Story

A story describes one piece of behavior in the business’s own words: given this history, when this command is sent, then these events are recorded. It’s also a test. It runs your real model in memory, so what it describes is what the model does.

CommandAggregateEventRejection

Read one

This story says an overdraft is refused, and what the refusal reports:

packages/domain/test/withdrawals.test.ts

it.effect("an overdraft is rejected with the evidence", () =>
  story(
    Story.given(account, opened, new MoneyDeposited({ amount: 50, balanceAfter: 50 })),
    asSam,
    Story.when(account, new WithdrawMoney({ amount: 80 })),
    Story.rejected(new InsufficientFunds({ balance: 50, requested: 80 })),
  ),
);

given writes history straight onto the account, without sending commands. when sends one command, and the checks after it say what must have happened.

It runs the whole model

Story.make builds a runner from your model, so a story runs everything a command sets off. The aggregate decides, views update, and policies react and send their own commands.

packages/domain/test/withdrawals.test.ts

const story = Story.make(Bank, {
  layer: BankHandlers.pipe(Layer.provide(MemoryLedgerStore)),
  externalSystems: [FraudCheck, Payments],
});

Two things stay under your control. The clock only moves when the story moves it, so deadlines and daily limits are testable. And every call to an external system waits until the story answers it, so you decide what the outside world says.

A passing story accounts for everything

A story fails if a command is rejected and no step checks the rejection, or if an external system call is still waiting at the end. So a story that passes has accounted for every outcome, not just the ones it mentions.

See Testing a model for the full set of checks, the clock, and answering outside systems.