In transaction processing, databases, and computer networking, the two-phase commit protocol (2PC) is a type of an atomic commitment protocol. It is a distributed algorithm that coordinates all the processes that participate in a distributed atomic transaction on whether to commit or abort (roll back) the transaction. The protocol achieves its goal even in many cases of system failure (involving either process, network node, communication, etc. failures), and is thus widely utilized.[1][2] However, it is not resilient to all possible failure configurations, and in rare cases user intervention is needed to remedy outcome.
The two phases of the algorithm are the commit-request phase, in which a coordinator process attempts to prepare all the transaction's participating processes (named participants or cohorts) to take the necessary steps for either committing or aborting the transaction, and the commit phase, in which, based on voting (either "Yes," commit, or "No," abort) of the cohorts, the coordinator decides whether to commit (only if all vote "Yes") or abort the transaction (otherwise), and notifies the result to the cohorts. The cohorts then follow with the needed actions (commit or abort) with their transactional resources and their respective portions in the transaction's output (if applicable).
The two-phase commit (2PC) protocol is not the same as two-phase locking (2PL), a concurrency control locking protocol.
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Assumptions
The protocol works in the following manner: one node is designated the coordinator, which is the master site, and the rest of the nodes in the network are designated the cohorts. The protocol assumes that there is stable storage at each node with a write-ahead log, that no node crashes forever, that the data in the write-ahead log is never lost or corrupted in a crash, and that any two nodes can communicate with each other. The last assumption is not too restrictive, as network communication can typically be rerouted. The first two assumptions are much stronger; if a node is totally destroyed then data can be lost.
The protocol is initiated by the coordinator after the last step of the transaction has been reached. The cohorts then respond with an agreement message or an abort message depending on whether the transaction has been processed successfully at the cohort.
Basic algorithm
Commit-request phase
- The coordinator sends a query to commit message to all cohorts and waits until it has received a reply from all cohorts.
- The cohorts execute the transaction up to the point where they will be asked to commit. They each write an entry to their undo log and an entry to their redo log.
- Each cohort replies with an agreement message (cohort votes Yes to commit), if the transaction succeeded, or an abort message (cohort votes No, not to commit), if the transaction failed.
Commit phase
Success
If the coordinator received an agreement message from all cohorts during the commit-request phase:
- The coordinator sends a commit message to all the cohorts.
- Each cohort completes the operation, and releases all the locks and resources held during the transaction.
- Each cohort sends an acknowledgment to the coordinator.
- The coordinator completes the transaction when acknowledgments have been received.
Failure
If any cohort sent an abort message during the commit-request phase:
- The coordinator sends a rollback message to all the cohorts.
- Each cohort undoes the transaction using the undo log, and releases the resources and locks held during the transaction.
- Each cohort sends an acknowledgement to the coordinator.
- The coordinator undoes the transaction when all acknowledgements have been received.
Disadvantages
The greatest disadvantage of the two-phase commit protocol is that it is a blocking protocol. A node will block while it is waiting for a message. Other processes competing for resource locks held by the blocked processes will have to wait for the locks to be released. A single node will continue to wait even if all other sites have failed. If the coordinator fails permanently, some cohorts will never resolve their transactions, causing resources to be tied up forever. The algorithm can block indefinitely in the following way:
If a cohort has sent an agreement message to the coordinator, it will block until a commit or rollback is received. If the coordinator is permanently down, the cohort will block indefinitely, unless it can obtain the global commit/abort decision from some other cohort.
When the coordinator has sent "Query-to-commit" to the cohorts, it will block until all cohorts have sent their local decision. However, if a cohort is permanently down, the coordinator will not block indefinitely: Since the coordinator is the one to decide whether the decision is 'commit' or 'abort' permanent blocking can be avoided by introducing a timeout: If the coordinator has not received all awaited messages when the timeout is over it will decide for 'abort'. This conservative behaviour of the protocol is another disadvantage: It is biased to the abort case rather than the complete case.
Implementing the two-phase commit protocol
Common architecture
In many cases the 2PC protocol is distributed in a computer network. It is easily distributed by implementing multiple dedicated 2PC components similar to each other, typically named Transaction managers (TMs; also referred to as 2PC agents), that carry out the protocol's execution for each transaction (e.g., The Open Group's X/Open XA). The databases involved with a distributed transaction, the participants, both the coordinator and cohorts, register to close TMs (typically residing on respective same network nodes as the participants) for terminating that transaction using 2PC. Each distributed transaction has an ad hoc set of TMs, the TMs to which the transaction participants register. A leader, the coordinator TM, exists for each transaction to coordinate 2PC for it, typically the TM of the coordinator database. However, the coordinator role can be transferred to another TM for performance or reliability reasons. Rather than exchanging 2PC messages among themselves, the participants exchange the messages with their respective TMs. The relevant TMs communicate among themselves to execute the 2PC protocol schema above, "representing" the respective participants, for terminating that transaction. With this architecture the protocol is fully distributed (does not need any central processing component or data structure), and scales up with number of network nodes (network size) effectively.
This common architecture is also effective for the distribution of other atomic commitment protocols besides 2PC, since all such protocols use the same voting mechanism and outcome propagation to protocol participants.[1] [2]
Protocol optimizations
Database research has been done on ways to get most of the benefits of the two-phase commit protocol while reducing costs by protocol optimizations [1] [2] and protocol operations saving under certain system's behavior assumptions.
Presume abort and Presume commit
Presumed abort or Presumed commit are common such optimizations.[2] An assumption about the outcome of transactions, either commit, or abort, can save both messages and logging operations by the participants during the 2PC protocol's execution. For example, when presumed abort, if during system recovery from failure no logged evidence for commit of some transaction is found by the recovery procedure, then it assumes that the transaction has been aborted, and acts accordingly. This means that it does not matter if aborts are logged at all, and such logging can be saved under this assumption. Typically a penalty of additional operations is paid during recovery from failure, depending on optimization type. Thus the best variant of optimization, if any, is chosen according to failure and transaction outcome statistics.
Tree two-phase commit protocol
The Tree 2PC protocol [2] (also called Nested 2PC, or Recursive 2PC) is a common variant of 2PC in a network, which better utilizes the underlying communication infrastructure. In this variant the coordinator is the root ("top") of a communication tree (inverted tree), while the cohorts are the other nodes. Messages from the coordinator are propagated "down" the tree, while messages to the coordinator are "collected" by a cohort from all the cohorts below it, before it sends the appropriate message "up" the tree (except an abort message, which is propagated "up" immediately upon receiving it, or if this cohort decided to abort).
The Dynamic two-phase commit (Dynamic two-phase commitment, D2PC) protocol[2] is a variant of Tree 2PC with no predetermined coordinator. Agreement messages start to propagate from all the leaves, each leaf when completing its tasks on behalf of the transaction (becoming ready), and the coordinator is determined dynamically by racing agreement messages, at the place where they collide. They collide either on a transaction tree node, or on an edge. In the latter case one of the two edge's nodes is elected as a coordinator (any node). D2PC is time optimal (among all the instances of a specific transaction tree, and any specific Tree 2PC protocol implementation; all instances have the same tree; each instance has a different node as coordinator): it commits the coordinator and each cohort in minimum possible time, allowing earlier release of locked resources.
See also
References
- ^ a b c Philip A. Bernstein, Vassos Hadzilacos, Nathan Goodman (1987): Concurrency Control and Recovery in Database Systems, Chapter 7, Addison Wesley Publishing Company, ISBN 0-20110-715-5
- ^ a b c d e f Gerhard Weikum, Gottfried Vossen (2001): Transactional Information Systems, Chapter 19, Elsevier, ISBN 1-55860-508-8
External links
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