Question

Both perform the same operation that is

MSDN

• Singleton: “”Single Call objects service one and only one request coming in…. “
• CAO: “Client-activated objects (CAO) are server-side objects that are activated upon request from the client….”

in both cases, data is not shared, but in singleton only once client can be connected at a time, why would someone want that
Under what scenario is Singleton useful and are there any more differences in them ?

Answer 1

You missread the lines. Here´s the definition from the MSDN. I highlight (bold) some diffrences between those objects.

• Single Call

  Single Call objects service one and only one request coming in. Single
  Call objects are useful in scenarios
  where the objects are required to do a
  finite amount of work. Single Call
  objects are usually not required to
  store state information, and they
  cannot hold state information between
  method calls. However, Single Call
  objects can be configured in a
  load-balanced fashion.

• Singleton Objects

  Singleton objects are those objects that service multiple clients
  and hence share data by storing state
  information between client
  invocations. They are useful in cases
  in which data needs to be shared
  explicitly between clients and also in
  which the overhead of creating and
  maintaining objects is substantial.

• Client-Activated Objects (CAO)

  Client-activated objects (CAO) are server-side objects that are
  activated upon request from the
  client. This way of activating server
  objects is very similar to the classic
  COM coclass activation. When the
  client submits a request for a server
  object using “new” operator, an
  activation request message is sent to
  the remote application. The server
  then creates an instance of the
  requested class and returns an ObjRef
  back to the client application that
  invoked it. A proxy is then created on
  the client side using the ObjRef. The
  client’s method calls will be executed
  on the proxy. Client-activated objects
  can store state information between
  method calls for its specific client
  and not across different client
  objects. Each invocation of “new”
  returns a proxy to an independent
  instance of the server type.

State Information is some data that you store in variables or properties of the object, to process the client request.

Since a Single call object is created when a client request it to do some work, and destroyed after it has done work, it cannot hold state information, cause each request creates a new object (It can load and store data in a datasource, to do it´s work).

A Singleton object is created only once (maybe at server starup) and lifes as long as the server process is running. It can store information in variables and properties to handle client requests, cause each client works with the same object and it isn´t destroyed after a client call.

An easy example to display the diffrence betwenn a single call and singleton object is to create a method Increment() to increment a variable (integer) in the object and write the variable to the Console. The single call object will always print the same value to the Console (1 if the variable starts with 0), while the singleton object will always print an incremented value (1, 2, 3, etc.) after each call.