Have You Heard about Event Sourcing?
I cut my teeth in programming in the BASIC language on a computer that had a tape recorder as the persistent memory store (if you don’t know what a tape recorder is Google it). From there I transitioned to VBA and VBScript which wasn’t a stretch because it was all procedural, chaining a bunch of instructions to make the computer do what I want.
During my VB scripting days I was exposed to ASP and relational databases through Access then SQL Server. Cobb’s third normal form was not that much of a stretch for me to grasp. When .Net 1.0 came along as soon as it was released I jumped from ASP to ASP.NET and I took all of my procedural habits along with me to VisualBasic.NET.
Then as I barely got started with .Net I heard all the buzz around C# and object orientation and I just didn’t get it. I tried to force my procedural understanding into an OOP hole. In ASP I would create separate scripts for little pieces of functionality I wanted to reuse. I thought I was getting the same benefit of reuse and object composition that everyone was raving about with C#. How little I knew.
Today I find myself in the same boat trying to understand Event Sourcing. I am trying to fit Event Sourcing into a relational hole, but this time I won’t spend a couple years just doing it absolutely wrong. My boss asked me to talk about Event Sourcing and I took it as an opportunity to learn more about it, even though I will likely never give the talk. I did quite a bit of research and this is more of a post on where you can find some useful info.
Everyone starts with Wikipedia definitions, not sure why, but here is Event Sourcing according to Wikipedia…wait, there isn’t a Wikipedia page for it (as of 8/26/2013). Even Martin Fowler has Event Sourcing as a Work-in-Progress on his EAA page on the subject. So why the hell are we talking about it?
Event sourcing is in production on some of the most data intensive systems on the planet. People way smarter than me advocate it. Also, sometimes it’s nice to be on the cutting edge of a movement as it forces you to innovate.
Event Sourcing is a data persistence technique that focuses on capturing the state of an application overtime. The states are captured in an event object and the objects are stored in sequential order according to the time that the state changed. Once the state is captured, it can’t be changed or undone, it is immutable. To correct a mistaken state change you have to issue a compensating state change to correct it. So, your persisted state is the gospel, if it was stored you can trust that it is true and wasn’t tampered with (outside of some malicious change to mess with you).
OK, I’m not sure about you, but when I learned this it blew my mind. The idea of persisting the entire history of the state of my application was a red or green pill moment for me. On one hand it seemed terribly inefficient to store the state of every object in my application especially since most of the discussions is about using NoSQL DB’s. How could you possibly query this data easily and what benefit does it get me? Then I learned about the ease of data recovery and production incident research and being able to replay events that happen in production last month on my local box today…what!
Then I had an epiphany. I have source control for my code, took me a little to get comfortable with it and it provided a lot of benefits for me and hopefully for you too. Event Sourcing is something a little like source control for application state. Actually, SVN is an example of Event Sourcing used in my environment at work today. So, this understanding made it a practical solution to me, but I was still unclear on real world usage and what scenarios would benefit most from Event Sourcing.
Being in the financial industry auditing is a big deal and Event Sourcing could provide an instant audit log of every transaction we record. Yet, the whole logging of every event seemed a little overkill. I won’t try to persuade you either way or actually try to explain it to you as I couldn’t do the subject justice, but I decided it was too much for my current projects. Actually, a couple videos by one of the originators of CQRS (another concept that I am researching) has a lot to offer on the subject of event sourcing. Its buried in these references, but its all related and in my opinion all fascinating. Especially, if you are into broadening your coding horizons.
Decoupled Logic 
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