Two-Phase Commit Processing › Two-Phase Commit within CA IDMS › Commit Optimizations

Commit Optimizations

Types of Optimizations

To minimize the cost of doing a two-phase commit operation, CA IDMS supports the Read Only, Single Agent, and Presumed Abort optimizations.

Read Only

The Read Only optimization reduces the number of communications needed to commit a distributed transaction. A participant that has not updated resources within the scope of the transaction can respond FORGET to a Prepare request. CA IDMS does not include such read-only participants in the second phase of the commit operation, thus eliminating at least one communication. Additionally, the read-only participant writes no journal records in support of the two-phase commit operation.

Single Agent

CA IDMS uses the Single Agent optimization to reduce the flows needed to commit a distributed transaction. At the point when a Prepare request is to be sent to the last remaining participant, if all other participants have responded FORGET or if this is the only participant in the transaction, then a OnePhaseCommit request is sent instead of a Prepare. This results in only a single communication with the participant to complete the commit operation. Furthermore, if there is only a single participant, the coordinator writes no journal records in support of the distributed transaction.

Presumed Abort

CA IDMS uses a Presumed Abort protocol to reduce journaling overhead. Simply put, this means that while a coordinator retains knowledge of a committed transaction until all of its participants indicate that they have completed the second phase of the commit operation, the coordinator can immediately forget transactions whose outcome is BACKOUT. Consequently, no journaling activity for a distributed transaction takes place at a coordinator until all Prepare votes have been collected and then only if the outcome is OK. The absence of knowledge of a transaction signifies that its outcome is BACKOUT.

The alternative to Presumed Abort is Presumed Nothing. Under this protocol, a coordinator retains knowledge of the outcome of a commit operation until all participants indicate that it can be forgotten, regardless of whether the final outcome is OK or BACKOUT. Consequently, a coordinator must journal the existence of a transaction prior to forwarding the first Prepare request, and it must retain knowledge of backed out transactions longer. CA IDMS does not support the Presume Nothing protocol.