2009-01-09

Domain Driven Design by Eric Evans - my book review

To me this book has been a huge disappointment. Someone told me "Pete, there's a name for what you do!" and pointed me to this book.

Now personally I prefer technical books, as I read the Ubiquitous Language part at the start I thought to myself "Not technical, but fair enough some people will get value from being taught how to ask questions" and I stuck with it.

The first thing I must say that I cannot stand and which happens a lot in this book is over emphasis. When you want to emphasise something it needs to stand out. This book not only emphasises whole paragraphs but does it far too often too. Being an Internet user for some time now when I read upper case letters the imagined vocalisation actually SHOUTS at me, when I read bold my brain vocalises it as a loud and punctuated word, so when I read a whole paragraph in bold I, READ, EACH, WORD, LIKE, THIS; it makes it difficult to read.

Another thing I don't like when reading something is reading it for the Nth time. I don't mind a bit of reiteration in the way of reading something and then at the end telling me it is another example of "X" but only if the example is so different that it probably didn't occur to me. When I am on page 400+ I really don't want to be reading more examples of what I was reading in the first chapter. It really switches my brain off when so far into the book I am reading yet another example of "The Ubiquitous Language" that was covered at the start of the book. To be honest I am finding it very difficult to motivate myself to read the remainder of the book.

So, what have I learned from the book? I would say I have learned 1 valuable thing. Many times in the past I have modeled a parent/child association such as PurchaseOrder, and then modeled a kin-like parent/child such as Invoice/InvoiceLine, where the invoice is always for a single order and each invoice line is for a specific order line. In these circumstances I would have both Invoice.Order and InvoiceLine.OrderLine, everytime I did this I would cringe, it just felt "wrong" or "messy". Enforcing the idea that I should have no direct associations to the aggregated parts of the Order means that now I merely have Invoice.Order, it's easy to see which order line the invoice line is for because they are ordered the same and both the order and invoice are immutible. Now, I don't agree with the idea of never referencing an aggregated part, but at least now I consider the option. It certainly cleaned up a 3 kin-like aggregate part of the model I am currently working on.

For someone who doesn't know how to talk the "language of the current domain" with a customer I can see that this book could be useful, and also for people who need some pointers on how to segment their apps a bit.

When I read about people mentioning the "map of the world" example being such an eye opener (or whatever other way they express their positive experience) it honestly amazes me. The idea that a "Customer" to company A is completely different from how company B sees one as a break through just makes me shake my head in disbelief. In some businesses a customer is a company, in others a person, in others it could be either, and in one domain I worked a customer could have been either

A: A company
B: A department
C: An individual
D: A non physical entity such as a business process

All of which could also be a "Supplier". This is because they saw their Customers and Suppliers as things that consume and things that produce.

On the whole I personally found the book to be "a whole lot of nothing much at all", some of the personal stories were interesting but I also mainly found it repetitive and boring; a very difficult read. Unless the last 100 pages or so have something knock-out in them I expect I will remain very disappointed with it, if I ever manage to read the end that is.

Some people seem to be quite religious about this book, referring to it as "The book" and I feel by posting this negative review I might be opening myself up to attacks from DDD-extremists :-)

The fact is that I use parts of the DDD approach (now I at least know by what name to refer to it), it's just the book...

3 comments:

Anonymous said...

"Pete, there's a name for what you do!"

Given that you already do everything in the book it is understandable that you would find it "a whole lot of nothing much at all".

I felt that I already did a lot of what was in the book. However, I (personally) think it provides a useful contribution in language alone. The clearly named concepts and patterns provide a useful addition to our shared design language.

The Ubiquitous Language is not really about asking questions. It is about making your conversations with domain experts reflect the code and vice-versa. This is very powerful but difficult process and one I would argue, rare in real-world software dev.

FYI - The last 100 pages is the best part of the book ;)

Ben Taylor said...

Whoops. I did not mean to make my comment anonymous. It was me!

rodrigow said...

Hi Pete!
I'm still reading this book and I learned a lot.

Just a point: the bold in the book. Eric wrote:
"A skimmer who already has some grasp of a topic should be able to pick up the main points by reading headings and bold text."

Regards,
Rodrigo