Tuesday 22 April 2008

Domain Driven Design (the book)

I just finished reading Domain Driven Design: tackling complexity in the heart of software by Eric Evans. This is a book that was recommended to me years ago but I was lazy and ignored the advice - and what good advice it was. Hopefully you are familiar with the concept of a domain model. Implementing a domain model is by far my preferred technique for dealing with business rules in software, probably because so much of it leverages the strengths of object oriented programming (and don't get me started about how much I love OO). The thing about domain models however, is that getting one up and running is only half the battle. I am big fan of the Martin Fowler book Patterns of Enterprise Application Architecture and much of it deals with accomplishing this task. But the thing is, there are already plenty of object-relational mappers avaliable, both commercial and free. Given this fact, its probably incorrect of me to refer to technical aspects of using a domain model as "half the battle" as it should make up a far smaller proportion. The real battle is putting a domain model to work, leveraging its strengths and guarding against anaemia. Designing a domain model of true value is no easy task and it is this particular problem that Evans focuses on in DDD. I really enjoyed the first half of this book, being the two sections Putting the domain model to work and The building blocks of a model driven design as I found then easy to follow and very relevant to the work I am doing. Some of the concepts described were familiar and worth reinforcement, but several were completely new. The second half of the book consists of the sections Refactoring towards deeper insight and Strategic design. I found these sections a bit more uneven than the first, and particularly dry in places. Ever caught yourself and realised that your eyes scanned the last half a page without any of it actually sinking in? This happened to me more than a few times, but I think its as much my own fault as it is Eric's. Putting aside these complaints, the second half of the book is still a very worthwhile read as it introduces many valuable concepts. Its probably worth mentioning that there is not a great deal of code in this book as it is much more concerned with theory than with the hands-on practicalities of developing with a domain model. I can recall a few instances where the book describes a mechanism that made a lot of sense, but it was not entirely clear how I would actually go about implementing it. I am considering picking up a copy of Applying Domain-Driven Design and Patterns by Jimmy Nilsson in the hope it might help fill in some of the gaps. It occurs to me that there might have been a better way to approach the topic of DDD and it might have made Evans a bit more $$. If you take the first two sections of the book and add a few choice selections from the remaining half, you've got an excellent text for introducing developers to domain modelling. You could hand a developer that wasn't experienced with using domain models a 200 page text and they would come back in a few days excited and with a firm knowledge foundation. The majority of the second half of the book consists of material that is more relevant to a developer in a leading or mentoring role and this would easily fit in a follow up, "advanced" book. But don't let my theories on how the material might have been better packaged distract you, because DDD is still a really good book. A few years ago, when I was reading CLR via C# by Jeffrey Richter, I felt kind of embarrassed that I had been doing all this .NET development without much of a clue of what was going on. Reading DDD made me feel much the same way about building applications that use domain models.

Wednesday 16 April 2008


RSS stands for Really Simple Syndication and is basically about recording information about updates to a website in a structured format. What you actually need to know about RSS is that most websites these days have RSS feeds and this means that you can do alot of web browsing from within an RSS reader. I use Google Reader because it works via a browser so I can access it from home, work, or my mobile phone and they all stay in sync. RSS is amazing. I thought it was cool when I first discovered it years ago because it meant I didn't actually have to go to my favourite sites to see if they had been updated. And yes, fundamentally, years later, that is still all I use it for. But the really important difference is that initially, I used it to make sure that I didn't miss a Penny Arcade comic while now I use it to plug my brain into lots of really smart people's brains (and also never miss a PA comic). Steve Yegge. Paul Graham. Reginald Braithwaite. These guys are brilliant. I just bought Reg a Darkhorse double espresso via his handy donation link and I would be doing the same for Steve if he made it similarly easy (Paul Graham doesn't need my money, his brilliance has already brought him fat wads of cash). I discovered these names via the ever growing torrent of information that hits in me in the face every morning, noon and night (yep, my brain needs its breakfast, lunch and dinner the same way my body does). There are many other great bloggers that I subscribe to, but the above 3 are special because they are inspirational. Their writing has changed the way I think of myself, my work, and my future. I have struggled in the past to come up with a singular example of why Steve's writing is so great and this evening I went through the same process with Reg. These people are inspirational because they each have their own particular perspective on software development and these perspectives are refined, logical, and exciting. Furthermore, these perspectives are communicated through the aggregate of their writing, rather than any individual piece. Much of my effort to generalize is RSS assisted. I have found the programming sub-reddit to be especially useful as it is focused on less common programming languages such as Ruby, Python, Lisp, Haskell, etc rather than the heavyweights (.NET and Java). My RSS subscription count has probably doubled since I discovered Reddit - its an excellent source of worthwhile blogs. The world wide web has had an incredible influence on my life and RSS has significantly changed how I consume the web's content. Do you ever stop to wonder "gee, how on earth did we ever do anything without the internet"? Well now I'm wondering how I ever got by without RSS.