Atomic Commit Protocols  

Abstract Category: I.T.
Course / Degree: M.Phil.
Institution / University: University of Manchester, United Kingdom
Published in: 2005

Thesis Abstract / Summary:

A transaction is a logical collection of tasks that must either complete successfully or unsuccessfully as a group. A system that provides tools to accomplish this is known as a transaction processing system.

In a distributed environment where the tasks that make up a transaction are distributed across physically distinct nodes, a protocol is required to decide if the transaction should be completed successfully (Commit) or cancelled (Abort); this protocol is called an Atomic Commit Protocol. The protocol that is used in most distributed transaction processing systems is Two Phase Commit Protocol (2PC). In 2PC a single coordinator is used to collect each participant’s status of the work; the coordinator decides to Commit the transaction if all participants have successfully completed their part of the transaction, otherwise the transaction is Aborted.
2PC is a blocking protocol, i.e. if a coordinator fails while the participants are waiting for a decision, all the participants remain in a blocked state till the coordinator recovers and terminates the transaction. As alternatives, Non-Blocking Atomic Commit (NB-AC) protocols have been suggested; all NB-AC protocols require a synchronous environment.

Paxos Commit is an Atomic Commit protocol that solves a weaker form of NB-AC, the protocol is non-blocking only if the network is non-faulty for long enough. It uses an instance of the Paxos Consensus Algorithm to achieve consensus on each participant’s status. Paxos Consensus is a fault tolerant consensus algorithm that uses 2F+1 acceptors (acceptors are processes that receive values from proposing processes and pass the first value to the Leader) and proceeds if F+1 of them are active.

The study investigates and develops an alternative algorithm, Modified Paxos, based on the Paxos consensus algorithm and a disk write optimisations to Paxos Commit that utilises the available redundancy to improve performance. The algorithms have been simulated in a framework designed to observe overall transaction commit times in varying failure conditions.

The results show that 2PC is faster than Paxos Commit and Modified Paxos in a fault free environment. This is true even in the case where the two Paxos consensus based commit algorithms are run with fault tolerance switched off. The disk write optimisation improves the performance of Paxos Commit for committed transactions by 46%. The Modified Paxos when compared with unoptimised Paxos Commit is 4.5% faster for committed transactions. Modified Paxos also has the advantage of not using a single Transaction Manager process, the current leader of the acceptor processes is used as the Transaction Manager.

Thesis Keywords/Search Tags:
Atomic Commit, Two Phase Commit, Paxos Commit

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Submission Details: Thesis Abstract submitted by Sumit Agarwal from United Kingdom on 11-Apr-2006 00:28.
Abstract has been viewed 2919 times (since 7 Mar 2010).

Sumit Agarwal Contact Details: Email: sumit.aga@gmail.com

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