What is the difference between PerCall, PerSession, and Single instance modes?

Question

ICSM Computer · Answer

In WCF, PerCall, PerSession, and Single are
instance context modes that define how and when WCF service instances are created and reused during client communication.
1. PerCall
Description: A new service instance is created for
every method call from any client.State: Not maintained between calls.Concurrency: No synchronization needed.Use case: Stateless operations (e.g., utility services, logging).
[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerCall)]
Feature
Value
Instance Scope
Per method call
State Retention
No
Scalability
High
2. PerSession
Description: A single instance is created for the
lifetime of a client session.State: Maintained throughout the session.Concurrency: Needed if multiple threads are involved.Use case: Services that require state tracking per client (e.g., shopping cart, workflow).
[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerSession)]
Feature
Value
Instance Scope
Per client session
State Retention
Yes (per client)
Scalability
Medium
Requires Sessions
Yes (NetTcp, etc.)
3. Single (Singleton)
Description: A single service instance is created and shared across
all clients and all calls.State: Shared across all clients.Concurrency: Critical — must handle simultaneous access.Use case: Centralized resource sharing, caching, or coordination services.
[ServiceBehavior(InstanceContextMode = InstanceContextMode.Single)]
Feature
Value
Instance Scope
Entire application
State Retention
Yes (shared)
Scalability
Low
Concurrency
Must be handled
Summary Table
Mode
Instance Created When
State Retained
Suitable For
Needs Concurrency Handling
PerCall
On every operation call
No
Stateless services
No
PerSession
Once per client session
Yes (per client)
Stateful workflows
Yes
Single
Once per application
Yes (shared)
Shared-state services
Yes
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What is the difference between PerCall, PerSession, and Single instance modes?

ICSM Computer

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1 Answers

1. PerCall

2. PerSession

3. Single (Singleton)

Summary Table

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Feature	Value
Instance Scope	Per method call
State Retention	No
Scalability	High

Feature	Value
Instance Scope	Per client session
State Retention	Yes (per client)
Scalability	Medium
Requires Sessions	Yes (`NetTcp`, etc.)

Feature	Value
Instance Scope	Entire application
State Retention	Yes (shared)
Scalability	Low
Concurrency	Must be handled

Mode	Instance Created When	State Retained	Suitable For	Needs Concurrency Handling
PerCall	On every operation call	No	Stateless services	No
PerSession	Once per client session	Yes (per client)	Stateful workflows	Yes
Single	Once per application	Yes (shared)	Shared-state services	Yes