Partitions

Activity diagrams tell you what happens, but they do not tell you who does what. In programming, this means that the diagram does not convey which class is responsible for each action. In business process modeling, this does not convey which part of an organization carries out which action. This isn't necessarily a problem; often, it makes sense to concentrate on what gets done rather than on who does what parts of the behavior.

If you want to show who does what, you can divide an activity diagram into partitions, which show which actions one class or organization unit carries out. Figure 11.4 (on page 122) shows a simple example of this, showing how the actions involved in order processing can be separated among various departments.

The partitioning of Figure 11.4 is a simple one-dimensional partitioning. This style is often referred to as swim lanes, for obvious reasons and was the only form used in UML 1.x. In UML 2, you can use a two-dimensional grid, so the swimming metaphor no longer holds water. You can also take each dimension and divide the rows or columns hierarchically.