XML accelerators help Web servers
By Eugene Kuznetsov

Traditional network accelerators speed network applications by moving content closer to users, as in the case of local or global caching, or by off-loading cryptographic functions from servers, as with Secure Sockets Layer accelerators.

However, the growing use of dynamic XML in applications is giving network administrators something new to worry about. XML is a verbose format that creates performance problems. This has spurred the emergence of a new type of network device: the XML accelerator.

XML accelerators off-load XML processing from application servers and Web servers. This is a dramatic departure from the past, given that XML processing traditionally has been considered to be the purview of the application, not the network infrastructure. The benefits of XML accelerators are faster response times and lower project costs.

There are many types of XML processing that can be performed at the network level - the two most common are XML redirection and Extensible Stylesheet Language Transformations (XSLT).

XML redirection is similar to URL-switching load balancing. The contents of incoming XML messages are examined and matched against user-defined patterns. XML documents matching a specific pattern can be sent to a corresponding server for further processing, while those matching another pattern can be "redirected" to another server.

This lets application developers and network administrators exercise tight control over which transactions are processed on which servers. Although the same functionality can be implemented in software running on the corresponding servers, there are benefits to performing XML routing in single-purpose XML accelerator devices. Some of the benefits are reduced latencies for XML redirection decisions and eased administration by virtue of separating routing functions from complex applications.

XSLT is a World Wide Web Consortium-specified processing language used to render XML content for HTML browsers and convert between different XML formats. The benefits of XSLT include cross-platform capability (it is supported under .Net and Java 2 Platform Enterprise Edition platforms) and that it is standard and mature (XSLT 1.0 specification was issued in 1999).

By separating presentation from content and data interchange from business logic, it is easier and less expensive to change the appearance of a Web site overnight or connect to new trading partners. But XSLT processing is resource-intensive, and the performance of some high-traffic applications can depend to a significant extent on XSLT processing speed. XML accelerators can be of considerable benefit by off-loading and accelerating XSLT processing beyond what is possible using purely software solutions.

XSLT acceleration for Web servers can be basically autoconfiguring if the site already is set up to use XSL stylesheet processing instructions. In this scenario, a special tag at the beginning of an XML document tells a user agent which XSL stylesheet should be applied to the document to make it viewable by the user.

The stylesheet is itself an XML document identified by the processing instruction using a URL. The XML accelerator intercepts the HTTP response, finds the processing instruction in the XML document, retrieves the XSL via the URL, applies it to the XML document and sends the resulting HTML to the end user's browser. Even if the site has thousands of pages and hundreds or thousands of XSL stylesheets, installing and configuring an XML acceleration solution should be simple because the accelerator "learns" the configuration as the processing instructions of XML documents pass through it.

On the other hand, XML redirection often involves more setup and configuration effort. Depending on the vendor, there may be custom scripting or routing rules language involved, but increasingly the trend is toward XPath-based redirection rules. Any existing routing or redirection functionality needs to be factored and included in the joint XML redirection rule set, which is input into the XML accelerator.

An incoming XML document is examined by the XML router and redirected in accordance with the rules in effect for the appropriate server. Technologically, the problem of XML redirection is not as complex as XSLT acceleration, which is part of the reason that the first XML accelerators offered only XML redirection.

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Kuznetsov founded DataPower Technology in 1999 to provide content-aware network infrastructure for next-generation protocols. He can be reached at eugene@datapower.com.