Domain modeling or data modeling
Domain modeling or data modeling?
(a) Users do not care “objects”. They care UI, and the fact that the data on the UI are saved in databases. So, by definition, UI and the key data are the “business language”; objects are not.
(b) The pragmatic inevitability of Anemic Domain Model http://www.martinfowler.com/bliki/AnemicDomainModel.html-- OK, just in case I am not clear enough, I am saying that although sounds terrible, anemic domain model is a good thing. -- Just as others pointed out, OR mapping encourages this. Again, a lot of people believe it is a bad thing (and therefore, OR mapping is a bad thing!), I believe it is a good compromise.
(c) Part of the reason of (b) is that > 80% of the function of an application is “data CRUD”; so, why bother domain model.
(d) Because of all above, domain model can be “skipped” at least for smaller projects; for larger projects, it is certainly a good exercise. However, even in the latter case, the first-cut of the major database tables should be done first; therefore, the key objects should be “copied” from data model. The key reason is, of course, users (see (a)). This is crucial. You may find ways to “bypass” the data modeling phase, and say that you start with a “domain model”. However, if you think of the reason how you get the major objects done, it is always “we need those key data”! OK, the value of being honest to ourselves? – Time saving: > 90% of the domain model can be done within one or two days, after data model is done. And data model is much easier to do, because you can get help from users directly. You may say, how about the rest 10%? – Those are the low-level objects, they are implementation oriented; and we should not talk about them with users anyway.
(e) Fit can fit in perfectly, even when we do not use domain modeling. The key is data.
posted by survic at
12:20 PM
108 Comments:
Nice post. After reading your post, I wrote this post.
http://vikasnetdev.blogspot.com/2006/07/domain-driven-design-vs-data-driven.html
I reached the conclusion that Domain Driven Design is like automated unit testing for project with multiple releases. It is very important and cost effective in long run. Doing Domain Model Design before Data Model Design or in parallel has some benefits. But important thing is whether you do it at all or not before you jump to coding
Nice posting. We are close to see the light :-)
1. I agree the importance of “custom classes” – I just use “one table, one class” rule for 80% (or 70%) of the application.
My point is that if we have done the 70% first by “copying” the data model, it is a much faster and therefore a better approach. My understanding is that you feel it is just a possible option, while I believe it should be formalized as a necessary first step.
2. I agree that we need “controller” or “process” class layer. As a matter of fact, in CSLA, I was the last straw of the people of asking Rocky about it and prompted Rocky to post the “workflow” (or “process”) class solution in CSLA. Frankly, to me, putting business logic UI is not just a crime, but a felony crime.
However, I must say that “domain modeling” per se is not the issue here -- I have sensed that we have a terminology issue here: in my mind, “domain modeling” per se does not include the “process” class part. To me “domain modeling” is about those “entity classes” (i.e., anemic domain model). Actually, this is implied in those J2EE architecture diagrams, and also implied in MS’s SOA/application diagrams (they are obviously from J2EE anyway).
However, I know this kind of terminology difference can make reasonable discussion impossible; so, let’s put it aside.
My point is, how about we use data modeling, and by assuming “one table, one class” most of time, we get a “free” entity model. Then, we assume there is another layer, and we use common sense/”best practice” to design/implement the “business logic” layer. I put “design/implement” here, because most of the time, the “controller”/”process” is straightforward – that is, once you get used to the “anemic domain model” – again, I believe the horrible name refers an actually good compromise: if you do SOA, you got to have “anemic domain model” (the best you can do in SOA is to translate the shallow logic in “anemic domain model” into javascript).
By doing the above, I can successfully avoid “domain modeling” – “See, Mom, no UML!”
3. You may say, come on, you are doing it in your mind; be honest to yourself, man! Yes, let’s drive it deeper in the same spirit of being honest to ourselves. The real thing is – by the way, thank you for quoting the example -- this really shows that a good example is better than thousand pictures ;-) -- that I do not believe users will say “Delete the rows. OK, whatever”. No, a real user will never say that – that is, under “normal” situations.
Yes, a bad developer will use data modeling incorrectly, and confuse users to such a degree that a user will say “whatever” to whatever. However, that can have in “domain modeling” also.
My point is, users will never understand objects with certainty; in a sense, even in “normal” situations, users will always say “whatever” in their mind when developers talk about “objects”. However, users do know, for absolutely certainty, that certain data must be persistent. Right, they do not know and do not need to know whether the data are in Oracle or SQL Server, or even in a flat file; but they do know they must be persistent and must be shared!
Users’ astute senses of what data must be persistent are the major (read as “only”) source for the first-cut of both data modeling and domain modeling. Because it is indeed data, so, it is only honest to say that we are doing data modeling first, regardless whether we call it data modeling or not.
Yes, we are getting close. Domain Driven Design is going to get two new practitioners or may be none. :)
http://vikasnetdev.blogspot.com/2006/07/domain-driven-design-vs-data-driven.html
While waiting for vikas's moer comments, I came across a post:
http://codebetter.com/blogs/jeremy.miller/archive/2006/07/05/147119.aspx
Here is my comment:
People do not like UML because just as Vikas pointed out: they do not like “domain modeling”; and the reason that they do not like “domain modeling” is that “data modeling” is sufficient already; the extra “domain modeling” is waste of time.
Note that UML is now a superset of ER, so, you may say, if we use UML to do the data modeling, then, people will love UML ;-).
I want to point out that excluding “data modeling” from “domain modeling”, or, thinking “domain modeling” is OO only, instead of both database and OO, is the leftover of an old myth that DABs are in charge of database table design.
Nowadays, because programming is so “easy” (because .net or java is so powerful), and also because SQL Server (or Oracle, DB2) is so “easy” also, developers are in charge of both data design and OO design (i.e. programming design). As a result, “domain modeling” includes “data modeling”. Also, data modeling should be done first. Further, in most projects, most OO modeling should be simply copied from the data model; and therefore a separate OO domain modeling per se is totally unnecessary.
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In my experience, you are correct that "data modeling" works fine for small to mid-size projects with minimal complexity, and you get a basic "domain model" for free. This is the essence of the ActiveRecord pattern and works quite well, with 1:1 ratio between objects and tables.
The problem is with more complex applications that have a lot of logic. At this point the data driven approach breaks down, because there is so much more behavior and business rules in the system. The data driven approach tends to cause developers to ignore all these "extra objects" that don't fit the usual CRUD pattern and 1:1 table/object ratio. If you have a good developer, they'll be able to switch gears to a DDD approach when the complexity warrants it, and create the extra objects in the model as necessary. If you don't have strong developers, chances are they will take a fairly procedural approach or possibly copy and paste a lot of logic around.
To play devil's advocate, why not start with a DDD approach on most projects? If its a small project and stays small, chances are you will get "your tables for free" and the CRUD stuff with it. Many ORMs will do the CRUD stuff with your objects and some will even generate your tables too (not that making tables and constraints is hard.)
In my early career, i focused strongly on a data driven and procedural approach to development. This worked fine for most of the simple applications that i was working on where the majority of logic was CRUD. But i kept painting myself into a corner on the more complex applications until I found OOP and DDD. I know you think that DDD is out of date, but i think it is still quite relevant as applications become more complex.
I also think that you are wrong that people only see UI and the data on it, so thats what we should focus on the most. If something calculated (like interest) is wrong, they'll notice that too, even though thats not data its behavior.
Also, users don't really understand relational databases or tables any more than they do objects.
I don't mean to offend you Survic, but you strike me as being where I was about 5 years ago before i started tackling complex line of business applications.
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