undefined | Better HN

0 pointsfeoren3y ago0 comments

I had the exact opposite reaction to this book. It seemed like great stuff when I first read it, but over time, I realized it's really misguided in a lot of ways:

- Aggregates are too heavy. You need to make the decision about what is or is not an aggregate way too early in the design process. Boundaries are fuzzy.

- Actual concepts don't exist in nicely packaged bounded contexts. Concepts overlap a lot. You need to make the decision about which concept fits into which bounded context too early in the design process. Boundaries are fuzzy. Things are kinda like other things. The definition of "Employee" is not the same in the Scheduling context as the HR context as the Payroll context, yet they do overlap a lot, and you can't just treat them as completely separate things. If you break everything down into tiny contexts to deal with this, you just make Contexts and Aggregates the same.

- Repositories are not original to DDD and I think are very likely to foster absolutely horrific SELECT N+1 or even SELECT N^2 or N^3 performance. You simply can't let one bounded context do all its expensive operations in a vacuum; not when you have lots of contexts and lots of operations. In a complex system, most parts need to be planning, not doing. The results of most operations should be a plan that you can compose with other plans, analyze, and possibly even have an "optimization pass" if you need one.

- Ubiquitous language is the right idea. If you take nothing else from DDD, take this.

0 comments

2 comments · 2 top-level

theptip3y ago

Fair enough; YMMV.

I've not found that Aggregates need to be designed at the beginning; I've found it works fine to define an Aggregate after you start seeing performance issues with query patterns (i.e. define an Aggregate and forbid direct access to sub-objects when you see pathological access patterns/deadlocking).

Personally I've found Repositories to be a good way of enforcing that N+1 queries DO NOT happen. For example, in Django you can have the repository run select_related/prefetch_related and `django-seal` on the ORM query to forbid unexpected queries. This somewhat neuters the flexibility of the ORM, which can be a big cost, but lets you build much more restrictive queries that are guaranteed to perform well. It's a trade-off and I don't think it's a clear win for every use-case, but particularly when dealing with Aggregates I think having a limited number of ways to query is beneficial. (This might mean you're running some sub-optimal SQL queries, over-fetching etc., but for most line of business applications, that's actually a viable trade in exchange for simpler domain interfaces and protection against N+1 queries.)

Regarding "planning" vs. "doing", that seems to fit quite well with doing most of your work in POJO/POPO domain models, then only rendering those into DB writes at the edges. I think Repos can help with that. (IME you get N+1 selects when you use ORM models that abstract away the SQL queries and have application code interacting directly with ORM models; if you remove the ORM from your core domain logic and force it to live at the edges in Repos, this is not possible.)

1 more reply

scruple3y ago

> Aggregates are too heavy. You need to make the decision about what is or is not an aggregate way too early in the design process. Boundaries are fuzzy.

I've had the exact same complaint. I think there's a lot of great stuff to take away from DDD -- I also hear myself frequently making the same point about it's ubiquitous language -- but going all-in on some of its concepts, especially early in a projects life, is probably always a mistake that may well end in disaster.

j / k navigate · click thread line to collapse