It is hard to separate DDD, Event Sourcing/CQRS - they seem to all be joined up concepts promoted by a small circle of people.
I worked on a blockbuster project financed by a local billionaire in a Gulf State where the entire shebang was “mandated” by the CTO and a well-known Scala consultancy. Event sourcing, CQRS, with DDD to define architecture.
Let me describe one simple issue that was almost intractably complex - user sign up.
We had one service which was authentication, and another which was user preferences.
The front end sends a create new user command (set up a new user in the auth system with a password, email, etc), we also needs now to send some kind of message to initialise the user info records (e.g. home address, telephone, language preference, etc.)
We need to implement a saga for this, e.g. implement a process coordinator. Or maybe the authorisation system should on first request where the data is not populated, fill in a blank, default record?
We genuinely achieved organisational paralysis over this, as there was no clear emergent right way to do this, and a bunch of ways that really smelled.
Additionally, regarding the a code organisation perspective, we had a repo that had the central commands and events defined as code, which every service linked to. So we had a huge central dependency which had a high velocity of change.
What was meant to be a distributed, loosely connected system was in fact the most coupled system I’ve ever worked on.
Architecture is about clear communication, and the developers were simply perplexed. We were bringing in 10 developers a month, and communicating the architecture was impossible. Lofty ideals were espoused.
The first basic rule of delivery in a software engineering project, KISS, was universally ignored, in favour of using “sexy” architecture.
Fitfully, the CEO was first sacked after 18 months at the helm, a few months later the CTO and 95% of the dev staff. The year long coding effort was turfed. Estimates are $50m-$100m was burnt.
The winner of course was the DDD consultant that mandated everything, whose word was seen as law, who didn’t actually seem to have very much pragmatic, practical experience - he was getting paid $2k/day, and found it difficult to listen to any idea which watered down his architecture in any way - I think he would eeked out enough cash from this shitshow to purchase a small dwelling.
The biggest losers were the hundreds of staff that had relocated to the region sometimes with families, that had made genuine plans to be in the region for years, that would have all had their visas cancelled.
I think the entire thing is a con - my experience was in ideologues and purists pushing it and making themselves niche consultancy/speaking/writing careers in it.
Now this is just one apocryphal story - but it really is an architectural style that can totally wreck a development or even a company - people are ideological about it - it is not low-risk, and the entire Saga/Process Coordinator stuff is just a tack-on to try to (unsuccessfully in my opinion) answer things that simply don’t work properly. The message I mean to communicate is that the async-everywhere nature of CQRS/ES is super complex where coordination is required.
If you are going for loosely connected services, I far more prefer the microservices/RPC architecture - such as Netflix - as the synchronous model with distributed load balancing is a lot more sound.