Actors and State Machines will solve State Management

Ever heard about Actors, the Actor model, Message-passing?

It's the next level of doing state management 💁🏼‍♂️

I got a glimpse into the future of state management with Actors (XState) this week.

Let me share this with you 👇

Tech stack

• XState: since v5 the main building block of XState is an actor. This week I learned why and how 💡
• Effect: combine the actor model of XState with Effect (services, layers, error handling) and you will obtain an immense power 🔥

Setup

No limit here. Both XState and Effect are 0 dependencies, plain typescript: they work everywhere 💁🏼‍♂️

I used Vite with React 👇

"dependencies": {
  "@xstate/react": "^4.0.3",
  "effect": "^2.3.2",
  "framer-motion": "^11.0.3",
  "nanoid": "^5.0.5",
  "react": "^18.2.0",
  "react-dom": "^18.2.0",
  "xstate": "^5.6.2"
}

Get started

This week project is an editor for animated code snippets 🎥

Sandro Maglione

@SandroMaglione

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Create a service with the new `Context.Tag` of @EffectTS_ 🏗️ 👉 Create a service class 👉 Define the interface 👉 Create a Layer 👉 Define layer implementation

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6:18 PM · Feb 13, 2024

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2 building blocks:

• A single XState machine to manage the state (inside machine 👇)
• Effect services and layers to organize dependencies

A single "machine" folder contains all the XState code, while Effect is used to organize services and layers ("Highlight" and "Highlighter")

Implementation

Now, what's the deal with actors? 🤔

Each actor has:

• Encapsulated (private) state
• Communication by sending and receiving events asynchronously
• Create (spawn/invoke) new actors

In practice this means:

• Manage internal state, without the risk of unintended mutations
• Clear logic using events: nothing changes if no events are sent
• Coordination and concurrency: each actor responsible for a subset of the system
• Easy to scale: create/spawn new actors for new requirements

[…] there is no such thing as a single source of truth in any non-trivial application. All applications, even front-end apps, are distributed at some level

Local reasoning solves state management

Since every application is distributed, we need a (reliable) way to manage complexity.

Solution: local reasoning 💡

Local Reasoning: property of some code wherein the correctness of the code can be inferred locally, without considering prior application state or all possible inputs (Source)

It makes sense: the system is a big-complex-distributed beast, but with local reasoning we only care about a small subset that works independently from the whole app.

Actors for state management

Example: event that needs to fetch some async resources, compute and update the current state 🤯

• What about errors? What if something goes wrong (it's async after all 💁🏼‍♂️)?
• What happens while the async request is processing?
• Who is responsible to update the state?

Actors solution:

1. Send an update event to the main actor
2. The actor spawns another actor responsible to process the request
3. The sub-actor works independently on the async task
4. When done, the sub-actor reports the output (fail or success) back to the parent actor
5. The parent actor gets the result and updates its own state

export const editorMachine = setup({
  actors: {
    // 4️⃣ Sub-actor works independently on the request
    onAddEvent: fromPromise<
      Partial<Context.Context>,
      { params: Events.AddEvent; context: Context.Context }
    >(({ input: { context, params } }) => Actions.onAddEvent(context, params)),
  },
}).createMachine({
  id: "editor-machine",
  context: Context.Context,
  initial: "Idle",
  states: {
    AddingLines: {
      invoke: {
        // 2️⃣ Spawn child actor
        src: "onAddEvent",
        input: ({ context, event }) => {
          if (event.type === "add-event") {
            // 3️⃣ Pass required parameters to the sub-actor
            return { context, params: event };
          }

          throw new Error("Unexpected event type");
        },
        // 5️⃣ Handle error result
        onError: {
            target: ".Idle",
        },
        // 5️⃣ Handle success result
        onDone: {
          target: "Idle",
          // 👇 Success: Update parent actor state (`assign`)
          actions: assign(({ event }) => event.output),
        },
      },
    },
    Idle: {
      on: {
        // 1️⃣ Send event to the main actor
        "add-event": {
          target: "AddingLines",
        },
      },
    },
  },
});

The sub-actor (onAddEvent) is independent: this unlocks local reasoning 🚀

// No need to know about the full app, just focus on your own internal logic (with `Effect`) 🪄
export const onAddEvent = (
  context: Context.Context,
  params: Events.AddEvent
): Promise<Partial<Context.Context>> =>
  Effect.gen(function* (_) {
    /// ...
  }).pipe(
    /// ...
    Effect.runPromise
  );

This scales to any level of complexity: you can spawn even more complex actors, each working independently, with their own state and clear events to communicate

Embrace this new power 🔥

I wrote a complete step-by-step article on how state machines and actors work in XState v5.

Don't miss this, it's the future (and present) of state management 🔥

Takeaways

• Actors and state machines will solve state management (at all levels of complexity)
• XState + Effect is the most powerful combination of libraries in Typescript
• Complexity cannot be avoided, but it can be managed 🛠️
• Every app is a distributed system: local reasoning to the rescue 🦸

You can read the full code on the open source repository 👇

Next week it's the week: the Effect Days are here 🚀

I will share my full experience at the conference and all behind the scenes 🔜

See you next 👋