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The role of the server

This provides a sketch of the complete lifecycle of our queue-based architecture for combining many small domain model updates into a single HTTP transaction. It combines the ability to execute fine-grained synchronization between the client and server domain models with the need to manage server traffic effectively. As we noted in section 5.3, it may provide a solution for JSF and similar frameworks, in which the structure of the user interface and interaction model is held tightly by the server. In our case, though, it simply provides an efficient way of updating the domain models across the tiers.

This concludes our tour of client/server communication techniques for this chapter and our overview of key design issues for Ajax applications. Along the way, we’ve developed the start of a pattern language for Ajax server requests and a better understanding of the technical options available to us for implementing these.

5.6 Summary

We began this chapter by looking at the key roles of the application server in Ajax, of delivering the client code to the browser, and supplying the client with data once it is running. We looked at the common implementation languages on the server side and took a tour of the common types of server-side frameworks of the day. These are largely designed to serve classic web applications, and we considered how they can adapt to Ajax. The server-side framework space is crowded and fast moving, and rather than looking at particular products, we categorized in terms of generic architectures. This reduced the field to three main approaches: the Model2 frameworks, component-based frameworks, and service-oriented architectures. SOA seems to provide the most natural fit for Ajax, although the others can be adapted with varying degrees of success. We looked at how to enforce good separation of concerns in an SOA by introducing Façades.

Moving down to the fine-grained details, we contrasted three approaches to fetching data from the server, which we labeled as content-centric, script-centric, and data-centric. These form a continuum, with classic web applications tending heavily toward the content-centric style and Ajax toward a data-centric style. In discussing data-centric approaches, we discovered that there is life beyond XML, and we took a look at JSON as a means of transmitting data to the client.

Finally, we described ways of sending updates to the server, using HTML forms and the XMLHttpRequest object. We also considered bandwidth management using a client-side queue of Command objects. This sort of technique can give a significant performance boost by reducing both server load and network

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traffic, and it is in keeping with what we have observed about best practice in SOA, moving from an RPC-style approach toward a document-based communication strategy.

This chapter concludes our coverage of the core techniques of Ajax. We’ve now covered all the basics and touched on quite a few advanced topics along the way. In the following three chapters we return to the theme of usability and add some polish to the technical wizardry that we’ve accomplished here, in order to highlight key issues that can differentiate a clever hack from something that the lay user will actually want to use.

5.7 Resources

Several web frameworks were discussed in this chapter. Here are the URLs:

■Struts (http://struts.apache.org)

■Tapestry (http://jakarta.apache.org/tapestry/)

■JSF (http://java.sun.com/j2ee/javaserverfaces/faq.html)

■PHP-MVC (www.phpmvc.net)

There are over 60 web frameworks for Java alone listed by the Wicket developers: (http://wicket.sourceforge.net/Introduction.html).

JSF is a broad category covering many individual frameworks and products. Kito Mann, author of JavaServer Faces in Action (Manning, 2004), maintains the definitive portal site for all things JSF at www.jsfcentral.com/. Greg Murray and colleagues of Sun’s Blueprints catalog discuss Ajax and JSF at https://bpcata- log.dev.java.net/nonav/ajax/jsf-ajax/frames.html. AjaxFaces is a commercial Ajaxenabled JSF implementation (www.ajaxfaces.com), and Apache’s Open Source MyFaces is looking at Ajax, too (http://myfaces.apache.org/sandbox/inputSuggestAjax.html).

Microsoft’s Atlas is still under development at the time of writing, but early releases are expected later this year (2005). Scott Guthrie is Project Manager of Atlas. His blog can be found at http://weblogs.asp.net/scottgu/archive/2005/06/28/ 416185.aspx.

You can find JSON-RPC libraries for a range of programming languages at www.json-rpc.org/impl.xhtml.