Apache Axis2 - XML Processing Model

XML Processing Model

As mentioned in Chapter 1, Axis2 is built on a completely new architecture as compared to Axis 1.x. One of the key reasons for introducing Axis2 was to have a better, and an efficient XML processing model. Axis 1.x used DOM as its XML representation mechanism, which required the complete object hierarchy (corresponding to incoming message) to be kept in memory.

This will not be a problem for a message of small size. But when it comes to a message of large size, it becomes an issue. To overcome this problem, Axis2 has introduced a new XML representation.

AXIOM (AXIs Object Model) forms the basis of the XML representation for every SOAP-based message in Axis2. The advantage of AXIOM over other XML InfoSet representations is that it is based on the PULL parser technique, whereas most others are based on the PUSH parser technique. The main advantage of PULL over PUSH is that in the PULL technique, the invoker has full control over the parser and it can request the next event and act upon that, whereas in case of PUSH, the parser has limited control and delegates most of the functionality to handlers that respond to the events that are fi red during its processing of the document.

Since AXIOM is based on the PULL parser technique, it has 'on-demand-building' capability whereby it will build an object model only if it is asked to do so. If required, one can directly access the underlying PULL parser from AXIOM, and use that rather than build an OM (Object Model).

SOAP Processing Model

Sending and receiving SOAP messages can be considered two of the key jobs of the SOAP-processing engine. The architecture in Axis2 provides two Pipes ('Flows'), in order to perform two basic actions. The AxisEngine or driver of Axis2 defines two methods, send() and receive() to implement these two Pipes. The two pipes are named InFlow and OutFlow. The complex Message Exchange Patterns (MEPs) are constructed by combining these two types of pipes. It should be noted that in addition to these two pipes there are two other pipes as well, and those two help in handling incoming Fault messages and sending a Fault message.

Extensibility of the SOAP processing model is provided through handlers. When a SOAP message is being processed, the handlers that are registered will be executed. The handlers can be registered in global, service, or in operation scopes, and the final handler chain is calculated by combining the handlers from all the scopes.

The handlers act as interceptors, and they process parts of the SOAP message and provide the quality of service features (a good example of quality of service is security or reliability). Usually handlers work on the SOAP headers; but they may access or change the SOAP body as well.