TX2 The Two Phase Commit Protocol in Java

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When a transaction is created using create, the transaction is ACTIVE This is the only state in which participants may join the transaction Attempting to join the transaction in any other state throws a CannotJoinException Invoking the manager's commit method causes the manager to move to the VOTING state, in which it attempts to complete the transaction by rolling forward Each participant that has joined the transaction has its prepare method invoked to vote on the outcome of the transaction The participant may return one of three votes: NOTCHANGED, ABORTED, or COMMITTED If a participant votes ABORTED, the manager must abort the transaction If prepare throws UnknownTransactionException or NoSuchObjectException, the participant has lost its state of the transaction, and the manager must abort the transaction If prepare throws RemoteException, the manager may retry as long as it wishes until it decides to abort the transaction To abort the transaction, the manager moves to the ABORTED state In the ABORTED state, the manager should invoke abort on all participants that have voted PREPARED The manager should also attempt to invoke abort on all participants on which it has not yet invoked prepare These notifications are not strictly necessary for the one parameter forms of commit and abort, since the participants will eventually abort the transaction either by timing out or by asking the manager for the state of the transaction However, informing the participants of the abort can speed up the release of resources in these participants, and so attempting the notification is strongly encouraged If a participant votes NOTCHANGED, it is dropped from the list of participants, and no further communication will ensue If all participants vote NOTCHANGED then the entire transaction was read only and no participant has any changes to roll forward The transaction moves to the COMMITTED state and then can immediately move to cleanup, in which resources in the manager are cleaned up There is no behavioral difference to a participant between a NOTCHANGED transaction and one that has completed the notification phase of the COMMITTED state If no participant votes ABORTED and at least one participant votes PREPARED, the transaction also moves to the COMMITTED state In the COMMITTED state the manager must notify each participant that returned PREPARED to roll forward by invoking the participant's commit method When the participant's commit method returns normally, the participant has rolled forward successfully and the manager need not invoke commit on it again As long as there exists at least one participant that has not rolled forward successfully, the manager must preserve the state of the transaction and repeat attempts to invoke commit at reasonable intervals If a participant's commit method throws UnknownTransactionException, this means that the participant has already successfully rolled the transaction forward even though the manager did not receive the notification, either due to a network failure on a previous invocation that was actually successful or because the participant called getState directly If the transaction is a nested one and the manager is prepared to roll the transaction forward, the members of the nested transaction must become members of the parent transaction This promotion of participants into the parent manager must be atomic all must be promoted simultaneously, or none must be The multi participant promote method is designed for this use in the case in which the parent and nested transactions have different managers
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TX2 The Two Phase Commit Protocol
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O'Reilly Network Safari Bookshelf Jini Specifications, The, Second Edition The promote method takes arrays of participants and crash counts, where crashCounts[i] is the crash count for parts[i] If any crash count is different from a crash count that is already known to the parent transaction manager, the parent manager throws CrashCountException and the parent transaction must abort The drop parameter allows the nested transaction manager to drop itself out of the parent transaction as it promotes its participants into the parent transaction if it no longer has any need to be a participant itself The manager for the nested transaction should remain available until it has successfully driven each participant to completion and promoted its participants into the parent transaction If the nested transaction's manager disappears before a participant is positively informed of the transaction's completion, that participant will not know whether to roll forward or back, forcing it to vote ABORTED in the parent transaction The manager may cease commit invocations on its participants if any parent transaction is aborted Aborting any transaction implicitly aborts any uncommitted nested transactions Additionally, since any committed nested transaction will also have its results dropped, any actions taken on behalf of that transaction can be abandoned Invoking the manager's abort method, cancelling the transaction's lease, or allowing the lease to expire also moves the transaction to the ABORTED state as described above Any transactions nested inside that transaction are also moved directly to the ABORTED state The manager may optimize the VOTING state by invoking a participant's prepareAndCommit method if the transaction has only one participant that has not yet been asked to vote and all previous participants have returned NOTCHANGED (Note that this includes the special case in which the transaction has exactly one participant) If the manager receives an ABORTED result from prepareAndCommit, it proceeds to the ABORTED state In effect, a prepareAndCommit moves through the VOTING state straight to operating on the results A getState call on the manager can return any of ACTIVE, VOTING, ABORTED, NOTCHANGED, or COMMITTED A manager is permitted, but not required, to return NOTCHANGED if it is in the COMMITTED state and all participants voted NOTCHANGED TX28 Crash Recovery Crash recovery ensures that a top level transaction will consistently abort or roll forward in the face of a system crash Nested transactions are not involved The manager has one commit point, where it must save state in a durable fashion This is when it enters the COMMITTED state with at least one PREPARED participant The manager must, at this point, commit the list of PREPARED participants into durable storage This storage must persist until all PREPARED participants successfully roll forward A manager may choose to also store the list of PREPARED participants that have already successfully rolled forward or to rewrite the list of PREPARED participants as it shrinks, but this optimization is not required (although it is recommended as good citizenship) In the event of a manager crash, the list of participants must be recovered, and the manager must continue acting in the COMMITTED state until it can successfully notify all PREPARED participants The participant also has one commit point, which is prior to voting PREPARED When it votes PREPARED, the participant must have durably recorded the record of changes necessary to successfully roll forward in the event of a future invocation of commit by the manager It can remove this record when it is prepared to successfully return from commit Because of these commitments, manager and participant implementations should use durable forms of RMI references, such as the Activatable references introduced in the Java 2 platform An unreachable manager causes much havoc and should be avoided as much as possible A vanished PREPARED participant puts a transaction in an untenable permanent state in which some, but not all, of the participants have rolled forward
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