Update 05/27/2019: “How to build an event store masterclass” is now available. Learn how to build an event store using C#. NET Core, DynamoDB, MySQL for read models, and more.
In the past few weeks, I have been working on creating an event store in AWS DynamoDB and AWS S3. I use an event store for the domain driven design (DDD) concept in that system. Specifically, one of my systems uses CQRS and Event Sourcing (which is awesome, btw).
The idea for the event store started when I wanted to create my own event store for several reasons. I played with EventStore from Greg Young available at https://eventstore.org/ I encountered errors when I tested EventStore and I expected 100% functionality without issues. Besides, I did not want to babysit another persistence mechanism. I just do not have time for that.
I have created an event store before that was based entirely on Redis. That had worked great and it was super-fast. I used https://redislabs.com/ service to allow zero-maintenance of a fully clustered Redis solution. This has been working for some time now. The only problem was that it can have inconsistencies when it comes to running many applications nodes that use the Redis event store. Long story short, this has to do with complex timing, concurrency violations, etc.
So, I thought there must be a better way. I want zero administration headaches ideally, but I want to take advantage of the consistency capability of the underlying storage mechanism. I want the event store to be a service to the application itself that runs within the same process of the application. So, no need to babysit a separate event store cluster. I want the event store conceptually living side by side within the application.
If I can only take advantage of the consistency of the persistent mechanism, the application can then handle error conditions etc. accordingly based on what the use cases are.
After some experimenting and doing quiet a few load tests using http://loader.io, I can now say that the Event Store in DynamoDB has been born. My first 10,000 user / min load test has passed with flying colors and reached API response times of less than 10 ms with sustained rates of 170 clients / second. All this was running on a single cheap t2.medium instance. DynamoDB had to increase the throttling automatically to take this kind of load. This is one of those cool features, btw.
The event store I created in DynamoDB uses two concepts, Aggregates and Change Sets. Aggregates are the aggregates in your DDD system but only a handful of meta data. Change sets are one ore more domain events that are created when your DDD system processes a command.
I know I can get much higher numbers even by adding additional nodes and doing further tweaking. I would have to do much more load testing of course and tweaking but so far this has been a tremendous success and I’m super excited to take advantage of DynamoDB.
Anyways, I wanted to share this information. Maybe in the in future I can go into much more details. Time is the only problem I have really.
Thomas Jaeger 