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distributed system (Fault Tolerance (Phases (Fault Detection, Fault…

- - - - measurement of whether a system is ready to be used immediately
    - - Prevents any unauthorized access
    - - measurement of whether a system can run continuously without failure
  - - - Creating multiple copies or replica of data items and storing them at different sites
      - Can be used only for deterministic processes.
      - If one site goes down, data is accessed from other so that performance in not affected
      - limitations
        
        Data consistency(Data should match)
        
        Degree of Replica(Exact Numbers of Replica).
      - Types
        
        Active Replication :
        
        Client’s request is processed by all the servers
        
        Requires an atomic broadcast protocol to forward the requests to all the servers in the same order
        
        Can be used only for deterministic processes
        
        Passive Replication
        
        Can be used for non deterministic processes also
        
        There is only one server that processes client’s request known as primary server
        
        Other servers act as back up servers
        
        Response time is high as there is only one server which process many client’s request
    - - Saving the state of a system when they are in a consistent state and storing it on a stable storage.
      - Types
        
        Uncoordinated Check pointing
        
        Coordinated Check points
        
        User Triggered
        
        Message Based Check pointing
- - - - Timing Failure
        
        Occurs when a server fails to respond within a particular amount of time.
      - Response failure
        
        Occurs when a server sends incorrect message in response to the client’s message.
      - Omission Failure
        
        Occurs when a server does not receive incoming requests from client or fails to send messages in response to clients request.
      - Crash Failure
        
        Occurs when a server crashes or any other hardware related problem occurs
  - - - Intermittent
      - Permanent
      - Transient