Client applications and WCF services communicate by passing XML messages. Message Exchange Patterns illustrate how clients and services communicate by passing messages. There are three message exchange patterns supported by WCF:
· Request – Reply Message Exchange Pattern
· One – WayMessage Exchange Pattern (also called Datagram)
· Duplex Message Exchange Pattern (also called Callback)
Request – Reply Message Exchange Pattern
After sending message to a WCF service, a client application waits for the reply in Request – Reply pattern. This is the default and classic pattern for both WCF and Web Services. Since the client has asked a question, and it expects an answer, it waits for the service to send a message back containing the answer.
The Request – Reply message exchange pattern is used when a client requests a service to perform an action, and it needs some confirmation. Fault exception is handled in Request – Reply pattern. The Request – Reply pattern is the most used message exchange pattern.
In Request – Reply Message Exchange Pattern, only the client can initiate a communication with the service.
One – Way Message Exchange Pattern
The One – Way Message Exchange Pattern is also known as Datagramor Fire and Forget. This pattern is used when a client requests the service to take an action but it does not need any confirmation. Thus neither the service sends a reply message not the client waits for a reply. A client sends a message (request) to a WCF service but the service does not send a reply message to the client. The client does not wait for the replyand proceeds with other processing work. Some scenario’s for one-way pattern includes logging in and out, maintenance tasks, heads down data entry and other repetitive tasks etc.
An implication of using one – way pattern is that even if a method call fails due to an exception or invalid data, the service does not notify the client. If the client needs to be notified about the failure of the service request, then this pattern should not be used. This pattern should be used when the client does not need to wait for a reply from the WCF service.
In One – Way Message Exchange Pattern, only the client can initiate a communication with the service.
Duplex Message Exchange Pattern
The Duplex Message Exchange Pattern is also known as Callback. Both the client and the service can initiate a communication in the duplex pattern. It can be used for transactions. The client calls the service, which after processing the request, callbacks the client with the reply. In the meanwhile, instead for waiting for the reply (as in Request – Reply pattern), the client proceeds with other tasks.
This pattern should be used when the client (after the client has called the service) expects a notification or alert from the WCF service.
Note: The One – Way and Duplex patterns provides opportunities to fine-tune the performance of applications and to add more flexibility regarding what happens after a client calls a service.
The presence of One – Way and Duplex patterns in WCF is a major advantage of WCF over Web Services.