In a decentralized variant of the two-phase commit protocol the participants communicate directly with one another instead of indirectly via the coordinator. In Phase 1, the coordinator sends its vote to all the participants. In Phase 2, if the coordinator's vote is No, the participants just abort the transaction; if it is Yes, each participant sends its vote to the coordinator and the other participants, each of which decides on the outcome according to the vote and carries it out. Calculate the number of messages and the number of rounds it takes. What are its advantages or disadvantages in comparison with the centralized variant?
What will be an ideal response?
In both cases, we consider the normal case with no time outs. In the decentralised version of the two-phase commit protocol:
No of messages:
Phase 1: coordinator sends its vote to N workers = N
Phase 2: each of N workers sends its vote to (N-1) other workers + coordinator = N*(N - 1). Total = N*N.
No. of rounds:
coordinator to workers + workers to others = 2 rounds.
Advantages: the number of rounds is less than for normal two-phase commit protocol which requires 3. Disadvantages: the number of messages is far more: N*N instead of 3N.
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