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Monday, September 1, 2014

What is Commitment ordering method for database concurrency control?

Commitment ordering or CO is a class that consists of techniques for implementing interoperable serializability in concurrency control mechanism of the transaction processing system, database systems and other applications related to database management. With the use of commitment ordering methods we can have non – blocking or optimistic implementations. With the advent of multi – processor CPUs, there has been a tremendous increase in the employment of CO in transactional memory (software transactional memory to be particular) and concurrent programming. In these fields CO is used as a means for having non – blocking serializability.
In a schedule that is CO compliant, there is compatibility between the chronological order of the events and precedence order of respective transactions. Conflict serializability when viewed with a broad meaning is nothing but CO. it is highly effective, offers high performance, reliability, distributable, scalable etc.; with these qualities it is a great way of achieving modular serializability across a heterogeneous database systems collection i.e., the one which contains database systems employing different concurrency control methods. A database system that is not CO compliant is linked to a CO component such as COCO – commitment order coordinator. The purpose of this component is put the commitment events in order so as to make the system CO compliant. This also removes access to data and any interference in the operation of transactions. All this leads to reduction of overhead and we get an appropriate solution for distributed serializability and global serializability. A fundamental part of this solution is the atomic commitment protocol or ACP which is used in breaking the cycles present in the conflict graph. This graph can either be a serializability graph or a precedence graph. If the concurrency control information is not shared among the involved database systems beyond ACP messages or if they don’t have any knowledge about the transactions, then for achieving global serializabiolity, CO becomes the absolutely necessary condition.
Another advantage of CO is that its local concurrency information distribution is not costly. This information includes Timestamps, tickets, relations, locks and the local precedence relations etc. it makes use of SS2PL property. SS2PL used with 2PC (two – phase commit protocol) becomes the de facto standard through which global serializability can be achieved. This also creates a transparent process through which the other CO compliant systems can join such global solutions. When a multi – database environment is based upon commitment ordering, the global deadlocks can be resolved automatically without requiring human intervention. This is an important benefit of having CO compliant systems. There is another concept where we intersect CO and strictness called as the strict commitment ordering or SCO. This results in a better overall throughput, shorter execution times for transactions and thus better performance when compared to the traditional SS2PL. The positive impact of having SCO can be felt during lock contention. The same database recovery mechanism can be used by both SCO and SS2PL by virtue of the strictness property. Today we have two major variants of CO namely:
- CO – MVCO and
- CO – ECO
The first one is the multi - version and the second one is called the extended version. Any concurrency control method that is relevant can be combined with these two for employing non – blocking implementations. Both make use of additional information for making relaxations to the constraints and for better performance. A technique is used by CO and variants called the Vote Ordering or VO – a container schedule set. In case of absence of concurrency control information sharing, global serializability can be guaranteed only if there is local VO. The inter – operation of CO and variants is quite transparent which makes automatic deadlock resolution possible also in the heterogeneous environments. 

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