This figure below presents the notion of exchange that is a main concept of SOA paradigm.
A exchange is composed of information to route exchange such as source address, destination endpoint and service name. It can also contains other metadata information to manage non-functional global contexts.

During the lifecycle of an exchange, it is composed of one or several messages:

• MessageIn: It represents the client request.
• MessageOut: It represents the provider response (used only for InOut exchange pattern).
• MessageError: It represents the provider error (used only for InOut exchange pattern).
• MessageAck: It represents the acknowledgement of provider (used only for InOnly exchange pattern).

A message contains these following items:

• An header: That contains meta-data to manage the routing of exchange or others non-functional aspects such as security, reliability or transactional features.
• A body: That contains the xml payload of client request or provider response.
• Attachements: That contains all attachments provided with the messages.

The model of this exchange is represented below: