CQRS Documents by Greg Young CQRS and Event Sourcing CQRS and Event Sourcing become most interesting when combined together. This chapter looks at the intersection of these two concepts within a system where Domain Driven Design has been applied. CQRS and Event Sourcing have a symbiotic relationship. CQRS allows Event Sourcing to be used as the data storage mechanism for the domain. One of the largest issues when using Event Sourcing is that you cannot ask the system a query such as “Give me all users whose first names are ‘Greg’”. This is due to not having a representation of current state. With CQRS the only query that exists within the domain is GetById which is supported with Event Sourcing. Event Sourcing is also very important when building out a non-trivial CQRS based system. The problem with integration between the two models is a large one. The maintaining of say relational models, one for read and the other for write, is quite costly. It becomes especially costly when you factor in that there is also an event model in order to synchronize the two. With Event Sourcing the event model is also the persistence model on the Write side. This drastically lowers costs of development as no conversion between the models is needed.
Figure 26 Stereotypical Architecture Sending Commands
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