result
// An instance of the service will be bound to each duplex session
[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerSession)]
public class CalculatorService : ICalculatorDuplex
{
    double result;
    string equation;
    ICalculatorDuplexCallback callback = null;

    public CalculatorService()
    {
        result = 0.0D;
        equation = result.ToString();
        callback = OperationContext.Current.GetCallbackChannel<ICalculatorDuplexCallback>();
    }

    public void Clear()
    {
        callback.Equation(equation + " = " + result.ToString());
        result = 0.0D;
        equation = result.ToString();
    }

    public void AddTo(double n)
    {
        result += n;
        equation += " + " + n.ToString();
        callback.Equals(result);
    }

    public void SubtractFrom(double n)
    {
        result -= n;
        equation += " - " + n.ToString();
        callback.Equals(result);
    }

    public void MultiplyBy(double n)
    {
        result *= n;
        equation += " * " + n.ToString();
        callback.Equals(result);
    }

    public void DivideBy(double n)
    {
        result /= n;
        equation += " / " + n.ToString();
        callback.Equals(result);
    }
}