Øredev 2011 in the rear-view mirror – Part 3
This is the third part of ny sum-up of Øredev 2011.
I will label each session with day:order to satisfy all structure freaks (myself included) that read this.
2:1 – Dan North – Embracing Uncertainty – the Hardest Pattern of All
If there is one session I really regret not attending to, it has got to be this one. Everyone I spoke to afterwards were totally blown away by Dan’s keynote about how we humans strive to avoid uncertainty at all cost, even in situations where uncertainty would be a much better mode to be in than being certain.
Uhm…I’ll just point you guys straight to Daniel Lee’s excellent blog, where he write more about this session.
The quote “Fear leads to risk, risk leads to process, process leads to hate… and suffering and Gantt charts”, “We would rather be wrong than be uncertain!” and the way Dan reasons about how faith become religion makes this the top session that will keep me waiting for the Øredev videos.
2:2 – Greg Young – How to not apply CQRS
I just love Greg. After this session, I stormed out of the room and bursted out “I think it is SO cool that Greg is, like, the Henry Rollins of software engineering”, on which a complete stranger turned around, equally happy, and almost shouts “I KNOW!”. A minute or so later, I happen to overhear a conversation, where one guy says “Wow, Greg looks JUST like Phil Anselmo”.
Yeah, Greg is the personification of all those metal vocalists we always wanted to be (or be friends with) and lets us embed ourselves in a thick layer of ignorance that allows us ignore that we are a developer converence. Oh no, we are almost those rock stars we dreamt of to become…and almost friends with Phil and Henry. I also think I saw a little bit of Mike Patton in his eyes, and a piece Christian Bale in the way he speaks.
All this despite the fact that Greg wears Five Finger Shoes. Five Finger Shoes!
That is quite an achievement.
But hey, what did he talk about, I hear you ask. Well, I frankly do not remember. No, that was a lie. Greg talked about how one good way to fail is to apply CQRS everywhere within a monolithic system. CQRS is not a top level architecture. It requires a bounded context. So, to apply CQRS in a non-core context, and without a bounded context, is an almost fail-proof way to fail. With CQRS applied everywhere, nothing can change.
Greg then went on to speak about CQRS and DDD, and the dangers of imaginary domain experts. Who can be considered to be a domain expert? The consultant who have worked with a system for a while, who maybe even answer to another consultant? Or is it an employee who have worked with the domain for a couple of years? How about twenty years?
This is really important, since CQRS is business-centric, which is why CQRS and DDD without domain expertise does not work. The result will become like playing the telephone game and translating Shakespeare with Google Translate. BA:s are really good at asking the right questions, but they are not domain experts.
As an exercise, Greg talked a bit about Programming as Analysis, which I’d like to try. The point is that you are supposed to build a system for a domain that you do not know. Now, timebox your access to the domain expert to two hours. That is it. In two hours, you have to find out everything you need to build your system. The entire system. Sure, you will fail. Fail hard. But, in doing so, you will come up with a bunch of new questions. So you throw everything away. Everything. Then you do it all again. Two hours. Build the entire system. Then again. Then again.
Greg finally summed up his session with pointing out the most certain way to fail – by learning CQRS by building a CQRS framework. Instead, focus on the business value of applying CQRS. He finished his great session with calling frameworks “crack for architects” and stating that frameworks are evil.
Great one, Greg. Thanks a million. Now, drop those five fingers and I would not hesitate to put a poster of you up on my wall.
2:3 – Rickard Öberg – Event Sourcing explained
Next to CQRS, which I have not had the pleasure of applying (maybe I should build my own CQRS framework…or?), Event Sourcing is something that I’d love to try out.
Rickard started with describing a common architecture:
Client <- Service facade <- Domain <- Storage
and how with Event Sourcing, things look a bit different:
Client <- Service facade <- Domain (Commands -> Events) <- Event storage
We do not store state – we store events, which is a huuuuge difference. By storing the events, we can replay all the events that has affected an entity during its lifetime, and in such a way build it up from scratch to its current state. In order to avoid really heavy operations, we can store snapshots as well. With snapshots, an event stack like this one (the latest event is topmost):
is handled by starting with the lastest event. As long as the item is not a snapshot, we keep it for later. Once we reach a snapshot, we grab that representation, then apply all the events that we have kept for later. This way, we do not have to replay the entire life of the entity, just a part of it.
That is basically it.
Rickard, however, then went on to talk a bit about his event sourcing framework. Obviously suffering after Greg’s recent framework burstout, Rickard had the balls to still insist that his framework was really good…and that he likes to use it. I for one believe him.
With event sourcing, report generation becomes a breeze. Also, you never loose data. If I add 5000 to my bank account then withdraw 3000, and no events are stored, all I know is that I now have 2000 more than before I started. Sure, maybe the system writes transactional details to the log, but with event sourcing, the event storage becomes the log.
Due to its log-like nature, event sourcing also simplifies debugging. Who did what when? With event sourcing, nothing gets thrown away. My events is what makes my entities look the way they do. If an event accidentally was not saved, that is bad, sure, but it is another problem. My entity does not know about it, since the event does not exist.
Event sourcing continues to interest me. Maybe one day, I will actually get around to use it?