As we have already discussed, the web services communicate with each other and the other programs by sending messages. If we consider two SOAP processing nodes, the communication pattern between the two entities can be defined as a message exchanging pattern (MEP). The primary message exchanging patterns are:
In a request-response pattern, when a source entity (service requester) transmits a message to a destination (service provider), the provider should respond to the requester. This is the most commonly used message exchanging pattern and we will use this in most of the examples in this book.
In the following diagram, a service requester sends a SOAP request message to a service provider:
Upon receiving the SOAP request message, the service provider responds with a SOAP response as shown in the following diagram:
When a response to a request message is not expected from a web service (or service provider), it is known as a fire and forget message exchange pattern. For example, if we send a ping request to a web service, we do not expect a response message back.