what is exact use of instance modes in wcf

we normally use PerCall when we want our service instances to be created and disposed off for each incoming request. this scenario is good in many ways when we want each client to have its own service instance and that instance is working on an exclusive thread. it does not mean multiple clients can not call the services at once. it only means each client will have its own copy of service instance. use PerCall for higher scalability and throughput.

single is like a singleton. only one instance is created to service all incoming requests from all the clients. concurrent calls can be made to singleton service just like PerCall but developer has the responsibility to manage concurrency and global state management. less scalable.

PerSession becomes PerCall if there binding is not sessionfull. PerSession maintains the session for each client channel given the sessionafull binding is used. PerSession is default service behavior.