7. Two-phase validation protocol

In a distributed context, the validation of a global request that is broken down into local sub-requests requires the implementation of a Two Phase Commit Protocol (2PC).

A global control module (GCM) supervises the execution of the query and determines whether it has been completed correctly. To do this, this control module communicates with local control modules (LCMs), specific to each site where the subqueries are executed. Depending on whether or not the subqueries have been executed correctly, the MCG may or may not signal the correct end of the query to the user.

The following pseudo-code explains how these modules work. Correct execution of the query is validated by a COMMIT operation, which makes the modifications made to the database effective. Incorrect execution triggers a restore operation (ROLLBACK) which undoes the modifications...