Projection

A projection writes events somewhere else, in batches. It's replayable, so writes must be idempotent.

Import

import { projection, trycatch } from '@marcelsud/accord'

Usage

const Totals = projection(
  'Totals',
  { from: Account, on: [Deposited], batch: { size: 100 } },
  (batch) =>
    trycatch(() => {
      for (const { source, event } of batch) increment.run(source.id, event.amount)
    })
)

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

The body receives an array of envelopes, batched per source instance, and returns a Result.

Options

from

Aggregate | View.

on

Event[]. The events to receive.

uses optional

Record<string, Dependency>.

version optional

number, default 1. A new version replays all source history.

batch optional

{ size: number }, default { size: 100 }.

Idempotency

• Use stable event IDs in destination upserts.
• Bumping the version replays history. Migrate or clear the destination first, or write to a new versioned destination, otherwise counters double.

Atomic SQL effects

With the SQLite store, write through storage.database inside the handler. The batch's SQL changes and its checkpoint commit or roll back together.