.Net (>=3.0) message exchange patterns in WCF:
request/reply, one-way, and duplex (duplex can be used to achieve event behavior between client and server.)

To allow the service to be accessed in a concurrent mode, either we have to set up the ConcurrencyMode in ServiceBehavior to Multiple (if you are expecting a large amount of concurrent visit), in this instance, we will need to handle data integrity in the multi-threading enviroment and other locks in the service implementation ourselves; the other way is to set up the ConcurrentcyMode in ServiceBehavior to Reentrant. In this case sevice will still be running single thread mode, but with the WCF automatic lock discharged.