Activity Diagrams

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Activity Diagrams

• in UML

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Definition

• Activity diagrams represent the dynamics of the
  system.
• They are flow charts that are used to show the
  workflow of a system.
• They show.
• The flow of control from activity to activity in
  the system,
• What activities can be done in parallel.
• Alternate paths through the flow.
• They can show the flow across use cases or within
  a use case.

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Activity Diagram

• Core symbol is an activity.
• An activity is some task which needs to be done.
• Each activity can be followed by another activity
  (sequencing).
• Triggers from the activity may be guarded as in
  state diagrams.

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Decision Activities

• Diamond.
• Each trigger coming from it has a guard.
• Synchronisation bar.
• All triggers from this attach to activities that
  can occur in parallel, with no specific sequence,
  or concurrently.
• The next synchronisation bar closes the
  concurrency.
• Iteration is represented by a on the trigger.

or
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Activity Diagrams for Use Cases

• They can be used for describing either
• Use cases or
• Complicated methods

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no coffee
no cola
Find Beverage
found coffee
Add water to reservoir
Put coffee in filter
Get cups
Get can of cola
Put filter in machine
Turn on machine
Brew coffee
Drink Beverage
Pour coffee
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Use Case for Order Processing (Generic system)

• When we receive an order, we check each line
  item on the order to see if we have the goods in
  stock. If we do, we assign the goods to the
  order. If this assignment sends the quantity of
  those goods inn stock below the reorder level, we
  reorder the goods. While we are doing this, we
  check to see if the payment is OK. If the
  payment is OK and we have the goods in stock, we
  dispatch the order. If the payment is OK but we
  dont have the goods, we leave the order waiting.
  If the payment isnt OK, we cancel the order.

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Combining Use Cases

• The end point of an activity diagram is the point
  at which all triggered activites have been run
  and there are no more left to do.
• Dead ends (e.g. Reorder item) can occur.
• Sometimes dead ends meet up with other use cases
  (e.g. Check line item).

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Receiving Supply

• When a supply delivery comes in, we look at the
  outstanding orders and decide which ones we can
  fill from this incoming supply. We then assign
  each of these to its appropriate orders. Doing
  this may release those orders for dispatching.
  We put the remaining goods into stock.

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Drawback

• Activity diagrams tell you what is happening, but
  not who does what.
• In domain modeling, this diagram type does not
  convey which people or departments are
  responsible for each activity.
• In programming, it does not convey which class is
  responsible for each activity.

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Swimlanes

• Arrange activity diagrams into vertical zones
  separated by dashed lines.
• Each zone represents the responsibilities of a
  particular class or department.

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When to Use Activity Diagrams

• Do use them for
• Analysing Use Cases.
• Understanding workflow across many Use Cases.
• Dealing with multi-threaded applications.
• Dont use them
• to see how objects collaborate.
• to see how an object behaves over its lifetime.

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Activity Diagram for Receiving an Order
Receive order
for each line item on order
Check line item
Authorise Payment
failed
in stock
Cancel Order
succeeded
Assign to Order
need to reorder
Reorder item
stock assigned to all line items and
payment authorised
Dispatch Order
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Activity Diagram for receiving Supply
Receive Supply
Choose outstanding order items
for each chosen order item
Assign goods to order
all outstanding order items filled
Add remainder to stock
Dispatch Order
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Combined Activity Diagram
Receive order
Receive Supply
for each line item on order
Choose outstanding order items
Check line item
Authorise Payment
in stock
for each chosen order item
failed
succeeded
Assign to Order
Assign goods to order
Cancel Order
need to reorder
Reorder item
stock assigned to all line items and
payment authorised
all outstanding order items filled
Dispatch Order
Add remainder to stock
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With Swimlanes
Stock Manager
Order Processing
Finance
Receive order
Receive Supply
for each line item on order
Choose outstanding order items
Check line item
Authorise Payment
in stock
for each chosen order item
failed
succeeded
Assign to Order
Assign goods to order
Cancel Order
need to reorder
Reorder item
stock assigned to all line items and
payment authorised
all outstanding order items filled
Dispatch Order
Add remainder to stock
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Understanding Workflows

• Each activity represents the performance of a
  group of actions in a workflow.
• Once the activity is complete, the flow of
  control moves to the next activity or state
  through a transition.
• If an outgoing transition is not clearly
  triggered by an event, then it is triggered by
  the completion of the contained actions inside
  the activity.
• A unique activity diagram feature is a swimlane
  that defines who or what is responsible for
  carrying out the activity or state.
• It is also possible to place objects on activity
  diagrams.
• The workflow stops when a transition reaches an
  end state.

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Activity Diagram Tools

• You can use the following tools on the activity
  diagram toolbox to model activity diagrams
• Activities
• Decisions
• Object
• Object Flow
• States
• Start State
• End State
• Swimlanes
• Synchronizations
• Transitions

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Placing an Activity diagram

• Where?
• You can attach activity diagrams to most model
  elements in the use case or logical views.
• Activity diagrams cannot reside within the
  component view.
• Why?
• Very effective in illustrating the workflow of
  various events in a use-case diagram.
• You can use activity diagrams to specify and
  define each event in a use-case diagram.

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Creating an activity diagram

• Modelling a workflow in an activity diagram
• Identify a workflow objective.
• Decide the pre and post-conditions of the
  workflow.
• Define all activities and states.
• Define any objects that are created or modified.
• Decide on responsibility for performing the
  activities.
• Connect all elements on the diagram with
  transitions.
• Place decisions on the diagram.
• Evaluate your diagram for concurrent workflows.
• Set all actions, triggers and guard conditions in
  the specifications of each model element.

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Identify a workflow objective.

• "What needs to take place or happen by the end of
  the workflow? What needs to be accomplished?
• For example, if your activity diagram models the
  workflow of ordering a book from an online
  bookstore, the goal of the entire workflow could
  be getting the book to the customer.
• What is the goal in the claims system?
• What are the goals in the tennis club?

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2. Workflow pre and post-conditions

• Define pre and post conditions of the workflow
  through a start state and an end state.
• In most cases, activity diagrams have a
  flowchart structure so start and end states are
  used to designate the beginning and ending of the
  workflow.
• Start and end states clarify the perimeter of the
  workflow.
• What is the pre-condition of the main workflow in
  
• The claims system?
• The tennis club?

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Define activities and states

• Define and recognize all activities and states
  that must take place to meet your objective.
  Place and name them on the activity diagram in a
  logical order.
• Are there intermediate states in the claims /
  tennis club systems?

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Identify persistent object operations

• Define and diagram any objects that are created
  or modified within your activity diagram.
• Connect the objects and activities with object
  flows.

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Add swimlanes

• Decide who or what is responsible for performing
  the activities and states through swimlanes.
• Name each swimlane and place the appropriate
  activities and states within each swimlane.
• Identify swimlanes for
• The tennis club workflow
• The claims workflow

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Finish diagram

• Connect all elements on the diagram with
  transitions.
• Place decisions on the diagram where the workflow
  may split into an alternate flow.
• E.g. based on a Boolean expression, the workflow
  could branch to a different workflow.
• Evaluate your diagram and see if you have any
  concurrent workflows. If so, use
  synchronizations to represent forking and
  joining.
• Set all actions, triggers and guard conditions in
  the specifications of each model element.