Patterns of Distributed Systems

Problem

If multiple servers allow the same key to be updated, it’s important to detect when the values are concurrently updated across a set of replicas.

Solution

Each key value is associated with a version vector that maintains a number for each cluster node.

In essence, a version vector is a set of counters, one for each node. A version vector for three nodes (blue, green, black) would look something like [blue: 43, green: 54, black: 12]. Each time a node has an internal update, it updates its own counter, so an update in the green node would change the vector to [blue: 43, green: 55, black: 12]. Whenever two nodes communicate, they ...