Common Mechanisms of UML

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 

1. A message may be passed from one passive object to another 
2. A message may be passed from one active object to another 
3. A message may be passed from an active object to a passive object 
4. 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 

Sequential - Callers must coordinate outside the object so that only one flow is in the object at a time
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 

1. sendTime 
2. receiveTime
3. 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