What is Instance Management?

Instance Management is the name for a set of techniques used by WCF to bind client requests to service instances, governing which service instance handles which client request. This is a service side implementation detail. Reference: Programming WCF Services

Why we need Instance Management

We need Instance Management because applications differ too much in their needs for scalability, performance, throughput, transactions and queued calls. So one solution for all applications doesn't fit.
3 ways of instance management
WCF framework has provided 3 ways by which we can manage WCF service instance creation.

Per-Call instance mode

One instance of the service class is created per incoming request. A service instance is created only when the client call is in progress and it will be destroyed after response is sent back to client.
Every time the client sends a new request either from the same proxy or a different proxy, a new service instance is always created.
Per-Call instance mode in WCF
This represents the process of handling the request from client using Per-Call instance mode.
WCF Per-Call instance mode

Implementation of Per-Call services


To configure a service type as a per-call service, we need to apply the 'ServiceBehavior' attribute with the 'InstanceContextMode' property set to InstanceContextMode.PerCall.
- Service code -
  1. [ServiceContract]
  2. interface IMyContract
  3. {
  4. [OperationContract]
  5. int MyMethod( );
  6. }
  7. [ServiceBehavior(InstanceContextMode = InstanceContextMode.PerCall)]
  8. class MyService : IMyContract,IDisposable
  9. {
  10. static int m_Counter = 0;
  11. public int MyMethod( )
  12. {
  13. m_Counter++;
  14. return m_Counter;
  15. }
  16. }
- Client code -
  1. MyContractClient proxy = new MyContractClient( );
  2. Console.WriteLine("Counter: " + proxy.MyMethod());
  3. Console.WriteLine("Counter: " + proxy.MyMethod());
  4. Console.WriteLine("Counter: " + proxy.MyMethod());
  5. Console.WriteLine("Counter: " + proxy.MyMethod());
  6. Console.ReadLine();
Possible Output
Counter: 1
Counter: 1
Counter: 1
Counter: 1
Important points of per-call instance mode

Per-Session instance mode

One instance of the service class is created per client session. Here, a logical session is maintained between client and service.
Each proxy will connect to new instance.
WCF Per-Session instance mode

Implementation of Per-Session services

To configure a service type as a per-session service, we need to apply the 'ServiceBehavior' attribute with the 'InstanceContextMode' property set to InstanceContextMode.PerSession.

- Service code -
  1. [ServiceContract]
  2. interface IMyContract
  3. {
  4. [OperationContract]
  5. int MyMethod( );
  6. }
  7. [ServiceBehavior(InstanceContextMode = InstanceContextMode.PerCall)]
  8. class MyService : IMyContract,IDisposable
  9. {
  10. static int m_Counter = 0;
  11. public int MyMethod( )
  12. {
  13. m_Counter++;
  14. return m_Counter;
  15. }
  16. }
- Client code -
  1. MyContractClient proxy = new MyContractClient( );
  2. Console.WriteLine("Counter: " + proxy.MyMethod());
  3. Console.WriteLine("Counter: " + proxy.MyMethod());
  4. Console.WriteLine("Counter: " + proxy.MyMethod());
  5. Console.WriteLine("Counter: " + proxy.MyMethod());
  6. Console.ReadLine();
Possible Output
Counter: 1
Counter: 2
Counter: 3
Counter: 4
Important points of per-session instance mode

Single instance mode

One instance of the service class handles all incoming requests. This implements a singleton. When a service is configured as a singleton, all clients connect to the same shared service instance; it is not a factor, to which endpoint of the service they are connected.
All client requests will go to same instance.
WCF Single instance mode

Implementation of Single instance services

To configure a service type as a single instance service, we need to apply the 'ServiceBehavior' attribute with the 'InstanceContextMode' property set to
InstanceContextMode.Single.
- Service code -
  1. [ServiceContract]
  2. interface IMyContract
  3. {
  4. [OperationContract]
  5. int MyMethod( );
  6. }
  7. [ServiceBehavior(InstanceContextMode = InstanceContextMode.PerCall)]
  8. class MyService : IMyContract,IDisposable
  9. {
  10. static int m_Counter = 0;
  11. public int MyMethod( )
  12. {
  13. m_Counter++;
  14. return m_Counter;
  15. }
  16. }
- Client code -
  1. Console.WriteLine("Client 1 making call to service.");
  2. Console.WriteLine("Counter: " + proxy1.MyMethod());
  3. Console.WriteLine("Counter: " + proxy1.MyMethod());
  4. Console.WriteLine("Counter: " + proxy1.MyMethod());
  6. Console.WriteLine("Client 2 making call to service.");
  7. MyContractClient proxy2 = new MyContractClient( );
  9. Console.WriteLine("Counter: " + proxy2.MyMethod());
  10. Console.WriteLine("Counter: " + proxy2.MyMethod());
  11. Console.ReadLine();
Possible Output
Client 1 making call to service....
Counter: 1
Counter: 2
Counter: 3
Client 2 making call to service....
Counter: 4
Counter: 5
Important points of Single instance mode
When to use per-call, per-session and single instance mode
Different instance mode in WCF