input: [Category] Concurrency Management [Description] By default WCF will not let a service callback within a service operation to its clients. The reason is that by default the service uses single-threaded access – only one client is allowed to call it at a time. If the service were to allow to call back to its client it could result with a deadlock if the client will call to the service as a result of the callback to the client. To allow callbacks, you need to configure the service for reentrancy – that is, release the lock before the callback, as show in the demo. [Last Updated] 9/7/2006 [Title] 
output: Callbacks and Reentrancy