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CQRS: Command Query Responsibility Segregation

Posted By: brian_ritchie     Posted Date: August 31, 2010    Points: 0   Category :ASP.Net
Jason Gerard and I were talking about document databases the other day, and he said "Have you heard about CQRS?"  This was news to me, so I started doing some research.  Wikipedia, which usually provides good information, said the following "CQRS is simply the creation of two objects where there was previously only one. The separation occurs based upon whether the methods are a command or a query (the same definition that is used by Meyer in Command and Query Separation, a command is any method that mutates state and a query is any method that returns a value)."Ok, that was clear as mud. Maybe a picture will help.  This diagram is from Udi Dahan's excellent presentation on CQRS at the London .NET User Group.  Udi is an amazing presenter & his presentation is well worth the watch.  He is also the guy behind NServiceBus.Here are the basics as I understand them:In a multi-user environment, data on the screen is always stale. Due to this fact, we don't need a complicated ORM to pull "live" data out of our OLTP database.  Instead, we can use a persistent view model to pre-cache the data needed by the UI.The user interface needs to capture the user's intent, not just their input.  It can then build up commands that are submitted asynchronously to the services layer.  This is a more imperative way of doing things and provides the oppo

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