In computer science, an XML Pipeline is formed when XML (Extensible Markup Language) processes, sometimes called XML transformations, are connected together.
For instance, given 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.
Linear Operations
Simple pipelines like the one described above are called linear; a single input document always goes through the same sequence of transformations to produce a single output document.
Examples of operations on linear pipeline may 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
Some standards also categorize transformation as macro (changes impacting an entire file) or micro (impacting only an element or attribute)
XML Pipeline languages
XML pipeline languages are used to define pipelines. A program written with an XML pipeline language is implemented by software known as an XML pipeline engine, which creates processes, connects them together and finally executes 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) http://www.w3.org/Submission/xpl/http://www.w3.org/TR/xml-pipeline/ is specified in a W3C Submission and a component of Orbeon Presentation Server OPS (now called Orbeon Forms). This specification 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, among other functionality, the declaration of XML pipelines. Cocoon sitemaps are one of the earliest implementations of the concept of XML pipeline.
smallx XML Pipelines are used by the smallx project.
ServingXML defines a vocabulary for expressing flat-XML, XML-flat, flat-flat, and XML-XML transformations in pipelines.
PolarLake Circuit Markup Language used by PolarLake's runtime to define XML pipelines. Circuits are collections of paths through which fragments of XML stream (usually as SAX or DOM events). Components are placed on paths to interact with the stream (and/or the outside world) in a low latency process.
Pipe Granularity
Different XML Pipeline implementations support different granularity of flow.
Document: Whole documents flow through the pipe as atomic units. A document can only be in one place at a time. Though usually multiple documents may be in the pipe at once.
Event: Element/Text nodes events may flow through different paths. A document may be in concurrently flowing through many components at the same time.
1998 Stefano Mazzocchi releases the first version of Apache Cocoon, one of the first software programs to use XML pipelines.
1998 PolarLake build XML Operating System, which includes XML Pipelining.
2002 Notes submitted by Norman Walsh and Eve Maler from Sun Microsystems, as well as a W3C Submission submitted in 2005 by Erik Bruchez and Alessandro Vernet from Orbeon, were important steps toward spawning an actual standardization effort. While neither submission directly became a W3C recommendation, they were considered key sources of inspiration for the W3C XML Processing Working Group.
September 2005 W3C XML Processing Working Group started. The task of this working group was to create a specification for an XML pipelining language.
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