WCF Interview Questions on Instancing Modes

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What are the instancing modes available in wcf?
1. Percall
2. PerSession
3. Single

What are the advantages of using Percall instancing mode in WCF?
In a Percall instance mode, a new service object is created for each call. The following are the advantages and Disadvantages of Percall instance mode.

Advantages:
Less memory consumption
Service instances are freed
Concurrency is not an issue
PerCall services increase overall throughput

Disadvantages:
State not maintained between calls


What are the four session types available in WCF?
Transport session
Reliable sessions
Secure sessions
Application sessions

What are the advantages and disadvantages of using PerSession instancing mode in WCF?
A new service object gets created for each client/proxy. The following are the advantages and Disadvantages of PerSession instance mode.

Advantages:
State maintained by service instance

Disadvantages:
Less throughput, greater memory consumption
Concurrency issues for multithreaded clients

Name a few bindings that can support PerSession instancing mode?
1. NetTcpBinding
2. NetNamedPipeBinding
3. WSHttpBinding
4. WSFederationHttpBinding
5. WSDualHttpBinding

What is the default Application Session timeout in WCF?
Session lifetime lasts 10 minutes by default, for Application Session.

What are the advantages and disadvantages of using Single, Instancing mode in WCF?
In a single instancing mode, a single service object is created for all calls from all clients and sessions This type of wcf service is also called as singleton service. The following are the advantages and Disadvantages of Single instance mode.

Advantages:
State maintained by service instance

Disadvantages:
Least throughput
Potentially greater memory consumption
Concurrency issues

What are the general guidelines for choosing an instancing mode?
In general, for scalability and throughput, prefer to use PerCall services where ever possible. Use PerSession services only when necessary, but keep in mind the overhead of sessions and session timeouts. Try to avoid singletons almost always.Singleton services, could be useful on client machines for shared functionality. These are only general guidelines, and your selection depends on what you are trying to achieve.


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