DRAG DROP
The UploadOrder() method in the UploadCallbackService service is not implementing the callback behavior
defined in the IUploadCallBackService interface. You need to modify the class to implement the required
callback behavior. What should you do? (To answer, drag the appropriate code segments to the correct
location or locations in the answer area. Each codesegments may be used once, more than once, or not at all.
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Answer: 
Explanation:
Why ConcurrencyMode.Single?
Nothing in the brief specifies concurrency on the upload service. By default the concurrency mode would already be Single so why bother with the attribute if setting it to the default? Also, there is a possibility that the client may be attempting to upload more than one order at the same time, in which case setting the concurrency mode to Single would pointlessly cripple the service throughput (there is nothing in the Upload method that requires synchronization as far as I can see)..
Have I missed something?
I’m wondering the same thing.
is multiple por que permite ejecutarse en diferente hilos. as no single por que saldria un a exceptio este permite ejecutar en un solo procesos y solo hilo ejecutandolas las peticiones una detras de otra para que pueda continuar.
it depends of binding type. See
http://stackoverflow.com/questions/6065379/understanding-wcf-servicebehaviorproperty-concurrencymode
“default value is PerCall for bindings not supporting sessions (basicHttpBinding, webHttpBinding, wsHttpBinding without security context or reliable session) or PerSession for bindings supporting sessions (netTcpBinding, netNamedPipeBinding, wsHttpBinding in default configuration using security context).
ConcurrencyMode defines how can an instance of the service be used if multiple requests want to access it. For example PerCall instancing with Multiple concurrency is the same as Single concurrency because each instance is used to serve exactly one request regardless the concurrency setting”