Suppose you have a cluster of database servers in different datacenters, and somehow the connection between the datacenters go down. If you want to ensure high availability and allow your users to reach and modify the data while there is no connection, then you have to mark each modifications somehow so that later when the connection is up and the servers synchronize they are automatically able to merge most of the changes and find out conflicting cases.<p>That problem is solved by vector clocks and the most prominent application of them is Amazon's Dynamo system. See also eventual consistency for more information.