XSLT Overview

Commit scripts, op scripts, and event scripts can be written in the Extensible Stylesheet Language Transformations (XSLT), which is a standard for processing Extensible Markup Language (XML) data developed by the World Wide Web Consortium (W3C) and accessible at http://www.w3c.org/TR/xslt .

XSLT is a natural match for the Junos OS, with its native XML capabilities. XSLT performs XML-to-XML transformations, turning one XML hierarchy into another. XSLT offers a great degree of freedom and power in the way in which it transforms the input XML, allowing everything from making minor changes to the existing hierarchy (such as pruning or adding) to building a completely new document hierarchy.

Because XSLT was created to allow generic XML-to-XML transformations, it is a natural choice for both inspecting configuration syntax (which the Junos OS can easily express in XML) and for generating errors and warnings (which the Junos OS communicates internally as XML). XSLT includes powerful mechanisms for finding configuration statements that match specific criteria. XSLT can then generate appropriate XML from these configuration statements to instruct the Junos user-interface (UI) components to perform the desired behavior. The Junos XML management protocol defines XML elements for error, warning, and system log (syslog) messages.

Although XSLT provides a powerful scripting ability, its focus is specific and limited. It does not make the Junos OS vulnerable to arbitrary or malicious programmers. XSLT restricts programmers from performing haphazard operations, such as opening random Transmission Control Protocol (TCP) ports, forking numerous processes, or sending e-mail. The only action available in XSLT is to generate XML, and the XML is interpreted by the UI according to fixed semantics. An XSLT script can output only XML data, which is directly processed by the UI infrastructure to allow only the specific abilities listed above—generating error, warning, and system log messages, and persistent and transient configuration changes. This means that the impact of commit scripts, op scripts, and event scripts on the device is well-defined and can be viewed inside the command-line interface (CLI), using commands added for that purpose.

This chapter contains some overview material, intended as a brief introduction to XSLT. This chapter is not a comprehensive user guide for XSLT or XML Path Language (XPath). If you are already knowledgeable about XSLT, you can skip this chapter.

XSLT is a language for transforming one XML document into another XML document. The basic model is that an XSLT engine (or processor) reads a script (or style sheet) and an XML document. The XSLT engine uses the instructions in the script to process the XML document by traversing the document’s hierarchy. The script indicates what portion of the tree should be traversed, how it should be inspected, and what XML should be generated at each point. For commit scripts, op scripts, and event scripts, the XSLT engine is a function of the Junos OS management process (mgd).

Figure 1 shows the relationship between an XSLT commit script and the XSLT engine.

XSLT has seven basic concepts, summarized in Table 3.