export class MyService extends WorkerEntrypoint {
  sum(a, b) {
    return a + b;
  }
}

let three = await env.MY_SERVICE.sum(1, 2);

• For starters, you can pass Structured Clonable types as the params or return value of an RPC. (That means that, unlike JSON, Dates just work, and you can even have cycles.)
• You can additionally pass functions in the params or return value of other functions. When the other side calls the function you passed to it, they make a new RPC back to you.
• Similarly, you can pass objects with methods. Method calls become further RPCs.
• RPC to another Worker (over a Service Binding) usually does not even cross a network. In fact, the other Worker usually runs in the very same thread as the caller, reducing latency to zero. Performance-wise, it’s almost as fast as an actual function call.
• When RPC does cross a network (e.g. to a Durable Object), you can invoke a method and then speculatively invoke further methods on the result in a single network round trip.
• You can send a byte stream over RPC, and the system will automatically stream the bytes with proper flow control.
• All of this is secure, based on the object-capability model.
• The protocol and implementation are fully open source as part of workerd.

// OLD STYLE: HTTP-based service bindings.
export default {
  async fetch(req, env, ctx) {
    // Call the auth service to authenticate the user's cookie.
    // We send it an HTTP request using a service binding.

    // Construct a JSON request to the auth service.
    let authRequest = {
      cookie: req.headers.get("Cookie")
    };

    // Send it to env.AUTH_SERVICE, which is our service binding
    // to the auth worker.
    let resp = await env.AUTH_SERVICE.fetch(
        "https://auth/check-cookie", {
      method: "POST",
      headers: {
        "Content-Type": "application/json; charset=utf-8",
      },
      body: JSON.stringify(authRequest)
    });

    if (!resp.ok) {
      return new Response("Internal Server Error", {status: 500});
    }

    // Parse the JSON result.
    let authResult = await resp.json();

    // Use the result.
    if (!authResult.authorized) {
      return new Response("Not authorized", {status: 403});
    }
    let username = authResult.username;

    return new Response(`Hello, ${username}!`);
  }
}

// OLD STYLE: HTTP-based auth server.
export default {
  async fetch(req, env, ctx) {
    // Parse URL to decide what endpoint is being called.
    let url = new URL(req.url);
    if (url.pathname == "/check-cookie") {
      // Parse the request.
      let authRequest = await req.json();

      // Look up cookie in Workers KV.
      let cookieInfo = await env.COOKIE_MAP.get(
          hash(authRequest.cookie), "json");

      // Construct the response.
      let result;
      if (cookieInfo) {
        result = {
          authorized: true,
          username: cookieInfo.username
        };
      } else {
        result = { authorized: false };
      }

      return Response.json(result);
    } else {
      return new Response("Not found", {status: 404});
    }
  }
}

// NEW STYLE: RPC-based service bindings
export default {
  async fetch(req, env, ctx) {
    // Call the auth service to authenticate the user's cookie.
    // We invoke it using a service binding.
    let authResult = await env.AUTH_SERVICE.checkCookie(
        req.headers.get("Cookie"));

    // Use the result.
    if (!authResult.authorized) {
      return new Response("Not authorized", {status: 403});
    }
    let username = authResult.username;

    return new Response(`Hello, ${username}!`);
  }
}

// NEW STYLE: RPC-based auth server.
import { WorkerEntrypoint } from "cloudflare:workers";

export class AuthService extends WorkerEntrypoint {
  async checkCookie(cookie) {
    // Look up cookie in Workers KV.
    let cookieInfo = await this.env.COOKIE_MAP.get(
        hash(cookie), "json");

    // Return result.
    if (cookieInfo) {
      return {
        authorized: true,
        username: cookieInfo.username
      };
    } else {
      return { authorized: false };
    }
  }
}

Let's get fancy! Or should I say… classy?

• Get or update the user's profile info.
• Send the user an email notification.
• Append to the user's activity log.

import { WorkerEntrypoint, RpcTarget } from "cloudflare:workers";

// `User` is an RPC interface to perform operations on a particular
// user. This class is NOT exported as an entrypoint; it must be
// received as the result of the checkCookie() RPC.
class User extends RpcTarget {
  constructor(uid, env) {
    super();

    // Note: Instance members like these are NOT exposed over RPC.
    // Only class (prototype) methods and properties are exposed.
    this.uid = uid;
    this.env = env;
  }

  // Get/set user profile, backed by Worker KV.
  async getProfile() {
    return await this.env.PROFILES.get(this.uid, "json");
  }
  async setProfile(profile) {
    await this.env.PROFILES.put(this.uid, JSON.stringify(profile));
  }

  // Send the user a notification email.
  async sendNotification(message) {
    let addr = await this.env.EMAILS.get(this.uid);
    await this.env.EMAIL_SERVICE.send(addr, message);
  }

  // Append to the user's activity log.
  async logActivity(description) {
    // (Please excuse this somewhat problematic implementation,
    // this is just a dumb example.)
    let timestamp = new Date().toISOString();
    await this.env.ACTIVITY.put(
        `${this.uid}/${timestamp}`, description);
  }
}

// Now we define the entrypoint service, which can be used to
// get User instances -- but only by presenting the cookie.
export class AuthService extends WorkerEntrypoint {
  async checkCookie(cookie) {
    // Look up cookie in Workers KV.
    let cookieInfo = await this.env.COOKIE_MAP.get(
        hash(cookie), "json");

    if (cookieInfo) {
      return {
        authorized: true,
        user: new User(cookieInfo.uid, this.env),
      };
    } else {
      return { authorized: false };
    }
  }
}

export default {
  async fetch(req, env, ctx) {
    // `using` is a new JavaScript feature. Check out the
    // docs for more on this:
    // https://developers.cloudflare.com/workers/runtime-apis/rpc/lifecycle/
    using authResult = await env.AUTH_SERVICE.checkCookie(
        req.headers.get("Cookie"));
    if (!authResult.authorized) {
      return new Response("Not authorized", {status: 403});
    }

    let user = authResult.user;
    let profile = await user.getProfile();

    await user.logActivity("You visited the site!");
    await user.sendNotification(
        `Thanks for visiting, ${profile.name}!`);

    return new Response(`Hello, ${profile.name}!`);
  }
}

name = "app-worker"
main = "./src/app.js"

# Declare a service binding to the auth service.
[[services]]
binding = "AUTH_SERVICE"    # name of the binding in `env`
service = "auth-service"    # name of the worker in the dashboard
entrypoint = "AuthService"  # name of the exported RPC class

Wait, how?

Did you spot the security?

More security: Named entrypoints

export class AuthService extends WorkerEntrypoint {

entrypoint = "AuthService"  # name of the exported RPC class

What about type safety?

import { WorkerEntrypoint } from "cloudflare:workers";

// The interface that your server-side entrypoint implements.
// (This would probably be imported from a .d.ts file generated
// from your server code.)
declare class MyEntrypointType extends WorkerEntrypoint {
  sum(a: number, b: number): number;
}

// Define an interface Env specifying the bindings your client-side
// worker expects.
interface Env {
  MY_SERVICE: Service<MyEntrypointType>;
}

// Define the client worker's fetch handler with typed Env.
export default <ExportedHandler<Env>> {
  async fetch(req, env, ctx) {
    // Now env.MY_SERVICE is properly typed!
    const result = await env.MY_SERVICE.sum(1, 2);
    return new Response(result.toString());
  }
}

import { DurableObject } from "cloudflare:workers";

export class Counter extends DurableObject {
  async increment() {
    // Increment our stored value and return it.
    let value = await this.ctx.storage.get("value");
    value = (value || 0) + 1;
    this.ctx.storage.put("value", value);
    return value;
  }
}

let stub = env.COUNTER_NAMESPACE.get(id);
let value = await stub.increment();

interface Env {
  COUNTER_NAMESPACE: DurableObjectNamespace<Counter>;
}

Eliding awaits with speculative calls

// Makes three round trips.
let foo = await stub.foo();
let baz = await foo.bar.baz();
let corge = await baz.qux[3].corge();

// Same thing, only one round trip.
let foo = stub.foo();
let baz = foo.bar.baz();
let corge = await baz.qux[3].corge();