Cart Kata using Axon 4.1

Requirement

docker-compose
java 8
yarn

Run

Start Redis, Redis GUI, AxonServer: docker-composer up
Build app: run ./mvnw package
Start domain: java -Dspring.profiles.active=domain -jar target/axon-0.0.1-SNAPSHOT.jar
Start query: java -Dspring.profiles.active=query -jar target/axon-0.0.1-SNAPSHOT.jar
Start client: java -Dspring.profiles.active=client -jar target/axon-0.0.1-SNAPSHOT.jar
start front-end: cd front-end && yarn dev
Go to: http://localhost:3000

How to

Problematics

Data synchronization

The problem with CQRS/Event Sourcing is when you write data (by sending a command) is not transactional, you don't know when your view/projection will be updated. When your front-end app send a CreateCart http request, client app will just acknowledge the command if everything went well or not, but you have no clue if your query app have build the projection.

Solutions:

In your front-end app, retry request until the data is available. PRO: simple, easy? CON: Lot of request, flood network until the data is available
In your client app, you can make the command transactional by adding in the transaction the time to put the projection updated. PRO: like the old way (meh!) CON: difficult and you need to have a compatible infrastructure
In your client app, expose a subscriber (like realtime database from firebase) end point so front-end app can be advertise on every change. PRO: clean, scalable CON: difficult and you need to have a compatible infrastructure

I prefer the last solution, it's more elegant, clean and scalable. The good things is we can do it with the actual infrastructure: Redis and AxonServer

You can configure Redis by setting the option notify-keyspace-events (in ./docker-compose.yml) that send a message on every set/add/update on redis data.
AxonServer give us subscriptionQuery that allow us to subscribe to every change on a query
In front-end app we will use Server Sent Event to receive in real time data changed

BONUS: with this solution all the devices connected to the same cart will be updated in real time.