CQRS à la Greg Young

Greg has taken Domain-Driven Design from how Eric Evans describes it in his book and has adapted mostly the technical implementation of it. Command Query Separation (CQS) was originally thought of by Bertrand Meyer and is applied at object level Bertrand defines CQS as: every method should either be a command that performs an action, or a query that returns data to the caller, but not both. In other words, asking a question should not change the answer. Greg however takes this same principle but he applies it to the whole architecture of a system, clearly separating the write side (Commands) from the read side (Queries) of the system.