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What are the basic elements of an Activity Diagram?

Posted by Chris Adams

Article Rating // 42820 Views // 1 Additional Answers & Comments

Categories: Business Analysis, Systems Analysis, Unified Modeling Language (UML)


UML defines a specific notation and set of rules for creating Activity Diagrams. The following are the most commonly used elements:

  • Initial Node - The initial node represents the starting point of the activity diagram.
  • Activity Final Node - The activity final node represents the termination point of the activity.
  • Action Node - An action node is a type of activity node that represents a single action or behavior of the activity being modeled.
  • Activity Edge - An activity edge creates a directed connection between two activity nodes.  It represents the path that a token can take between two activity nodes.
  • Decision - A decision has one flow entering and several exiting.  The exiting flows each have a condition that must be met in order to traverse the flow.
  • Merge - A merge has several flows entering and one exiting.  The merge denotes that multiple parallel flows are merging at a single point.  Only one flow must reach the merge point in order to continue to traverse the flow to the next activity.
  • Fork - A fork has one flow entering and several exiting.  A fork denotes that several processes are occurring in parallel.
  • Join - A join has several flows entering it and one exiting it.  A join denotes that multiple parallel flows are merging at a single point.  All flows going into the join must be completed before the next activity can start.

There are others, but these eight symbols constitute the basic notation used by nearly every Activity Diagram.



Nathan Caswell posted on Saturday, May 2, 2009 1:23 PM
Activity diagrams beyond basic flow charts are not well standardized so it is important to recognize the basic differences.

In UML 1.x activity flow is an alternate view of a state diagram. Transitions in the state diagram are defined by and Event, Condition, and Action. In the activity diagram
- completion of an action causes an event
- diamond decision points represent the conditions associated with that event
- activity boxes represent actions
- start, stop, and external events are represented by variously decorated circles.

There are a variety of notations that utilize this basic representation of event, condition, and action.

UML 2 adds fork/join bars for parallel execution. While the notation is reminiscent of Petri net's (and tokens) the semantics refer to compound events, i.e. there is no explicit semantics to account for token consumption and emission.

Activity diagrams using BPMN extend the notion of compound events to include conditions. For example n of m input events have occurred, it is 0600 on Tuesday. Special circles are used to indicate various common cases. Each circle, including start, stop, intermediate, and external variations represent events that occur when the input conditions are satisfied. Additionally, activities may generate conditional events, e.g. order entry compete & price>$1000.

Workflow diagrams, such as Websphere Business Modeler (WBM or WBI Modeler) move the compound events to inputs on the activity and may allow an activity to generate both message and context output data and events.
Nathan Caswell
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