Apache Axis2 - Information Model

Information Model

As shown in the figure below, the information model consists of two hierarchies: "Description hierarchy" and "Context hierarchy". The Description hierarchy represents the static data that may come from different deployment descriptors.

If hot deployment is turned off, then the description hierarchy is not likely to change. If hot deployment is turned on, then we can deploy the service while the system is up and running. In this case, the description hierarchy is updated with the corresponding data of the service. The context hierarchy keeps run-time data. Unlike the description hierarchy, the context hierarchy keeps on changing when the server starts receiving messages.

These two hierarchies create a model that provides the ability to search for key value pairs. When the values are to be searched for at a given level, they are searched while moving up the hierarchy until a match is found. In the resulting model, the lower levels override the values present in the upper levels. For example, when a value has been searched for in the Message Context and is not found, then it would be searched in the Operation Context, and so on. The search is first done up the hierarchy, and if the starting point is a Context then it would search for in the Description hierarchy as well.

This allows the user to declare and override values, with the result being a very flexible configuration model. The flexibility could be the Achilles heel of the system, as the search is expensive, especially for something that does not exist.

Deployment Model

The previous versions of Axis failed to address the usability factor involved in the deployment of a Web Service. This was due to the fact that Axis 1.x was released mainly to prove the Web Service concepts. Therefore in Axis 1.x, the user has to manually invoke the admin client and update the server classpath. Then, you need to restart the server in order to apply the changes. This burdensome deployment model was a definite barrier for beginners. Axis2 is engineered to overcome this drawback, and provide a flexible, user-friendly, easily configurable deployment model.

Axis2 deployment introduced a J2EE-like deployment mechanism, wherein the developer can bundle all the class files, library files, resources files, and configuration files together as an archive file, and drop it in a specified location in the file system.

The concept of hot deployment and hot update is not a new technical paradigm, particularly for the Web Service platform. But in the case of Apache Axis, it is a new feature. Therefore, when Axis2 was developed, "hot" deployment features were added to the feature list.

• Hot deployment: This refers to the capability to deploy services while the system is up and running. In a real time system or in a business environment, the availability of the system is very important. If the processing of the system is slow, even for a moment, then the loss might be substantial and it may affect the viability of the business. In the meanwhile, it is required to add new service to the system. If this can be done without needing to shut down the servers, it will be a great achievement. Axis2 addresses this issue and provides a Web Service hot deployment ability, wherein we need not shut down the system to deploy a new Web Service. All that needs to be done is to drop the required Web Service archive into the services directory in the repository. The deployment model will automatically deploy the service and make it available.
• Hot update: This refers to the ability to make changes to an existing Web Service without even shutting down the system. This is an essential feature, which is best suited to use in a testing environment. It is not advisable to use hot updates in a real-time system, because a hot update could lead a system into an unknown state. Additionally, there is the possibility of loosening the existing service data of that service. To prevent this, Axis2 comes with the hot update parameter set to FALSE by default.