Common Mechanisms of UML

Forking and Joining

• Forks are used for showing parallel execution of activities 
• A fork may have one incoming transition and two or more outgoing transitions each of which represents an independent flow of control A fork represents the splitting of a single flow of control into two or more concurrent flows of control 
• Below the fork, the activities associated with each of these paths continues in parallel 
• A join may have two or more incoming transitions and one outgoing transition 
• Above the join, the activities associated with each of these paths continues in parallel 
• At the join, the concurrent flows synchronize, meaning that each waits until all incoming flows have reached the join, at which point one flow of control continues on below the join 
• The forking and joining of the parallel flows of control are specified by a synchronization bar 
• A synchronization bar is rendered as a thick horizontal or vertical line

Swimlanes

• Swimlanes partitions activity diagrams into groups having activity states where each group represents the entity responsible for those activities 
• Each swimlane has a name unique within its diagram 
• Swimlane represents a high-level responsibility for part of the overall activity of an activity diagram 
• Each swimlane is implemented by one or more classes

Object flow

• Object flow indicates the participation of an object in a flow of control. It shows the objects created as a result of an activity, and in which activities those objects are used 
• It is represented with the help of dependency relationships 

Activity diagram is used: 

• To model a workflow 
• To model an operation