Last Resource Commit Optimization
In some cases it may be necessary to enlist participants that are not two-phase commit aware into a two-phase commit transaction. If there is only a single resource then there is no need for two-phase commit. However, if there are multiple resources in the transaction, the Last Resource Commit optimization (LRCO) comes into play. It is possible for a single resource that is one-phase aware (i.e., can only commit or roll back, with no prepare), to be enlisted in a transaction with two-phase commit aware resources. The coordinator treats the one-phase aware resource slightly differently, in that it executes the prepare phase on all other resource first, and if it then intends to commit the transaction it passes control to the one-phase aware resource. If it commits, then the coordinator logs the decision to commit and attempts to commit the other resources as well.