CQRS is not a pattern that encompasses the whole application. It is a concept that builds on Domain Driven Design (DDD). And an important strategic concept of DDD is the so-called Bounded Context. In a typical application there are multiple bounded contexts, any of which can be implemented the way it makes sense. For instance … Read more
If you validate the username using the read model before you send the command, we are talking about a race condition window of a couple of hundred milliseconds where a real race condition can happen, which in my system is not handled. It is just too unlikely to happen compared to the cost of dealing … Read more
Commands can be rejected. Events have happened. This is probably the most important reason. In an event-driven architecture, there can be no question that an event raised represents something that has happened. Now, because Commands are something we want to happen, and Events are something that has happened, we should be using different verbs when … Read more
Following the advice in Tackling Complexity in CQRS by Vladik Khononov suggests command handling can return information relating to its outcome. Without violating any [CQRS] principles, a command can safely return the following data: Execution result: success or failure; Error messages or validation errors, in case of a failure; The aggregate’s new version number, in … Read more
CQS (Command Query Separation) and CQRS (Command Query Responsibility Segregation) are very much related. You can think of CQS as being at the class or component level, while CQRS is more at the bounded context level. I tend to think of CQS as being at the micro level, and CQRS at the macro level. CQS … Read more
The event store should not need to know about the specific fields or properties of events. Otherwise every modification of your model would result in having to migrate your database (just as in good old-fashioned state-based persistence). Therefore I wouldn’t recommend option 1 and 2 at all. Below is the schema as used in Ncqrs. … Read more
I am one of the original authors of Kafka. Kafka will work very well as a log for event sourcing. It is fault-tolerant, scales to enormous data sizes, and has a built in partitioning model. We use it for several use cases of this form at LinkedIn. For example our open source stream processing system, … Read more