ddd-ts-kit/application-command-query-separation

Architecture id: application/command-query-separation (added by this plugin). Default severity: warn in recommended, error in strict.

Applies to: files under application/queries/ of a context, tests excluded.

Why

A query answers and never changes state: it can then be retried, cached and run on a replica. A query that saves is a command with a return value.

Reported

• a call of save, delete or publish on something held by the handler (this.invoices.save(...), this.#bus.publish(...))

Incorrect:

export class SearchInvoicesHandler {
  async execute(query: SearchInvoicesQuery): Promise<SearchInvoicesResult> {
    await this.audit.save(query);
    return this.reader.read(query);
  }
}

Correct:

export class SearchInvoicesHandler {
  async execute(query: SearchInvoicesQuery): Promise<SearchInvoicesResult> {
    return this.reader.read(query);
  }
}

Details

Pragmatic by design: the rule matches method names on dependencies reached through this. A local Set whose delete is called is not reported.

Options

[{ "mutators": ["save", "delete", "publish"] }] replaces the list of method names.

rules."application/command-query-separation" in ddd-ts-kit.config.json set to off silences the rule, and // ddd-ts-kit-disable-next-line application/command-query-separation -- <reason> silences one line, exactly as for ddd-ts-kit check.

Limits

• A mutation behind another name (upsert, record) needs the mutators option.
• A dependency passed as an argument instead of injected is not followed.
• The other half of the principle, a command returning more than an identifier, is left to review.

With the CLI checker

New with this plugin: the CLI checker has no equivalent, because the shape of a class needs a syntax tree.