UML activity diagram
UML activity diagrams are a kind of behavior diagram, i.e. is a kind of UML behavioral modeling. UML activity diagrams are somewhat similar UML state machine diagrams. Both are somewhat similar to Petri nets.
Activity modeling means to describe sequencing and conditions of actions. Such descriptions commonly are control flow and object flow models.
Simple Activity diagrams consist of:
- Initial node.
- Activity final node.
- Activities in between
<uml> Begin.start; Activity.eat("Eat something really good", "from the fridge"); Activity.read("Read a book"); End.finish;
topToBottom(20)(start, eat, read, finish);
drawObjects(start, eat, read, finish);
clink(transition)(start, eat); clink(transition)(eat, read); clink(transition)(read, finish); </uml>
“The starting point of the diagram is the initial node, and the activity final node is the ending. An activity diagram can have zero or more activity final nodes. In between activities are represented by rounded rectangles.” (Wikipedia, retrieved 11:08, 4 June 2008 (UTC).
3 Architecture of activity diagrams
3.1 Basic Nodes
According to Conrad Bock (2003), there there are three kinds of nodes in activity models:
- Action nodes operate on control and data values that they receive, and provide control and data to other actions.
- Control nodes route control and data tokens through the graph. These include constructs for choosing between alternative flows (decision points), for proceeding along multiple flows in parallel (forks), and so on.
- Object nodes hold data tokens temporarily as they wait to move through the graph. Below, the notation for some of the activity nodes to be discussed. Contrary to the names, control nodes coordinate both data flow and control flow in the graph, and object nodes can hold both objects and data
- Action nodes
<uml redraw> Activity.do("Do something nice"); drawObjects(do); </uml>
- Represented by a rectangle with rounded corners (drawn in slightly different ways depending on the software used ...)
- Action nodes should have a label
- Object nodes
- Represented by a rectangle with a label (i.e. UML class boxes)
<uml> EClass.obj(iClassNameOnly)("Blog entry")()(); drawObjects(obj); </uml>
- Decision and merge control node
- Represented by a lozange
<uml> Branch.test; drawObject(test); </uml>
- Fork and join control node
- Forks and joins are represented by a horizonal or vertical line (and incoming and outgoing flows)
- One or more activities can fork.
- One or more activities can join
Fork.forkA("h", 100); Fork.forkB("v", 20); leftToRight(10)(forkA, forkB); drawObjects(forkA, forkB);
- Initial node
- Represented by a fat black dot
- There can be only one initial (starting) node
<uml redraw> Begin.b; drawObject(b); </uml>
- Final nodes
- A fat black dot inside a circle (bull's eye symbol)
- A circle with an x represents the end of a flow (not the whole activity)
<uml redraw> End.e; FlowFinal.f; leftToRight(20)(e,f); drawObjects(e,f); </uml>
Below is a complete example (copied June/2008 from the MetaUML website):
User eats until he has had enough. Then he both reads a book and listens to some music, until he stops.
<uml> Begin.b; Activity.eat("Eat something good", "from the kitchen"); Branch.enough; Fork.fork("h", 50); Activity.read("Read a book"); Activity.listen("Listen to music", "(and ignore it)"); Fork.join("h", 50); End.e;
leftToRight.top(10)(read, listen); Group.readListen(read, listen);
leftToRight(30)(b, eat); topToBottom(20)(eat, enough, fork, readListen, join, e);
drawObjects(b, eat, enough, fork, readListen, join, e);
clink(transition)(b, eat); clink(transition)(eat, enough); link(transition)(pathStepX(enough.e, eat.e, 80)); clink(transition)(enough, fork); clink(transition)(fork, read); clink(transition)(fork, listen); clink(transition)(read, join); clink(transition)(listen, join); clink(transition)(join, e);
item(iGuard)("still hungry")(obj.sw = enough.e + (20, 0)); item(iGuard)("had enough")(obj.nw = enough.s + (0, -4)); </uml>
Flows (or edges) describe connections between 2 actions. These edges can be drawn with arrows in various ways.
Usually in activity diagrams, actions are simply connected through an unlabelled arrow. In addition, you also can include an object in the flow. These objects can carry data. Finally, instead of objects one can use "pins".
The control flow is modeled in terms of tokens. The start node will create a token which then goes to next action. After the action executes, the token will go to the next action. When it encounters a fork, the fork will create a token for each of its outbound flows. The opposite happens for joins. It will produce an outbound token once all inbound tokens arrived.
- Simple flows
- Simple flows are represented by an arrow from an activity showing parameters with pins from a node to another
<uml> Activity.A("Do A ..."); Activity.B("Do B ..."); leftToRight(20)(A,B); drawObjects(A,B); clink(transition)(A,B); </uml>
- Simple flows with connectors
An arrow to a connector (a small circle with a letter) and then from a same connector to a activity node does the same job as a simple arrow. (not shown here)
- Flows with objects
- Activity node to object node to activity node (with arrows). Object nodes
<uml> Activity.A("Write down impressions"); Class.obj("Blog entry")()(); Activity.B("Reflect on the written"); leftToRight(30)(A,obj,B); drawObjects(A,obj,B); clink(transition)(A,obj); clink(transition)(obj,B); </uml>
- Flows with pins
- These are similar to flows with objects. A pin represents data needed and data produced.
- The flow is represented by an arrow and a pin is a small rectangle added to an activity rounded rectangle (no picture here)
- Decision flows
- Outgoing arrows from decision nodes are usually labeled.
Actions in a diagram also can respond to signals (instead of the flow)
- Time signals
- Represented like an hour glass
- Input Accept signal (accept)
- Represented by a concave pentagon (a smashed-in rectangle)
- Output signal (send)
- Represented by a convex polygon
3.4 Partitions and subactivities
- Activities can be decomposed into subactivities. They can have input or output parameters.
- A rake in an action node signals a subactivity
- The subsidiary activity diagram has an input and an output parameter (object nodes)
- An activity can be partitioned in a swimlanes
- Each swimlane (partition) typically represents a player or role (e.g. learner A, learner B, group A, teacher). See below for some examples
4 Typical patterns
(stub section, needs to be completed).
- Accept Event Action
- An action that waits for the occurrence of an event meeting specified conditions
- Activity Final
- Data Store
- Decision Node
- Flow Final
- Fork Node
- Join Node
- Merge Node
- Object Node
- Send Signal Action
- Object Flow
5.1 A simple example
The following example taken from D7.2 UNFOLD outcomes 2 contains the following elements:
- A start node
- An end node
- Two join/break (the fat lines)
- Three activities
5.2 Educational examples
Activity diagrams can be used to describe learning designs of CSCL scenarios. A good example are collaborative learning flow pattern (Hernández-Leo et al., 2005b).
Below some more pictures that show that activity diagrams are popular withing the IMS Learning Design community.
The next example shows a diagram for competency-based learning with two major alternatives, advising-then-anticipating and anticipating-then-advising. (IMS LD Best Practice specification)
- Summaries of UML activity diagrams
- Introductions to activity diagrams
- UML 2 Activity Diagrams, and UML 2 Activity Diagram Guidelines by Scott W. Ambler, Ambysoft.
- UML 2 Activity and Action Models by Conrad Bock, U.S. National Institute of Standards and Technology
- Activity Diagram, UML 2 Diagrams, Visual Paradigm Gallery.
- Poseidon user guide - Activity diagram (Gentleware)
- Unified Modeling Language (UML), version 2.1.2. There are two specifications that comprise the UML 2.1.2 specification: Superstructure and Infrastructure. There are also two specifications that relate to the UML2 specification (Diagram Interchange and Object Constraint Language).
- Bock, Conrad (2003). "UML 2 Activity and Action Models", Journal of Object Technology, vol. 2, no. 4, July-August 2003, pp. 43-53. http://www.jot.fm/issues/issue_2003_07/column3
- Fowler, M. (2000). UML distilled (3rd ed.). Upper Saddle River, NJ: Addison-Wesley. ISBN 0321193687
- Hernández-Leo, D., Asensio-Pérez, J.I., Dimitriadis, Y., Bote-Lorenzo, M.L., Jorrín-Abellán, I.M., Villasclaras-Fernández, E.D. (2005b). Reusing IMS-LD Formalized Best Practices in Collaborative Learning Structuring. Advanced Technology for Learning 2(4):223-232. http://dx.doi.org/10.2316/Journal.208.2005.4.208-0865 - PDF
- IMS Global Learning consortium (2003). IMS Learning Design Best Practice and Implementation Guide, HTML, retrieved 11:08, 4 June 2008 (UTC).