Policy

A policy says "when this happens, do that". It reacts to new events only and never replays history.

Import

import { on, policy } from '@marcelsud/accord'

Usage

const NotifyDeposit = policy(
  'NotifyDeposit',
  { from: Account, uses: { notifications: Notifications } },
  on(Deposited, (event, { notifications }, { id, source }) =>
    notifications.send({
      eventId: id,
      accountId: source.id,
      amount: event.amount,
      balance: event.balance,
    })
  )
)

const Bank = model('Bank', { aggregates: { account: Account }, policies: [NotifyDeposit] })

Each handler receives:

1. the tagged event payload,
2. the declared services plus execution context,
3. the envelope.

Omit parameters you don't use. Return a Result or a promise of one. Add more on() handlers to react to other events from the same source.

Options

from

Aggregate | View. The source.

uses optional

Record<string, Dependency>. Services the handlers need. See Service.

version optional

number, default 1.

Delivery

• Policies run when bank.flush() is called.
• New policies start at the current position. Existing policies resume where they left off.
• They skip history, including events forwarded while a view rebuilds.
• Delivery is at-least-once. Use the envelope id as an idempotency key.
• Duplicate event tags and empty handler lists are rejected.

Effect handlers

on(Deposited, effectify((event, services, envelope) => Effect.succeed(undefined)))

See Effect.

Contract form

policy(name, { from, on, uses? }, body) is also supported. The body receives (envelope, context).