Lecture 4: Activity Diagrams

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CS2341

• Lecture 4 Activity Diagrams
• Robert Stevens
• http//www.cs.man.ac.uk/stevensr

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User Requirements and Business Process

• User requirements interviews conveys (or implies)
  the main processes within the problem domain
• Processes are visible from the business viewpoint
• Are important at all levels of system development
• The system is a model of the business process

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On Processes

• Processes are problematic too
• Many are non-deterministic
• Many are dynamic - they change rapidly
• They often involve humans
• Difficult to gather
• User interface is an important aspect
• The system model can change the process

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Process is...

• Structured change, i.e. there is a pattern of
  events which an observer may recognise across
  different actual examples of the process
• A specific ordering of work activities across
  time and place a structure for action.
  Davenport, 1993
• A bundle of goal directed actions that are
  performed by actors. Bauer et al, 1993

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On Processes

• In general a process should only exist in order
  to meet some goal, when you identify a process
  try to identify a quantifiable goal as well
• Then identify the Roles which are to be played by
  actors in order to achieve that goal
• Your interviews have to uncover actors,
  activities and their performance

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Representing Process

• Many different types of process, both natural
  (e.g. fossilisation) and artificial (e.g.
  articulated by some agent to effect a
  transformation)
• Notations for capturing models of processes
• Many different types of representation
• narrative, mathematics especially process
  algebras
• diagrammatic techniques
• DFDs, RADs, UML Activity Diagrams
• software enactment Simulations

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Process and UML

• Activity Diagrams
• designed to be a simplified look at what happens
  during a process
• highlights the activities within a process
• First stage in capturing what the system will
  model
• Embarking on a progression from real world to
  system world

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Activity Diagrams

• Are an extension of state diagrams
• State diagrams highlight states and represent
  activities as arrows between states
• Activity diagrams highlight activities
• Need to represent
• Indicate start and end
• Actors and activities
• Different roles
• Decisions and transitions
• Branches and concurrency

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Activity Diagrams I

• Each activity is represented by a rounded
  rectangle
• The processing within an activity completes and
  then an automatic transition to the next activity
  occurs.

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Activity Diagrams II

• An arrow represents the transition from one
  activity to the next.
• The starting point of an activity diagram is
  represented by a filled-in circle.
• The end point is represented by a bullseye

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Basic Flows
Thus
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Decisions

• Most processes come to a point where a choice
  need to be made
• This is known as a decision point, one path leads
  to one set of activities, the other to another
  set.
• These two paths are mutually exclusive.

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Decision points I
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Concurrency

• Clearly some processes possess concurrent rather
  than alternative paths
• Such paths run at the same time (ie are
  concurrent) and then come together again
• The split and merge are represented as solid bold
  horizontal lines

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Concurrency II
Thus
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Signals

• During a sequence of activities, it is possible
  to send a signal. When received, the signal
  causes activities to take place.
• The symbol for sending a signal is a convex
  pentagon
• The symbol for receiving a signal is a concave
  polygon
• Indicate input and output events
• When received activity takes place

Order a drink
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Signals
Thus
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Swimlanes I

• In fact in the previous example we really now
  need to introduce a new concept
• The signal has, as a side effect, introduced the
  notion of two actors in this process
• the poor student
• the rich friend
• We often need this concept of ROLES

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Swimlanes II

• The activity diagram adds this dimension of
  visualising roles
• To do this, you separate the diagram into
  parallel lanes called swimlanes
• Each swimlane shows the name of the role at the
  top, and presents the activities of each role.
• Transitions can take place between swimlanes

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Swimlanes III
Thus

Poor student Rich Friend
Call for a drink
Pay for a drink
Enjoy free drink
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Activity Diagrams

• Can be used at all stages of development since
  they support the notion of hybrid diagrams
• In general we can use any sensible UML symbol in
  an activity diagram
• You can even show an activity diagram for a
  process inside an object symbol

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Activity diagrams

• We will be using them as a means of capturing the
  business processes associated with the travel
  shop
• A model of the travel shop in the real world
• But beware .. We are at this stage only trying
  to understand the business process
• We do not want to produce a flowchart of a
  software algorithm
• Used to check understanding with the client
• Use during the user requirements sessions

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Scenario Definition

• User requirements session to establish the
  context for the system
• The results of the interactive session with the
  client are recorded in Activity Diagrams which
  model
• The basic business processes
• By the use of swimlanes, the main roles that are
  associated with these processes

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Whats been achieved?

• Nothing so far has referred specifically to the
  proposed computer system
• We are understanding the business and the problem
  domain
• Only when this is completed shall we turn our
  attention to gathering system requirements and
  the envisioning of the proposed computer system
• Process model to system model