[vorpalhex]

I've worked in mature CQRS/ES services at scale, I've also helped convert other systems both into proper CQRS, and out of proper CQRS.

Like anything else, it's one tool in the tool bag, but like that giant pipe wrench that you're always looking for a reason to use, it's almost always the wrong tool for the situation. CQRS carries horrific complexity and requires commitment to a handful of golden rules or the entire thing comes crashing down.

> I don't think people are qualified to reject this pattern

I don't think most people are qualified to know what the hell this pattern truly is, much less put it into proper operational use. I've seen plenty of people attempt to use CQRS when they should have stuck to a simple CRUD model. At one point I was adjacent to a team that had built 8+ services to handle one or two trivial business processes.

CQRS isn't a fine wine or stinky cheese. If it smells funny for your case, it probably isn't the right choice.

[mason55]

> requires commitment to a handful of golden rules or the entire thing comes crashing down

What are the golden rules?

[dmitryminkovsky]

Two bigs ones for me:

- Don't use the read side from the write side.

- Each aggregate should own its data (or more abstractly, the basics of DDD, bounded contexts[0]).

[0]: https://cqrs.nu/Faq

[eithed]

Every state change needs to be reflected as event. Does your function need to save something to DB? Then your functionality should create event which will perform the DB save.

[stingraycharles]

> CQRS carries horrific complexity and requires commitment to a handful of golden rules or the entire thing comes crashing down.

As with all software architecture, you need to adopt a concept to the problem at hand: following the "rules to the letter" is hardly useful. Most successful CQRS systems are those that do not follow every rule (e.g. let command handlers return response data make for a much more convenient workflow).

[vorpalhex]

In general yes, but if you say, start using your read data on your write side, then you're in for a very bad time. Likewise you are going to have some very serious timing woes if you violate your domains.

It's easy to read this and go "of course" for a small conceptual system, but in practice even really experienced engineers want to break those rules in production systems - and it's a lot easier to break those rules than fix the system to respect them.

[stingraycharles]

In this specific case I'm talking about e.g. returning errors in case command validation failed. Officially it's supposed to be returned asynchronously through events (and then correlate by the command identifier), but it can be much more pragmatic (and reliable even) to just return an error from the command handler in these cases.

[vorpalhex]

Your writes are always going to return a write model, and that won't account for indexing problems when you don't return an error - so your clients may naively equate a non-error response as a success. If you're the one writing the client, maybe that's not a problem. If you have several consumers who don't know the ins and outs of your system, that might be a very serious problem.

This is the problem with CQRS. The answer to so many of these little tweaks depends on a ton of system knowledge and contracts and promises, and it's really really easy to make the wrong choice.