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

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re: I see your point now, I made the assumption that we were talking about a CQRS event driven system, as opposed to a CQRS system where the query side...

No, sorry, you don't understand. Query loads a projection, which is built from the aggregate. You shouldn't load the whole aggregate to query; if you do it's not really CQRS. You denormalise your whole data model; storing projections to minimise impedance mismatch and to maximise query. Quite often that means rdbms for command side and docdb for query. Your infrastructure updates the projections in the docDb a/synchronously using publisher/subscriber model on the aggregate root.

Secondly, it's not about simplicity, it's about the right data model for the right problem. CQRS+ES is just another tool to fit certain problems. I've seen ridiculously complex calculator APIs written in C that run pretty much in memory, dumping usage stats out to postgres. Highly available and enterprise but not simple and not CQRS worthy either.

Don't fixate on one architecture. If you do, you'll find yourself trying to force everything into that form; even when it doesn't fit.

Commands model the process, not the events. The events are the data as stored. Commands create events. You don't model your domain around events but around commands. If you asked a client to write down user stories to work out your aggregates then your end up with commands first (what they are trying to do) then the events to hold the data comes next. Group commands together and you get aggregates.

How the commands then persist the data isn't important. Whether you hydrate your aggregate through events and handlers or through a document/rdbms load is up to the best fit model. Event Store, for example, assumes that your events are immutable. Fine for some applications but not all.

I hope that makes sense.

Hi Rob, thank you for that. I hadn't thought of modelling a CQRS system like that, it's a really nice solution that solves a lot of problems. There's a lot of food for thought above and I appreciate you taking the time to lay it all out for me, and those reading.

The problem that I have is the statement that events, and I then assume that you mean commands also, are not necessarily immutable. Events happen in past tense. They are by definition immutable. A command is issued, whether or not anything happens because of it is irrelevant. The command was issued. Immutable.

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