Guideline: Modeling Functional Flows
This guideline provides help on modeling functional flow using activity diagrams.
Main Description

Functional Decomposition

Repeat the decomposition until you feel the leaf functions can be performed by a single system component.  It is better to stop decomposing too soon, and perform further decomposition later during architecture design as required.  Rule of thumb is to decompose to no more than three levels.

Use of swim-lanes

There is always a discussion whether actor swim lanes should be shown in an activity diagram. In many cases this may lead to "messy" diagrams. Focus of the activity diagram should be on the system's internal functional flow. Actor swim lanes and associated activities should be considered only when they support the understanding of system requirements and readability can be maintained.

A recommended alternative is shown in the activity diagram below. The interactions with the environment are documented in the description of the relevant action (e.g. readSecurityCard).

Note: If an activity diagram contains too much detail, some actions may be placed in a Reference Activity Diagram. Do not use SubActivity Diagrams because these can contain actions only for a single swim lane. In the later white-box activity diagrams the actions may span a number of swim lanes.