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Lecture 7: Robustness (Crash (Reliability (Availability (Expectation value…

- - - - Mean time between failures
    - - Mean time to recover
    - - Expectation value for the service to be available
      - A[%] = MTBF / (MTBF + MTTR) * 100
- - - - receive out-of-band information about server's state
    - - Cannot distinguish (without knowledge)
- - - - Coordinated checkpoint
      - Message logging
      - independent checkpoint
- - - - 2PC:
        2 phases commit
        
        Phase 1b Upon receiving a vote-request every participant either returns to coordinator
        
        a vote-commit
        
        vote-abort and stops computation.
        
        Phase 2a Coordinator receives all votes
        
        if all are vote-commit answers, sends a
        global-commit
        
        otherwise sends a global-abort
        
        Phase 1a Coordinator sends a vote-request to participants.
        
        Phase 2b Each participant needs to receive a global-commit or a global-abort and acts accordingly.
        
        Transaction logging: all participants record
        
        Until finally it has received a global-commit or global-abort message
        
        Purrge: Garbage Collection
      - 3PC: 3 phases commit
        
        Phase 1a Coordinator sends a vote-request to participants.
        
        Phase 1b Upon receiving a vote-request a participant either returns to the coordinator:
        
        vote-commit
        
        vote-abort
        
        Phase 2a Coordinator receives all votes
        
        if all are vote-commit answers, sends a prepare-commit to all participants;
        
        otherwise sends a global-abort
        
        Phase 2b Each participant voted vote-commit now waits for:
        
        a global-abort (and halts)
        
        prepare-commit. It replies ready-commit and thus the coordinator knows the process is no longer uncertain.
        
        Phase 3a Prepare-to-commitC: Coordinator waits for all participants have sent ready-commit and answers global-commit.
        
        Phase 3b Prepare-to-commitP: Participants waits for global-commit and carry on.
        Receiving this, it knows that no other processes are uncertain and finally commits without violating None Blocking.
    - - Need to reach a unique decision (result) among the involved processes (nodes)
      - need robust procedure to find and commit this decision