Process and Threads
- A heavyweight flow, which executes concurrently with other processes is known as 'Process'
- A lightweight flow which executes concurrently with other threads within the same process is known as 'Thread'
- An object which owns a process or thread and initiate control activity is known as 'Active Object'
- A class whose instances are active objects is known as 'Active Class'
- Graphically, active class is represented as a rectangle with thick lines and, processes and threads are represented as stereotyped active classes
- Flow of Control
- Classes and Events
- Communication
- Synchronization
Flow of Control
- In a sequential system, there is a single flow of control. i.e., one thing, and one thing only, can take place at a time
- In a concurrent system, there is multiple simultaneous flow of control i.e., more than one thing can take place at a time
Classes and Events
- Active classes are just classes which represents an independent flow of control
- Same properties are shared as like other classes
- Flow of control starts when an active object is created and it terminates when destroyed
- Stereotypes of active classes are
«process»
«thread»
- All the threads that live in the context of a process are peers of one another
Communication
- The four possible combinations of interaction with both active and passive objects are
- A message may be passed from one passive object to another
- A message may be passed from one active object to another
- A message may be passed from an active object to a passive object
- A message may be passed from a passive object to an active one
- In inter-process communication there are two possible styles of communication
One active object might synchronously call an operation of another
One active object might asynchronously send a signal or call an operation of another object
- Full arrow represents synchronous message and half arrow represents asynchronous message
Synchronization
- Synchronization means arranging the flow of controls of objects so that mutual exclusion will be guaranteed
- In object-oriented systems these objects are treated as a critical region
- The three methods to handle synchronization are
Guarded - Multiple flow of control is sequentialized with the help of object's guarded operations. Therefore, it becomes sequential
Concurrent - Multiple flow of control is guaranteed by treating each operation as atomic
- Synchronization is represented in the operations of active classes with the help of constraints
Time and Space
- Timing mark is a denotation for the time at which an event occurs
- Graphically represented as a small hash mark (horizontal line) on the border of a sequence diagram as shown in image
- A time expression evaluates to an absolute or relative value of time
Time
- Passing of messages represents the dynamic aspect of a system
- Messages in an interaction are represented with signal or a call
- The three functions of messages are
- sendTime
- receiveTime
- transmissionTime
Location
- In distributed systems, components are fixed at the time when loaded
- Components may migrate from node to node in other systems
- Artifacts are used to manifest components and classes
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