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Commit minutes regulate fixing (Commit) of data, which are to be changed by (distributed) a transaction for example in a data base management system.

Necessity and requirements

Commit minutes are necessary in distributed systems and data base management systems, in order to reach consistency, if several processes (called in English in this connection also agents) change data together and from each other dependently. A co-ordinator (usually the process, which introduces fixing) catches up in addition the agreement or refusal for fixing the data modifications of all processes involved. Only if all processes agree, the data modifications of the entire transaction are fixed; if one or more processes reject, the transaction is broken off and rejected the data modifications prepared so far.

Commit minutes describe thus, how processes communicate with one another over a co-ordinator, as logs information (logged) becomes and as data are finally fixed. Different error situations are intercepted by minutes, like e.g. Crash of the co-ordinator during a phase (dependent on minutes).

Distributed transactions should '' the ACID '' - characteristics fulfill (exactly the same as in non-distributed systems): A'tomicity, Consistency (consistency), isolation and D'urability

Minutes

Commit minutes differ in their phase's number. It determines the possibilities of information exchange and the error tolerance.

A phase Commit minutes

Rather theoretical a phase Commit minutes are certain by the decision of the co-ordinator. The only phase is the Commit phase (also called conclusion phase), in which the co-ordinator communicates to the processes involved whether the transaction is broken off fixed (doCommit) or (doAbort). The participating processes do not have further influence on the decision.

The disadvantage is in the missing tuning, which lends practice relevance to this minutes only if the co-ordinator from its knowledge could make this decision. This is possible only if the individual processes would fix in any case their partial transaction. The only abort reason would be then an abort on the part of the releasing Client process - thus the process, which arranged the transaction. At pure in phase minutes the small data traffic would be favourable.

Two phase Commit minutes

Two phase Commit minutes permit to communicate to all servers involved with the co-ordinator, in order with the completion of the transaction to a common decision to come (Commit or toilet).

  • Tuning phase: The co-ordinator sends a message to all processes involved canCommit for the determination of the readiness of each individual process for fixing for the distributed transaction. Each addressed computer communicates its individual answer to it.
  • Conclusion phase: Due to the received answers the co-ordinator decides whether the transaction can become or be broken off successfully final (doCommit) must (doAbort). All processes involved adhere to this decision.

With the two phase Commit the fundamental problem exists that a disturbance between tuning phase and conclusion phase can cause an uncertain condition. Example: Process 1 and process 2 announce in the tuning phase of OK ONE; the co-ordinator demands by means of the instruction doCommit to lock the transaction. Process 1 locks the transaction, process 2 cannot however because of a disturbance the transaction arisen between tuning phase and conclusion phase any longer lock. As consequence the transaction is accomplished in process 1, while process 2 waits for the message of the co-ordinator and in this uncertain condition blocked.

A further problem results from a loss of the co-ordinator during the tuning phase: All processes involved will have answered sometime and wait now for the co-ordinator. The processes are in an uncertain condition and block.

Three phase Commit minutes

Three phase Commit minutes are an extension of two phase Commit minutes. It permits to communicate likewise to all processes involved with the co-ordinator, in order with the completion of the transaction to a common decision to come (Commit or toilet).

  • Choice phase:

(A) The co-ordinator sends a prepare message to all Participant (B) to each participant answers with his vote message (or vote toilet vote commit). If its answer vote toilet reads, then the participant goes university-laterally into was entitled over.

  • Decision preparatory phase:

(A) The co-ordinator collects all vote messages of the participants. If all are expressed in vote commit, then it sends a prepare ton commit message to all participants. Otherwise it decides on global toilet and divides this all participants along (B) each participant, who vote commit with answered, waits for one prepare ton commit or global toilet message of the co-ordinator. If a global toilet arrives, then it decides on toilet. Otherwise it answers the co-ordinator with a confirmation (ready ton commit) and changes into the pre commit condition.

  • Decision phase:

(A) The co-ordinator collects all confirmations. If it received all, then it decides the participants on global commit and divides this the other participants along (B) waits for the commit decision of the co-ordinator and implements then the appropriate release operation.


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