Monday, October 17, 2011

Lets use EventSourcing on the Cloud, another Amazon project with Whale in the name

So, I'm playing with Event Sourcing, and I cooked up my own, mostly because I want to be able to explain this well to those who ask.

Let's explain in code with a really simple domain that everyone will understand -- Bank Accounts, you better have one!

How does one get a Bank Account? They open it, right? So I made this event.

Now lets make a quick domain POCO domain object for an account. The ctor will take this event, and do some cool stuff with state. Looks like this.

Two interesting things to note here. First, that I'm mutating my state in the Apply(AccountOpened) method, not just in the ctor. This is because that method will be called later when we get the Account from a repository. Yeah, it's a bit different, but honestly, to me the benefits WAY outweigh this single "drawback" to me. What benefits, you ask? Glad you did... We'll get there in a minute.
Second difference is that UncommittedEvents object... We'll also get to that in a minute. For now, lets write a quick integration test and watch WhaleEs in action. First, we bootsrap WhaleEs, like so.

I'm just parsing some text file that has my Amazon info in it here, then calling WhaleEs fluent config API so that I can get an Instance of Repository.
Next I'll create an account and save it.
Like so.

Kinda boring, huh? Let's go look at the guts of Repository, cause I know that's what you wanna see.

Just a little reflection magic here. I really don't wanna make people reference my library and ruin the POCO on their AR objects, so I just let them set the method name that contains the list of Uncommitted objects. That's pretty much the convention everyone is using for ES so I'm sure we'll be ok with that. Anyway, I'm calling that getter, then persisting the events it returns (calling the object that I blogged about last).
The get method is uses similar logic. It simply pulls the event stream for the AR with that Id, then uses reflection to call Apply(event,true) for every event, rebuilding state. It sends the extra true parameter for isReplaying, that way the AR won't add the event to it's UncommittedEvents property. Here it is.

Not to horribly complicated actually. Lets see if it works.

Sweet! Console outputs 100, just like I opened the account with.  Let's play with this a little more. Make a couple deposits and withdraws.

This works great, test passes, and that proves that we're actually rebuilding state. "So what Elliott! I could do that by just saving state, you twit!" you say.
Yes, tis true, my dear friend, you certainly could. However requirements change. Let's say they did, and know we want a list of all activity that ever occurred on the account. In good 'ole state land, we'd just cross our fingers that when we originally designed the system we had the good sense to actually save Deposit and Withdraw objects to some persistence, maybe we would, but maybe we wouldn't. Point is, working this way, even though we didn't it is very easy to add an "ActivityList" property to our Account object. Just modify the Apply methods for the appropriate events.... Like so.

Add a quick test like this.

And we see this....

Account Opened On 10/5/2011 2:35:43 PM with $1000
Deposit made on 10/5/2011 2:35:43 PM $45
Withdraw on 10/5/2011 2:35:43 PM $17

Anyway, I know there's a ton of writings on this stuff, but I hope this helps someone somewhere.
Feel free to yank the code down from github at

Have fun!


  1. An interesting take on ES, certainly - I like the simplicity of it. Is this going on to become a real project, or is it just an exposition of ES?

    You mention keeping track of the activity internally, but one of the things I really liked about using ES was that we could view the history externally just by inspecting the repository.

  2. Thanks... I wouldn't recommend keeping track of auditing like I did in this example. I just wanted to demonstrate how ES can eliminate a lot of friction. I didn't see the point in building a readmodel for the blog like I would in a real world app. The point of this blog was to demonstrate the simplest possible use of event sourcing.

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