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.

1 Linear Operations
2 Non-linear
3 XML Pipeline languages
4 Standardization
5 XML Pipeline History
6 See also
7 External links

[edit] Linear Operations

Simple pipelines like the one describe above one are called linear, as 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:

Identity transform - makes a verbatim copy of its input to the output
Transform - execute a transform on the input file using a specified XSLT file. Version 1.0 or 2.0 should be specified.
Rename - renames elements or attributes without modifying the content
Replace - replaces elements or attributes
Insert - adds a new data element to the output stream at a specified point
Delete - removes an element or attribute (also known as pruning the input tree)
Wrap - wraps elements with additional elements
Reorder - changes the order of elements

[edit] Non-linear

Non-linear operations on pipelines 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)

[edit] XML Pipeline languages

XML pipeline languages are used to define pipelines. A program written with an XML pipeline language is interpreted by software known as an XML pipeline engine, which creates processes, connects them together and finally executes the pipeline. Existing XML pipeline languages include:

XProc: An XML Pipeline Language is currently a W3C Working Draft [1]
W3C XML Pipeline Definition Language is specified in a W3C Note and Sun provides an implementation (reference needed).
W3C XML Pipeline Language (XPL) Version 1.0 (Draft) [2] [3] 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.

[edit] Standardization

There is currently no widely used standard for XML pipeline languages. However a the w3c XProc standard was in Working Draft stage as of November 2006 [4] [5]. This work is considered on-going.

[edit] XML Pipeline History

1972 Douglas McIlroy of Bell Laboratories adds the pipe operator to the UNIX command shell. This allows the output from one shell program to go directly into input of another shell program without going to disk. This allowed programs such as the UNIX awk and sed to be specialized yet work together [6] [7]. For more details see Pipeline (Unix).
1993 Sean McGrath developed a C++ toolkit for SGML processing [8]
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.