Domain Events

Events are facts: things that already happened.

Basic Usage

Define a plain struct and implement DomainEvent with a stable KIND.

#[derive(Clone, Debug, Serialize, Deserialize)]
pub struct OrderPlaced {
    pub product_id: String,
    pub quantity: u32,
    pub unit_price: i64,
}

impl DomainEvent for OrderPlaced {
    const KIND: &'static str = "order.placed";
}

Rules of Thumb

• Use past tense names (OrderPlaced, not PlaceOrder)
• Keep KIND stable forever once persisted
• Keep payload focused on a single fact

Why First-Class Structs?

This crate keeps events as standalone structs rather than forcing enum variants:

• Reuse across aggregates
• Decoupled projections by event type
• Smaller, explicit imports

The aggregate derive still generates an internal event enum for replay and storage routing.

KIND Naming Guidance

const KIND: &'static str = "order.placed"; // Good
const KIND: &'static str = "OrderPlaced";  // Acceptable
const KIND: &'static str = "placed";       // Too generic

Event Design Guidelines

1. Include values needed to replay decisions later.
2. Keep aggregate IDs in the envelope, not event payload.
3. Prefer multiple specific events over one broad event.

Trait Reference

pub trait DomainEvent {
    const KIND: &'static str;
}

Next

Commands — Handling requests that produce events