Distributed System Characteristics

Common Characteristics

Certain common characteristics can be used to assess distributed systems

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

Resource Sharing

• Ability to use any hardware, software or data anywhere in the system.
• Resource manager controls access, provides naming scheme and controls concurrency.
• Resource sharing model (e.g. client/server or object-based) describing how

• resources are provided,
  • they are used and
  • provider and user interact with each other.

Openness

• Openness is concerned with extensions and improvements of distributed systems.
  • Detailed interfaces of components need to be published.
  • New components have to be integrated with existing components.
  • Differences in data representation of interface types on different processors (of different vendors) have to be resolved.

Concurrency

Components in distributed systems are executed in concurrent processes.

• Components access and update shared resources (e.g. variables, databases, device drivers).
  • Integrity of the system may be violated if concurrent updates are not coordinated.
    • Lost updates
    • Inconsistent analysis

Scalability

• Adaption of distributed systems to
• accomodate more users
• respond faster (this is the hard one)
  • Usually done by adding more and/or faster processors.
  • Components should not need to be changed when scale of a system increases.
  • Design components to be scalable

Fault Tolerance

Hardware, software and networks fail!

• Distributed systems must maintain availability even at low levels of hardware/software/network reliability.
• Fault tolerance is achieved by

• recovery
• redundancy

Transparency

Distributed systems should be perceived by users and application programmers as a whole rather than as a collection of cooperating components.

• Transparency has different dimensions that were identified by ANSA.
• These represent various properties that distributed systems should have.