2.2. Add Domain Events

Define domain events to represent past-tense historical facts.

Once a command is successfully validated, the aggregate root must emit one or more Domain Events.

An event represents a historical fact—something that has already happened within our system. Because they represent the past, domain events are always named using the past tense (e.g., AccountOpened, MoneyDeposited).

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Defining Domain Events

In our framework, domain events are represented as a simple Rust enum, and we implement the DomainEvent trait to supply metadata about the event schema.

use ddd_cqrs_es::DomainEvent;

// =========================================================================
// Define Domain Events (The Facts)
// =========================================================================
// Events must represent historical facts that have already occurred. They
// must be stable, immutable, and serializable.
#[derive(Clone, Debug, PartialEq)]
pub enum BankAccountEvent {
    AccountOpened {
        account_id: String,
        owner: String,
    },
    MoneyDeposited {
        amount: u64,
    },
    MoneyWithdrawn {
        amount: u64,
    },
}

impl DomainEvent for BankAccountEvent {
    // Unique identifier for the event schema, useful for adapters or databases
    fn event_type(&self) -> &'static str {
        match self {
            BankAccountEvent::AccountOpened { .. } => "bank_account_opened",
            BankAccountEvent::MoneyDeposited { .. } => "money_deposited",
            BankAccountEvent::MoneyWithdrawn { .. } => "money_withdrawn",
        }
    }
}

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Event Design Guidelines

When designing events for your business domain, adhere to these guidelines:

1. Past Tense: Events represent facts that are set in stone. Always name them in the past tense (AccountOpened, MoneyDeposited).
2. Immutable: Events represent history and should never be modified once committed. In Rust, derive Clone, Debug, and PartialEq on your events, and keep them free of pointers, mutable references, or stateful handles.
3. Self-Contained: Events must carry all the parameters that represent the delta of the change. A MoneyDeposited event must carry the amount that was deposited so that replays and read model projections can calculate the resulting balance.