FAULT TOLERANCE

• Being fault tolerant is strongly related to what are called dependable systems.

• Dependability covers a number of useful requirements for distributed systems including the following :

  • Availability
  • Reliability
  • Safety
  • Maintainability

SCALABILITY

• Scalability refers to the capability of a system to handle a growing amount of work or

• It's the potential to perform more total work in the same elapsed time when processing power is expanded to accommodate growth.

A system is said to be scalable if it can increase its workload and throughput when additional resources are added.

OPENNESS

The property of a system to work with a number and sort of components that is not set once and for all at design time.These system are fundementally unpredicatable.

RELIABILITY

• Reliability refers to the property that a system can run continuously without failure.

This means that : -

• reliability is defined in terms of a time interval instead of an instant in time.
• A highly-reliable system is one that will most likely continue to work without interruption during a relatively long period of time.

For example

• If a system goes down for one millisecond every hour, it has an availability of over 99.9999 percent, but is still highly unreliable.

TRANSPARENCY

• Transparency is defined as the concealment from the user and the application programmer of the separation of components in a distributed system, so that the system is perceived as a whole rather than as a collection of independent components.

• Eight forms of transparency:

  • Access transparency

  • Location transparency

  • Concurrency transparency

  • Replication transparency

  • Failure transparency

  • Mobility transparency

  • Performance transparency

  • Scaling transparency

Transparency in Distributed Systems

• Access transparency: enables local and remote resources to be accessed using identical operations.

• Location transparency: enables resources to be accessed without knowledge of their physical or network location (for example, which building or IP address).

• Failure transparency: enables the concealment of faults, allowing users and application programs to complete their tasks despite the failure of hardware or software components.

• Mobility transparency: allows the movement of resources and clients within a system without affecting the operation of users or programs.

• Performance transparency: allows the system to be reconfigured to improve performance as loads vary. Scaling transparency: allows the system and applications to expand in scale without change to the system structure or the application algorithms.

• Concurrency transparency: enables several processes to operate concurrently using shared resources without interference between them.

• Replication transparency: enables multiple instances of resources to be used to increase reliability and performance without knowledge of the replicas by users or application programmers.