Martin Fowler

Problem

Consider a key-value store where values are stored with a timestamp to designate each version. Any cluster node that handles the client request will be able to read the latest version using the current timestamp at the request processing node.

In the following example, the value ‘Before Dawn’ is updated to value “After Dawn” at time 2, as per Green’s clock. Both Alice and Bob are trying to read the latest value for ‘title’. While Alice’s request is processed by cluster node Amber, Bob’s request is processed by cluster node Blue. Amber has its clock lagging at 1; which means that when Alice reads the latest value, it delivers the value ‘Before Dawn’. Blue has its clock at 2; when Bob reads the latest value, it returns the value as “After Dawn”

This violates a consistency known as external consistency. If

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