Nested two phase commit protocol:
Several optimizations of the two-phase commit protocol are possible. As described above, commit needs 4N messages if there are N participants. The coordinator invokes every participant once to take the vote and once to broadcast the result. If invocation as well as return are expensive (example go over thin wires) then a more economical protocol may be desired.
If the participants are able to be linearly ordered then a simpler as well as faster commit protocol that has 2N calls and returns is possible. This protocol is described the nested two-phase commit. The protocol works as follows-
a) Every participant is given a sequence number in the commit call order.b) Particularly each participant knows the name of the next participant as well as the last participant knows that he is the last.
Commit consists of participants consecutively calling one another (N-l calls) after performing phase 1 commit. At the end of the calling sequence every participant will have successfully completed phase 1 or else some participant will have broken the call chain. Therefore the last participant can perform phase 2 and returns success. Every participant keeps this up so that in the end there are N-l returns to give a grand total of 2(N-1) calls as well as returns on a successful commit. There is one last call necessary to signal the coordinator (last participant)that the commit completed thus that restart can ignore redoing this transaction. If some participant doesn’t succeed in phase 1 then he issues abort as well as transaction undo is started.
The following is the algorithm of every participant:
COMMIT: PROCEDURE;PERFORM PHASE 1 COMMIT;IF FAIL THEN RETURN FAILURE;IF I_AM_LAST THENWRITE_LOG(PHASE12)FORCE;ELSEDO;CALL COMMIT(I+l);IF FAIL THENDO;ABORT;RETURN FAILURE;END;END;PERFORM PHASE 2 COMMIT;IF I_AM_FIRST THENINFORM LAST THAT COMMIT COMPLETED;RETURN SUCCESS;END;
The following gives a depiction of a three deep nest:
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