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.

Work with legacy

Metrics

Admin

• Swagger UI: http://localhost:8080/swagger-ui.html

• Access AxonServer UI: http://localhost:8024

• Access to Redis UI: http://localhost:9090