While CQRS seems to be very useful tool for dealing with complex domains, I kinda got burned by it more times than I would like to admit. I'm talking about traditional interpretation, CQRS+ES+DDD ala Greg Young. The biggest frustration I'm facing is how it claims that readmodel can be somewhat an afterthought, yet how it still remains the crucial part, and unless you take a good care of it, you gonna have a bad time.
CQRS lets you defer decisions, not avoid them. You may think that choosing the reporting database is exactly the kind of decision CQRS lets you avoid. Well, not entirely. It allows you to offload the decision to someone else who is responsible for reporting. But still it's gonna be you for small one-man projects. And you'd better damn know well your database, as there are tons of nasty surprises awaiting with these beasts.
It is completely unsuitable for CRUD scenarios. If there is no complex processing of data coming in, using CQRS is certainly overkill. If your commands map 1:1 to your events map 1:1 to your read model, it is just common sense then this is a nonsense. Whoever says there is no overhead in doing this and you'd better just do it everywhere has no clue or is lying.
Naive implementations of read model only take you so far. There are some people around boasting of how CQRS+ES allowed them to get rid of the database. So this is really neat point with ES, that your primary data model (Events) is very simple. You can put serialized events anywhere you want, no need to tie yourself to any particular DB. On the reporting side however, this is entirely different story. Unless you are writing most primitive and thus probably useless app, just storing your (demormalized) data in files is not gonna work. Where is fulltext search? Where are those adhoc queries business always bugs you about? Or even, where is the list of users sorted by date of registration? Pretty tricky to implement when all you have is partitioning by id, eh?
So, while I believe this is the way we will be building software in the future, the fact remains, CQRS just lets you ask the right questions at the right time, but never be fooled that this makes the answers somehow less important.
