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The 2PC protocol responds in differenct ways to various types of failures: Failure of a participating site If the coordinator Ci detects that a site has failed, it takes these actions: If the site fails before responding with a ready T message to Ci , the coordinator assumes that it responded with an abort T message If the site fails after the coordinator has received the ready T message from the site, the coordinator executes the rest of the commit protocol in the normal fashion, ignoring the failure of the site When a participating site Sk recovers from a failure, it must examine its log to determine the fate of those transactions that were in the midst of execution when the failure occurred Let T be one such transaction We consider each of the possible cases: The log contains a <commit T> record In this case, the site executes redo(T) The log contains an <abort T> record In this case, the site executes undo(T) The log contains a <ready T> record In this case, the site must consult Ci to determine the fate of T If Ci is up, it noti es Sk regarding whether T committed or aborted In the former case, it executes redo(T); in the latter case, it executes undo(T) If Ci is down, Sk must try to nd the fate of T from other sites It does so by sending a querystatus T message to all the sites in the system On receiving such a message, a site must consult its log to determine whether T has executed there, and if T has, whether T committed or aborted It then noti es Sk about this outcome If no site has the appropriate information (that is, whether T committed or aborted), then Sk can neither abort nor commit T The decision concerning T is
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postponed until Sk can obtain the needed information Thus, Sk must periodically resend the querystatus message to the other sites It continues to do so until a site that contains the needed information recovers Note that the site at which Ci resides always has the needed information The log contains no control records (abort, commit, ready) concerning T Thus, we know that Sk failed before responding to the prepare T message from Ci Since the failure of Sk precludes the sending of such a response, by our algorithm Ci must abort T Hence, Sk must execute undo(T) Failure of the coordinator If the coordinator fails in the midst of the execution of the commit protocol for transaction T, then the participating sites must decide the fate of T We shall see that, in certain cases, the participating sites cannot decide whether to commit or abort T, and therefore these sites must wait for the recovery of the failed coordinator If an active site contains a <commit T> record in its log, then T must be committed If an active site contains an <abort T> record in its log, then T must be aborted If some active site does not contain a <ready T> record in its log, then the failed coordinator Ci cannot have decided to commit T, because a site that does not have a <ready T> record in its log cannot have sent a ready T message to Ci However, the coordinator may have decided to abort T, but not to commit T Rather than wait for Ci to recover, it is preferable to abort T If none of the preceding cases holds, then all active sites must have a <ready T> record in their logs, but no additional control records (such as <abort T> or <commit T>) Since the coordinator has failed, it is impossible to determine whether a decision has been made, and if one has, what that decision is, until the coordinator recovers Thus, the active sites must wait for Ci to recover Since the fate of T remains in doubt, T may continue to hold system resources For example, if locking is used, T may hold locks on data at active sites Such a situation is undesirable, because it may be hours or days before Ci is again active During this time, other transactions may be forced to wait for T As a result, data items may be unavailable not only on the failed site (Ci ), but on active sites as well This situation is called the blocking problem, because T is blocked pending the recovery of site Ci Network partition When a network partitions, two possibilities exist: 1 The coordinator and all its participants remain in one partition In this case, the failure has no effect on the commit protocol 2 The coordinator and its participants belong to several partitions From the viewpoint of the sites in one of the partitions, it appears that the sites in other partitions have failed Sites that are not in the partition containing the coordinator simply execute the protocol to deal with failure of the coordinator The coordinator and the sites that are in the same partition as
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