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
- Per-Session
- Single
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 ----------------------------
[ServiceContract]
interface
IMyContract
{
[OperationContract]
int MyMethod(
);
}
[ServiceBehavior(InstanceContextMode
= InstanceContextMode.PerCall)]
class
MyService :
IMyContract,IDisposable
{
static
int m_Counter = 0;
public
int MyMethod( )
{
m_Counter++;
return
m_Counter;
}
}
-----------------------------
Client code --------------------------
MyContractClient proxy =
new MyContractClient(
);
Console.WriteLine("Counter:
" + proxy.MyMethod());
Console.WriteLine("Counter:
" + proxy.MyMethod());
Console.WriteLine("Counter:
" + proxy.MyMethod());
Console.WriteLine("Counter:
" + proxy.MyMethod());
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 ----------------------------
[ServiceContract]
interface
IMyContract
{
[OperationContract]
int MyMethod(
);
}
[ServiceBehavior(InstanceContextMode
= InstanceContextMode.PerCall)]
class
MyService :
IMyContract,IDisposable
{
static
int m_Counter = 0;
public
int MyMethod( )
{
m_Counter++;
return m_Counter;
}
}
-----------------------------
Client code --------------------------
MyContractClient proxy =
new MyContractClient(
);
Console.WriteLine("Counter:
" + proxy.MyMethod());
Console.WriteLine("Counter:
" + proxy.MyMethod());
Console.WriteLine("Counter:
" + proxy.MyMethod());
Console.WriteLine("Counter:
" + proxy.MyMethod());
Console.ReadLine();
--Possible Output
Counter: 1
Counter: 2
Counter: 3
Counter: 4
Important points of per-session instance mode
-
Each proxy will be connected to a new
dedicated service instance.
-
Service instance is allocated when client
creates a new proxy and deallocated when client destroys this proxy object.
-
Client session is per service endpoint per
proxy.
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 ----------------------------
[ServiceContract]
interface
IMyContract
{
[OperationContract]
int MyMethod(
);
}
[ServiceBehavior(InstanceContextMode
= InstanceContextMode.PerCall)]
class
MyService :
IMyContract,IDisposable
{
static
int m_Counter = 0;
public
int MyMethod( )
{
m_Counter++;
return m_Counter;
}
}
-----------------------------
Client code --------------------------
Console.WriteLine("Client
1 making call to service…. ");
Console.WriteLine("Counter:
" + proxy1.MyMethod());
Console.WriteLine("Counter:
" + proxy1.MyMethod());
Console.WriteLine("Counter:
" + proxy1.MyMethod());
Console.WriteLine("Client
2 making call to service…. ");
MyContractClient proxy2 =
new MyContractClient(
);
Console.WriteLine("Counter:
" + proxy2.MyMethod());
Console.WriteLine("Counter:
" + proxy2.MyMethod());
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
-
Only one service instance is created. It is
created when the service host is created.
-
This service instance lives forever and is
only destroyed when service host shuts down.
-
Only one service instance is created. It
doesn't matter whether the requests coming from different endpoints or
different proxies.
When to use per-call, per-session and single
instance mode
![Different instance mode in WCF]()