ddd-ts-kit/event-sourcing-state-in-apply

Architecture id: event-sourcing/state-in-apply. Default severity: error in recommended, error in strict.

Applies to: domain/aggregates/ of a context, tests excluded: the classes that extend EventSourcedAggregate.

Why

An event-sourced aggregate is rebuilt by replaying its events through its apply methods. A command that also writes the state itself leaves a state no replay rebuilds: the aggregate behaves one way just after the command and another way once loaded.

Reported

• outside the constructor, the static methods and the apply methods (apply<Fact>, and restore for a snapshot), an assignment or an increment that starts at this
• there, a mutating call on something of this (push, splice, sort, set, add, delete, clear...)
• in an apply method, this.record(..): a replay would record the event again

Incorrect:

noteNoShow(appointmentId: string, now: Date): void {
  this.state = { noShows: this.state.noShows + 1 };
  this.record(new NoShowNoted(this.id, now, appointmentId));
}

Correct:

noteNoShow(appointmentId: string, now: Date): void {
  this.record(new NoShowNoted(this.id, now, appointmentId));
}

private applyNoShowNoted(event: NoShowNoted): void {
  this.state = { noShows: this.state.noShows + 1 };
}

Options

None.

rules."event-sourcing/state-in-apply" in ddd-ts-kit.config.json set to off silences the rule, and // ddd-ts-kit-disable-next-line event-sourcing/state-in-apply -- <reason> silences one line, exactly as for ddd-ts-kit check.

Limits

• A write through a local alias (const state = this.state; state.items.push(x)) is not followed.
• A class that extends EventSourcedAggregate under another name (an alias, a base class of the project) is not recognised.

With the CLI checker

ddd-ts-kit check reports the same id from the text of the files: the writes it finds in the bodies of the methods of a class whose superclass is written EventSourcedAggregate. This rule is the authority.