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For instance, given a two transformations T₁ andT₂, 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

1 Pipeline languages

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

• W3C XML Pipeline Definition Language is specified in a W3C Note and Sun provides an implementation.
• Cocoon sitemap allows the declaration of linear pipelines
• XPL is the OXF XML Pipeline Language. It allows the declaration of complex pipelines with conditionals, tees and aggregations.

2 External links

• W3C XML Pipeline Definition Language
• Sun's XML Pipeline Definition Language Controller Implementation
• Cocoon sitemap
• XPL