System Analysis and Design

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System Analysis and Design

• Advanced Modeling

With notes from Kostas Kontogiannis
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More on state machines
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Basic UML Statechart Diagram
top state
State
Initial pseudostate
Trigger
Healthy
Guard
Transition
t 39/print(sick)
coughing
Sick
Final state
Action
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Execution semantic of a state machine
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Processes and Threads

• Active object
• Owns a process/thread.
• Can initiate control activity.
• Process
• Independent address space.
• Execute concurrently with other processes.
• Threads
• Hidden inside a process.
• Not independently scheduled by OS.

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Object and Threads

• Passive objects depend on external power (thread
  of execution)
• Active objects self-powered (own thread of
  execution)

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The Run-to-Completion Model

• A high priority event for (another) active object
  will preempt an active object that is handling a
  low-priority event

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Passive Objects Dynamic Semantics

• Encapsulation does not protect the object from
  concurrency conflicts!
• Explicit synchronization is still required

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Synchronization

• Active gt passive object communication lead to
  multiple flows of control in one object at the
  same time.
• State corruption may occur ? need for mutual
  exclusion.
• Treat an object as a critical region.
• Use synchronization to implement critical regions

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More on Activities
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Object Flow
Class State

• A special sort of step that represents the
  availability of a particular kind of object,
  perhaps in a particular state.
• No action or activity is invoked and control
  passes immediately to the next step.
• Places constraints on input and output parameters
  of steps before and after it.

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Object Flow

• Take Order must have an output parameter giving
  an order, or one of its subtypes.
• Fill Order must have an input parameter taking an
  order, or one of its supertypes.
• Dashed lines used with object flow have the same
  semantics as control flow

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Coordinating Steps

• Decision point and merge ( )
• Decision chooses between different flows
• Merge waits for one flow to arrive

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Convenience Features

• Forks and joins implies parallelism and
  synchronization
• Joins waits for all flows to arrive (sync)

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Convenience Features

• Fork transitions can have guards.

• Instead of doing this

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Convenience Features

• Partitions are a grouping mechanism.
• Swimlanes are the notation for partitions
• They may represent an object or set of object
  performing the contained actions.
• They do not provide domain-specific semantics.
• Tools can generate swimlane view from
  domain-specific information without partitions.

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Convenience Features
Signal

• Signal send icon

• translates to a transition with a send action.

• Signal receipt icon

• translates to a wait state (a state with no
  action and a signal trigger event).

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When to Use Activity Diagrams

• Use activity diagrams when the behavior you are
  modeling ...
• does not depend much on external events.
• mostly has steps that run to completion, rather
  than being interrupted by events.
• requires object/data flow between steps.
• is being constructed at a stage when you are more
  concerned with which activities happen, rather
  than which objects are responsible for them
  (except partitions possibly).

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More on UML..

• http//www.uml.org/