Wikipedia

XML pipeline

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In computer science, an XML Pipeline is formed when XML (Extensible Markup Language) processes, sometimes called XML transformations, are connected together.

For instance, given a two transformations T1 and T2, the two can be connected together so that an input XML document is transformed by T1 and then the output of T1 is fed as input document to T2. Simple pipeline like this one are called linear, as a single input document always goes through the same sequence of transformation to produce a single output document. Non-linear pipelines can include:

  • Conditionals — where a given transformation is executed if a condition is met while another transformation is executed otherwise
  • Loops — where a transformation is executed on each node of a node set selected from a document or a transformation is executed until a condition evaluates to false
  • Tees — where a document is fed to multiple transformations potentially happening in parallel
  • Aggregations — where multiple documents are aggregated into a single document
  • Exception Handling — where failures in processing can result an alternate pipeline being processed

Pipeline languages

XML Pipeline languages are used to define pipelines. The definition can be understood by a pipeline engine which, given a pipeline definition, can create the processes, connect them together and execute the pipeline. Existing XML pipeline languages include:

  • W3C XML Pipeline Definition Language is specified in a W3C Note and Sun provides an implementation.
  • W3C XML Pipeline Language (XPL) Version 1.0 (Draft) is specified in a W3C Submission and Orbeon PresentationServer (OPS) provides an implementation of an earlier version of the language. XPL allows the declaration of complex pipelines with conditionals, loops, tees, aggregations, and sub-pipelines.
  • Cocoon sitemaps allow the declaration of linear pipelines. Cocoon sitemaps are one of the earliest implementations of the concept of XML pipeline.

smallX is a tool which has pipelines as their main focus

Standardization efforts

A W3C Note submitted in 2002, as well as a W3C Submission submitted in 2005, are efforts toward spawning a standardization effort. It appears, as of September 2005, that work is going on at W3C to start an XML Processing Model working group, with the main task of specifying an XML pipelining language.

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