This paper is really the first instance of this idea:

D. McKelvie, C. Brew, and H. Thompson. 
Using SGML as a Basis for Data-Intensive NLP. 
In Proceedings of the fifth Conference on Applied Natural Language Processing (ANLP-97)
1997

Here's that article: anlp97.pdf

The pipes are "fat" because they pass XML between components.

Definition: A pipeline is a chaining of XML-in-XML-out components.

Note: The chaining doesn't have to be a simple "line" components strung output-to-input.