During this sprint I created two versions of a frontend for signing up for memberships. I’ve used two different methods for state management, namely MobX and useGlobalState. For the use case I was looking at MobX turned out to have worked much better.

I am also in the process of creating a backend in Kotlin. I decided that I wanted to tryout following Clean Architecture by Uncle Bob. My plan is to see how that architecture works, then add CQRS, and finally add on Event Sourcing.

My reasoning is that starting from a good but familiar architecture will give me a good foundation to both compare the different approaches, but also means that when I add on new parts of the architecture the code will still follow principles of good software.

What is Event Sourcing

Event Sourcing is a system where the source of truth is events. In other words it is possible to reconstruct the entire application state from a table containing all the changes to the state. This table is called an event store.

Do you need microservices?

Not really but it works nicely together.

Do you need CQRS?

Again no, but it makes sense that the service that writes events to the event store is different from the service that materialises a view that can be queried.

What language do I want to write services in?

Since I want to do functional programming there are some languages that are better than others for functional programming.

Reasonml

Reasonml has great support for functional programming, while having an OOP like syntax. I haven’t seen any examples of it being used to create a web service so the ecosystem might not be well served for that purpose but I will have to look into that further.

Node

Node is pretty good for microservices, and the tooling for serverless functions is very good.

Both javascript and typescript can do some functional programming. There are features that are missing compared to languages like Haskell and reasonml. For instance js & ts does not have pattern matching or typeclasses.

I might even use things like cloud functions to create event driven micro services

How to deal with errors and response-codes

When doing Event Sourcing there is an asynchronous nature to most commands and the web service might have to wait for another event to be dispatched from another service before knowing whether a command has succeeded. Finding a good way to manage that in the context of synchronous http call can be tricky and i don't yet know how to do that.