re: Event Sourcing: What it is and why it's awesome VIEW POST

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re: Hi Barry. I have quite controversial feelings about your article, it seems to be a bit opinionated, but that's a different matter. I have two que...
 

Hi Victor,

It is definitely opinionated, the goal of the above piece was to sell people on the idea of event sourcing, so they could see the value. My further articles get into more objective discussions of event sourcing and the issues you can encounter, e.g. dev.to/barryosull/immediate-vs-eve... and dev.to/barryosull/event-granularit....

As for your questions:

1 . you write : "It allows us to talk to the business in their language".

I think the article explains what I mean quite well. By speaking in terms of events you are actually modelling the domain language. Technology never enters the conversation, only the language used to describe important business state changes, i.e. events. That's how the business owners think about the domain and ES allows you to express that in a way that both developers and domain experts can conceptualise. ES isn't a technology, it's a technique, and one that is used in many mature domains (i.e. law and accounting).

2 . This one is super-practical. What do you use as Event Store?

This one is easier to answer. I use MySQL as the event store, as RDBMSs are optimised for append only operations (so super fast writes). I index by the aggregate ID as well, so that it's easy to fetch an aggreate's event stream, and also to lock and ensure only one aggregate instance is updated at a time (aggregates need to be immediately consistent). For projections I would suggest Kafka, it's great for creating and aggregating streams of events, making it easy to spin up projections on the fly (eventually consistent). You could also use Redis streams, but I don't have experience using that technology so I can't give advice, if the idea appeals though here's a list of the pros and cons: logz.io/blog/kafka-vs-redis/

 

Thanks for a quick reply, Barry!

I also was thinking about RDBMS as event store. MySQL has great performance, and with it's support for JSON datatype it becomes even easier to store arbitrary data. With metadata stored in regular columns (like created_at TIMESTAMP, event_type VARCHAR, aggregate_id INT) you also get lots of abilities to SELECT only required events in proper order. With indexes on metadata columns, SELECTs become fast (although it goes at a price of slightly degraded WRITE performance). Having atomicity in place is another big benefit of RDBMS.

As of business language: I literally think it's a matter of communication skills and domain knowledge spread within the team. At least it's much more about that that about technology in use. I've been talking to business leaving tech questions aside for years (and, well, that means, years before I first heard about ES)

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