Everything is an Actor

David Khourshid - @davidkpiano

👩‍💻

🧔

👩‍🍳

☕️❔

📝

💶❓

💶

📄✅

☕️

👩‍💻

🧔

I would like a coffee...

What would you like?

Actor

I would like a coffee, please.

Actor

💭

💬

Here you go. ☕️

Thanks!

https://en.wikipedia.org/wiki/Eventual_consistency

What is an actor?

Actor 👩‍💻

Actor 🧔

✉️

someActor.send({
  type: 'greet',
  value: 'Hei!',
  from: self
});

// ...

switch (message.type) {
  case 'greet':
    message.from.send({
      type: 'greet',
      value: '...',
      from: self
    });
  // ...
}

Send

Receive

The Actor Model

A Universal Modular ACTOR Formalism for Artificial Intelligence (1973)

Carl Hewitt, Peter Bishop, Richard Steiger

Alan Kay

Coined "OOP"
Emphasized message-passing
Basically the Actor Model

https://en.wikipedia.org/wiki/Actor_model

Rules of actors

🤖 All computation is performed within an actor

📩 Actors can communicate only through messages

💥 In response to a message, an actor can:

📬 Send messages to other actors

👨‍👧‍👦 Create a finite number of child actors

𝑓 (𝑥)

Behavior

State

🎭 Change its state/behavior

Rules of actors

✉️

🤖 All computation is performed within an actor

📩 Actors can communicate only through messages

💥 In response to a message, an actor can:

📬 Send messages to other actors

👨‍👧‍👦 Create a finite number of child actors

🎭 Change its state/behavior

Rules of actors

✉️

🤖 All computation is performed within an actor

📩 Actors can communicate only through messages

💥 In response to a message, an actor can:

📬 Send messages to other actors

👨‍👧‍👦 Create a finite number of child actors

🎭 Change its state/behavior

A → A'

Rules of actors

✉️

🤖 All computation is performed within an actor

📩 Actors can communicate only through messages

💥 In response to a message, an actor can:

📬 Send messages to other actors

👨‍👧‍👦 Create a finite number of child actors

🎭 Change its state/behavior

✉️

Rules of actors

✉️

🤖 All computation is performed within an actor

📩 Actors can communicate only through messages

💥 In response to a message, an actor can:

📬 Send messages to other actors

👨‍👧‍👦 Create a finite number of child actors

🎭 Change its state/behavior

🆕

Behavior

idle

making coffee

delivering coffee

☕️ ordered

☕️ made

☕️ delivered

{
  orders: [/* ... */],
  inventory: {
    // ...
  },
  sales: 134.65
}

Finite State (behavior)

Extended State (data)

Actor System

Root (Guardian) Actor

cafe

cafe/manager1

cafe/manager1/employee1

Address

📫

✉️

state 1

✉️

state 2

✉️

processing...

state ??

Mailboxes

✉️

Mailboxes

✉️

state 1

✉️

Mailboxes

✉️

state 1

✉️

state 2

✉️

Mailboxes

✉️

state 1

✉️

state 2

✉️

state 3

Mailboxes

✉️

state 1

✉️

state 2

✉️

state 3

Mailboxes

✉️

state 4

Location Transparency

✉️→

💻

🛰

ActorRef

✉️

Fault Tolerance

✉️→

💻

ActorRef

✉️

❌

💻

✉️

const popup = window.open(/* ... */);

popup.postMessage(/* ... */);

function receiveMessage(msg) {
  // do something with the message
}

window.addEventListener('message', receiveMessage);

function receiveMessage(msg) {
  // do something with the message

  msg.source.postMessage(/* ... */);
}

window.addEventListener('message', receiveMessage);

const popup = window.open(/* ... */);

popup.postMessage(/* ... */);

function receiveMessage(msg) {
  // do something with the message
}

window.addEventListener('message', receiveMessage);

function receiveMessage(msg) {
  // do something with the message

  msg.source.postMessage(/* ... */);
}

window.addEventListener('message', receiveMessage);

merge(...)

  scan(reducer, initial)

initial

pipe(

)

Behavior

Actors

Actor

Parent

Any event

SEND

RECEIVE

Any event

Actors: Promise

Promise

Parent

Resolved value
Rejected value

AbortController signal

SEND

RECEIVE

Actors: Observable

Observable

Parent

Multiple values
Completion signal
Error signal

Nothing *

SEND

RECEIVE

* RxJS has Subjects for this

https://github.com/tc39/proposal-observable

github.com/davidkpiano/xstate

npm install xstate

xstate.js.org

import { createMachine } from 'xstate';

const machine = createMachine({
  initial: 'idle',
  states: {
    idle: {
      on: { SEARCH: 'searching' }
    },
    searching: {
      on: {
        RESOLVE: 'success',
        REJECT: 'failure',
        SEARCH: 'searching'
      }
    },
    success: {
      on: { SEARCH: 'searching' }
    },
    failure: {
      on: { SEARCH: 'searching' }
    }
  }
});

import { createMachine } from 'xstate';

const machine = createMachine({
  // ...
});

const searchingState = machine
  .transition('idle', 'searching');

searchingState.value;
// => 'searching'

import { createMachine, interpret } from 'xstate';

const machine = createMachine({
  // ...
});

const service = interpret(machine)
  .onTransition(state => console.log(state))
  .start();

service.send('SEARCH');
// State {
//   value: 'searching',
//   ...
// }

import { createMachine, interpret } from 'xstate';
import { from } from 'rxjs';

const machine = createMachine(/* ... */);

const service = interpret(/* ... */).start();

// Services are already subscribable!
const state$ = from(service);

state$.subscribe(/* ... */);

import { createMachine } from 'xstate';

const cafeMachine = createMachine({
  // ...
  states: {
    // ...
    makingCoffee: {
      invoke: {
        id: 'espresso',
        src: espressoMachine,
        onDone: 'deliveringCoffee'
      }
    },
    deliveringCoffee: {
      // ...
    }
  }
});

Invoke a new "actor"

with espressoMachine behavior

"done.invoke.espresso"

import { createMachine } from 'xstate';

const cafeMachine = createMachine({
  // ...
  states: {
    // ...
    makingCoffee: {
      invoke: {
        id: 'espresso',
        src: espressoMachine,
        onDone: 'deliveringCoffee'
      }
    },
    deliveringCoffee: {
      // ...
    }
  }
});

cafeMachine

espressoMachine

done.invoke.espresso

import { createMachine, send } from 'xstate';

const cafeMachine = createMachine({
  // ...
  states: {
    // ...
    makingCoffee: {
      invoke: {
        id: 'espresso',
        src: espressoMachine,
        onDone: 'deliveringCoffee'
      },
      on: {
        CHANGE_ORDER: {
          actions: send((_, event) => event, {
            to: 'espresso'
          })
        }
      }
    },
    // ...
  }
});

cafeMachine

espressoMachine

CHANGE_ORDER

import { createMachine, forwardTo } from 'xstate';

const cafeMachine = createMachine({
  // ...
  states: {
    // ...
    makingCoffee: {
      invoke: {
        id: 'espresso',
        src: espressoMachine,
        onDone: 'deliveringCoffee'
      },
      on: {
        CHANGE_ORDER: {
          actions: forwardTo('espresso')
        }
      }
    },
    // ...
  }
});

cafeMachine

espressoMachine

CHANGE_ORDER

import { createMachine, sendParent } from 'xstate';

const cafeMachine = createMachine({
  // ...
});

const espressoMachine = createMachine({
  // ...
  states: {
    // ...
    broken: {
      entry: sendParent('MACHINE_BROKEN')
    },
    // ...
  }
});

cafeMachine

espressoMachine

MACHINE_BROKEN

import { createMachine, assign, spawn } from 'xstate';

const createOrder = (details) => {/* ... */}

const cafeMachine = createMachine({
  context: {
    orders: []
  },
  // ...
  on: {
    PLACE_ORDER: {
      actions: assign({
        orders: (context, event) => {
          const order = spawn(
            createOrder(event.details),
            event.orderId);
          
          return context
            .orders.concat(order);
        }
      })
    }
  }
});

order 1

order n

cafeMachine

. . .

import { createMachine, send } from 'xstate';

const cafeMachine = createMachine({
  // ...
  on: {
    CANCEL: {
      actions: send('CANCEL', {
        to: (_, event) => event.orderId
      })
    }
  }
});

order 1

order n

cafeMachine

. . .

CANCEL

import { createMachine } from 'xstate';
import { invokeObservable } from 'xstate/invoke';

function createOrder(details) {
  return /* ... */
}

const cafeMachine = createMachine({
  // ...

  invoke: {
    src: invokeObservable(() => /* ... */)
  },
  
  // ...

  on: {
    ORDER: {
      actions: assign({
        orders: (context, event, { spawn }) => {
          const order = spawn.from(
            createOrder(event.details), event.id);
          
          return context.orders
            .concat(order);
        }
      })
    }
  }
});

5.0

(tentative API)

Returns an actor creator

import { createBehavior, createSystem } from 'xactor';


const counter = createBehavior((state, message) => {
  if (message.type === 'add') {
    return {
      count: state.count + 1
    }
  }
  
  return state;
}, { count: 0 });


const counterSystem = createSystem(counter, 'myCounter');


counterSystem.send({
  type: 'add'
});

import { createBehavior, createSystem } from 'xactor';


const createTodo = (title) => createBehavior((state, message) => {
  // ...
}, { title })


const todos = createBehavior((state, message, ctx) => {
  switch (message.type) {
    case 'todo.create':
      const newTodo = ctx.spawn(createTodo(message.title), `todo-${state.todos.length}`);
      
      return {
        ...state,
        todos: [
          ...state.todos,
          newTodo
        ]
      };
  }
}, { todos: [] });


const todoSystem = createSystem(todos, 'myCounter');


todoSystem.send({
  type: 'todo.create',
  title: 'Learn the actor model!'
});

Thank you ReactiveConf!

David Khourshid - @davidkpiano

Everything is an Actor

By David Khourshid

Everything is an Actor

3,092

Everything is an Actor

What is an actor?

The Actor Model

Rules of actors

Rules of actors

Rules of actors

Rules of actors

Rules of actors

Behavior

Actor System

Mailboxes

Mailboxes

Mailboxes

Mailboxes

Mailboxes

Mailboxes

Location Transparency

Fault Tolerance

Actors

Actors: Promise

Actors: Observable

5.0

Thank you ReactiveConf!

Everything is an Actor

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