Monday, October 10, 2011

Quick and Dirty Event Source using Amazon S3 and JSON

So yeah, I'm doing it too. Writing an event store. Not really because I think that Jonathan Oliver's isn't good, just because I wanna wrap my head around it. Lemme break down Event Sourcing into as small as a nutshell as possible.

Event Sourcing is the storing of a "stream" of events that represent the history of all that has happened in a system, and using that to create state instead of simply storing the state.

If that doesn't make sense to you, well, that's not the purpose of this blog. Go google it a bit, and read some more, then come back. This particular  post is about the quick and dirty ES implementation I'm writing.  If you're too lazy for google, here's a few great links.

Here's the idea. Events are raised by Aggregate Roots, and I'll create one stream for each AR. I'll serialize the stream of events (for now using JSON) to a Amazon S3 bucket with a key of the type name of the AR. Seems pretty simple to me... Let's give it a whirl.

First, I'll create some test dummies for my AR and an Event. Like so

I think I'll make an EventSource object where T is the AR type... Here's a quick integration test - starting with actual integration because, well, because I don't wanna get bit by Amazon quirks that I'll miss in a unit test.

Not really a test, but I'll just look in S3 for the file for now... Like I said, this is quick and dirty! Let's go make this happen.  So first, we're gunna need a way to tell the serializer what types the actual events are, so we can't just dump them all in a file and persist it. I created a quick little EventEnvelope class that wraps that. Like so.
We'll just serialize a list of these to S3. Ok, lets do this.
Looks pretty simple... Run the test. No exceptions and yep, I see a file there.

The contents look like so...

Cool. Looks ok so far. Now lets make that GetEventStream method work. I'll add a test that'll just write out the TestEvent.What value that I just put up there.

Pretty simple... Now lets make it pass.

Pretty simple huh? We're just calling that ExistingEvents for method that yanks the file from s3 for that Id..
Run the test, and yep, I see "Blah" in my console.

Ok, so I'm not sure if this code will actually get used, because Jonathan Oliver did some really great ES stuf and he's working on S3/Simple DB implementations now, however, I really wanted to create a quick implementation for some POC stuff. I was surprised at the simplicity of this. Next up, lets see if I can write something to get some AR state working of a stream.

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