Mailbox

XState v5 Actor Mailbox - Queue and process messages sequentially for XState machines.

👉 Blog Post: From Message Chaos to Order: How I Rebuilt the Mailbox Library for XState@5, Huan, 2026, Ship.Fail

Overview

Mailbox implements the Actor Mailbox pattern on top of XState v5:

• Incoming messages are queued and processed one at a time
• Child machine signals readiness via Mailbox.actions.idle() to receive next message
• Supports reply actions to send responses back to message senders
• Handles dead letters when mailbox capacity is exceeded

Installation

npm install mailbox xstate

Requirements: Node.js >= 18, XState >= 5.0.0

Quick Start

import * as Mailbox from 'mailbox'
import { setup, assign } from 'xstate'

// Create a machine that processes work one item at a time
const workerMachine = setup({
  types: {} as {
    context: { result: string | null }
    events: { type: 'WORK'; data: string } | { type: 'DONE'; result: string | null }
  },
}).createMachine({
  id: 'worker',
  initial: 'idle',
  context: { result: null },
  states: {
    idle: {
      // RULE 1: Signal readiness in idle state
      entry: Mailbox.actions.idle('worker'),
      on: {
        WORK: 'processing',
      },
    },
    processing: {
      entry: assign({ result: ({ event }) => event.data }),
      after: {
        100: {
          target: 'idle',
          // RULE 2: Reply when done
          actions: Mailbox.actions.reply(
            ({ context }) => ({ type: 'DONE', result: context.result })
          ),
        },
      },
    },
  },
})

// Wrap with mailbox
const mailbox = Mailbox.from(workerMachine)

// Subscribe to replies
mailbox.subscribe({
  next: (event) => console.log('Reply:', event),
})

// Open and send messages
mailbox.open()
mailbox.send({ type: 'WORK', data: 'task1' })
mailbox.send({ type: 'WORK', data: 'task2' })
mailbox.send({ type: 'WORK', data: 'task3' })

// All 3 will be processed sequentially, each receiving a DONE reply

Key Rules

1. Idle Action: Your machine MUST call Mailbox.actions.idle('machine-id') in the entry action of the state where it's ready to accept messages.

2. External Transitions: Use external transitions to re-enter the idle state, triggering the entry action again.

3. Reply Action: Use Mailbox.actions.reply(event) to send responses back to the message sender.

API

Mailbox.from(machine, options?)

Wraps an XState machine with mailbox functionality.

const mailbox = Mailbox.from(machine, {
  capacity: 100,        // Max queue size (default: Infinity)
  logger: console.log,  // Custom logger
  clock: new SimulatedClock(), // For testing
})

mailbox.send(event)

Send an event to the mailbox queue.

mailbox.open() / mailbox.close()

Start/stop the mailbox actor.

mailbox.subscribe(observer)

Subscribe to outgoing events (replies).

mailbox.address

Get the mailbox address for external communication.

Actions

• Mailbox.actions.idle(id) - Signal the machine is ready for next message
• Mailbox.actions.reply(event) - Reply to the message sender
• Mailbox.actions.proxy(id)(target) - Forward events to another mailbox

Type Guards

• Mailbox.isMailbox(value) - Check if value is a Mailbox
• Mailbox.isAddress(value) - Check if value is an Address
• Mailbox.isMailboxType(type) - Check if event type is internal Mailbox type

Constants

• Mailbox.Type - Internal event types (ACTOR_IDLE, ACTOR_REPLY, DEAD_LETTER)
• Mailbox.Event - Event factory functions
• Mailbox.State - Mailbox states (Idle, Processing)

Validation

// Validate a machine satisfies the Mailbox protocol
Mailbox.validate(myMachine) // throws MailboxValidationError if invalid

RxJS Observable Support

Mailbox implements the Observable protocol for RxJS interoperability:

import { from } from 'rxjs'
import * as Mailbox from 'mailbox'

const mailbox = Mailbox.from(machine)
mailbox.open()

// Use RxJS operators
from(mailbox)
  .pipe(filter(e => e.type === 'DONE'))
  .subscribe(console.log)

Testing

Use SimulatedClock for deterministic tests:

import * as Mailbox from 'mailbox'

const clock = new Mailbox.SimulatedClock()
const mailbox = Mailbox.from(machine, { clock })

mailbox.open()
mailbox.send({ type: 'WORK' })

// Advance time
clock.increment(100)
await new Promise(r => setTimeout(r, 0))

// Assert results...

The Problem Mailbox Solves

XState machines process events immediately. When multiple events arrive while processing, they can be lost:

Customer A: MAKE_COFFEE  → Processing...
Customer B: MAKE_COFFEE  → LOST! (machine is busy)
Customer C: MAKE_COFFEE  → LOST! (machine is busy)

With Mailbox, events are queued and processed sequentially:

Customer A: MAKE_COFFEE  → Queued → Processing → Done
Customer B: MAKE_COFFEE  → Queued → Processing → Done
Customer C: MAKE_COFFEE  → Queued → Processing → Done

Breaking Changes (v1.0.0)

This version is a complete rewrite for XState v5. Breaking changes from v0.x:

• Requires XState v5 - No longer compatible with XState v4
• Removed duckularize() - Use native XState v5 typed events instead
• Removed wrap() - Use from() instead
• Removed internal context utilities - XState v5 handles this natively
• Simplified API - Cleaner, more focused interface

Migration from duckularize

Before (v0.x):

import { createAction } from 'typesafe-actions'
const Event = { DING: createAction('DING')() }
const duckula = Mailbox.duckularize({ id: 'test', events: Event, ... })

After (v1.0):

// Use plain objects and XState v5 native typing
const Type = { DING: 'DING' } as const
const Event = { DING: () => ({ type: Type.DING }) as const }
const machine = setup({
  types: { events: {} as { type: 'DING' } }
}).createMachine({ ... })

License

Apache-2.0