CQRS / Event Sourcing — Rust

Library

cqrs-rust-lib

Opinionated Deviations from Pure CQRS-ES

Pure CQRS-ES	This implementation
Snapshot = optional optimization	Snapshot = primary read model, written on every command
One command type per operation	CreateCommand + UpdateCommand per aggregate
Views updated after event persistence	ViewDispatcher fires after (event + snapshot) pair
No prescribed query layering	Query in domain + QueryBuilder in infrastructure

Aggregate Pattern

impl Aggregate for User {
    const TYPE: &'static str = "users";
    type CreateCommand = CreateUserCommands;
    type UpdateCommand = UpdateUserCommands;
    type Event = UserEvents;
    type Services = Arc<dyn UserServices + Send + Sync>;
    type Error = UserError;

    async fn handle_create(cmd: Self::CreateCommand, svc: &Self::Services) -> Result<Vec<Self::Event>, Self::Error>;
    async fn handle_update(&self, cmd: Self::UpdateCommand, svc: &Self::Services) -> Result<Vec<Self::Event>, Self::Error>;
    fn apply(&mut self, event: Self::Event) -> Result<(), Self::Error>;
}

Query / QueryBuilder Split

UserQuery — plain data, no DB knowledge — lives in domain. UserQueryBuilder — translates UserQuery to a DB Document — lives in infrastructure.

CqrsContext

Carries user identity for audit trails. Extracted in HTTP middleware, threaded through all commands.

ViewDispatcher

ViewDispatcher<A, V, Q> fires after (event + snapshot) pair is persisted.

impl View<Account> for Movement {
    const TYPE: &'static str = "movement";
    const IS_CHILD_OF_AGGREGATE: bool = true; // true = one record/event; false = one record/aggregate

    fn view_id(event: &EventEnvelope<Account>) -> String;
    fn update(&self, event: &EventEnvelope<Account>) -> Option<Self>; // None = ignore this event
}

update() must be pure — no I/O, no side-effects.

Rules

• Aggregate + View structs in domain crate — no infra imports
• QueryBuilder + ViewDispatcher wiring in infrastructure crate
• Events persisted via engine — no direct snapshot mutation
• CqrsContext propagated from entry point, never created ad-hoc in domain