Kata Event Sourcing et CQRS

Event Sourcing

1. Create an account with a simple operation 'deposit' without event

2. Transform this operation deposit to an event Note : an event describe a fact that already happened !

3. Persist account

4. Reload the state of an account

5. Create another operation like 'withdraw'

Command Query Responsibility Segregation

1. Make commands as input of your system for 'deposit' and 'withdraw'

2. Dispatch events synchronously, which are listened by a a reader

3. Make a projection of an account

4. how to solve concurrency ?

   • How to deal with a slow state rebuild ?
   • How to deal with two commands fired on a same account ?
   • How to deal with the reading order when two events come to the same account ?

5. Scalability : split them all !