6,662,196 - Collision avoidance in bidirectional database replication
Abstract:A bidirectional database replication system includes a plurality of nodes. Each transaction at an originating node is paused prior to a commit operation. Ready to commit tokens are sent to the other nodes in the system to determine if the other nodes are prepared for the commit operation for the paused transaction. If all of the ready to commit tokens properly return to the originating node from the other nodes, thereby indicating that the other nodes are prepared for the commit operation, then the transaction is commited. For lengthy transactions, ready to sync tokens are assigned at one or more predesignated intermediate points in the transaction, and propagate throughout the system in a similar manner. The transaction continues to execute as long as all ready to sync tokens properly return to the originating node. The pause-before-commit and sync point schemes are used to avoid collisions at any of the nodes.