In WCF there are three types of Instance Management

1.   Per Call

2.   Per Session

3.   Single

Per call
• You want a stateless services
• your service hold intensive resources like connection object and huge memory objects.
• Scalability is a prime requirement. You would like to have scale out architecture.
• Your WCF functions are called in a single threaded model.
 

Per session
• You want to maintain states between WCF calls.
• You want ok with a Scale up architecture.
• Light resource references
 

Single
• You want share global data through your WCF service.
• Scalability is not a concern.