5 Reconstructing by exchanging data

In our framework exchanging data between client and server is accomplished by means of XHTML forms. On the server side data are represented at a higher abstraction level using ActionForm objects: this helps in managing both exchange and validation of information.

During the execution of a generic reconstruction step the exchange of messages between client and server follows the procedure outlined below (Figure 3 for a schematic representation):

1. The client submits to the server the request for a page: for example, corner.jsp is the page associated to the corner extraction step;
2. If the user is logged on, the server returns the page in which the requested parameters can be set up: for example, the size of the search window;
3. After the input of parameters, the user submits the request to perform the elaboration: following our previous example, the corner extraction;
4. The request arrives at the server as a form submitted using the POST method: the request is then forwarded to the ActionServlet;
5. The ActionServlet, according to the configuration in the file struts-config, redirects the request to the associated Action object;
6. The Action object implements the application logic level: going back to the previous example, the execution of the corner-extraction algorithm;
7. According to the output of the algorithm and the settings stored in the file struts-config, the server sends to the client an updated page containing the results of the elaboration or an error message.

Figure 3: Summary of the message exchange during a step of the reconstruction process.