How to build Projections

Date: 2023-12-27

Consider a basic banking application with functionalities to deposit and withdraw funds:

pub enum BankAccountEvent {
    Deposited { amount: u32 },
    Withdrawn { amount: u32 },
}
 
pub struct BankAccountState {
    pub balance: u32,
}

The role of the State is to enforce one business rule: the balance cannot be negative. Its second role is to be stored as a snapshot to be retrieved without having to fetch all the past events.

Consider we want a simple Projection that sums the deposits and withdrawals. We cannot derive this Projection from the State. We would have to either change the State (which is forbidden because the domain should not depend on the application) or fetch all the events (which is what we tried to avoid in the first place by creating a state).

The application might want to have a "replay" feature to be able to rebuild the State or a Projection from scratch when there is an issue.