Command Query Responsibility Segregation
Introduction

In [information technology](/wiki/Information_technology "Information technology"), **Command Query Responsibility Segregation** (CQRS) is a [system architecture](/wiki/Systems_architecture "Systems architecture") that extends the idea behind [command–query separation](/wiki/Command%E2%80%93query_separation "Command–query separation") (CQS) to the level of services. Such a system will have separate interfaces to send queries and to send commands. As in CQS, fulfilling a query request will only retrieve data and will not modify the state of the system (with some exceptions like logging access), while fulfilling a command request will modify the state of the system.

Many systems push the segregation to the data models used by the system. The models used to process queries are usually called *read models* and the models used to process commands *write models*.

Although its origin is usually attributed to Greg Young in 2010, everything indicates that the precursor of CQRS was Udi Dahan who in August 2008 published on his blog a training course that aimed to apply CQRS together with SOA  and in more detail in December 2009 in the article Clarified CQRS.


What are read models and write models in a system that uses CQRS?