Enterprise Integration Patterns: Designing, Building, and Deploying Messaging Solutions (Addison-Wesley Signature Series (Fowler))

by Hohpe Gregor

Activities can be connected in serial fashion, or multiple activities can be executed in parallel using a fork and join construct. A fork allows multiple activities to execute at the same time. It is semantically equivalent to a Publish-Subscribe Channel (106) in a hard-wired Pipes and Filters (70) architecture. A join synchronizes multiple parallel threads of execution back into a single thread. Execution after a join can continue only if all parallel threads have completed their respective activities. In the Pipes and Filters (70) style, an Aggregator (268) often serves this purpose. The process template also must be able to specify a branch, or decision point, so that the path of execution can change based on the content of a message field. This function is equivalent to a Content-Based Router (230). Many modeling tools include the ability to design a loop construct, but this is really a special case of the branch. The following figure highlights the semantic similarities between a process definition (depicted as a UML activity diagram) and a Pipes and Filters (70) implementation using the patterns defined in this pattern language, even though the physical implementations are very different.