Introduction To Distributed System

Distributed System:
       A collection of autonomous computer system,

• Linked By Networks
• Using software to produce an integrated computer system

Size of Distributed System:

• Local Area Network (10's of hosts) 
• Metropolitan Area Networks (100's of hosts) 
• Wide Area Networks (internet) (1000's or 1,000,000's of hosts) 

Example of Distributed System:

• Distributed Unix
• WAN application
• eMail
• BBS- Bulletin Board Systems
• Gopher - Text Retrieval Service
• WWW - World Wide Web
• Multimedia Information Access and Conferencing Applications
• Banking
• Sensor Networks

Key Characteristics of Distributed System:

• Resource sharing
• Openness
• Concurrency
• Fault Tolerance
• Transparency
• Scalability

Resource Sharing:

1. Resource:
             Hardware - disks and printers,
   
   Software- files, windows, and data objects
2. Data sharing for
   Consistency - compiler's and libraries
   Exchange of information - databases
   Cooperative work - groupware
3. Hardware sharing for
   Convenience
   Reduction of cost

Openness: Open or Closed with Respect To Hardware and Software.

        

    UNIX is a relatively open operating system

          C language readily available

System calls documented

New hardware drivers were easy to add

Applications were hardware independent

IPC allowed extension of services and resources

Concurrency: 

Multi-programming
Multi-processing

Parallel executions in distributed systems

1. Many users using the same resources, application interactions
2. Many servers responding to client requests

Fault Tolerance: 

      Computers fail therefore we need: hardware redundancy software recovery

      Program recovery via the process group

Transparency: Transparency of

• access
• location
• concurrency
• replication
• failure
• migration
• performance
• scaling

Scalability: 

• software should not change to support growth 
• research area - for large, high-performance networks