Realtime apps in AWS with SST

Use SST to build and deploy a realtime chat app to AWS.

We are going to use SST to build and deploy a realtime chat app on AWS.

Before you get started, make sure to configure your AWS credentials.

1. Create a project

Let’s start by creating a Node.js app.

npx create-next-app@latest my-realtime-app
cd my-realtime-app

Init SST

Now let’s initialize SST in our app.

npx sst@latest init
npm install

Select the defaults and pick AWS. This’ll create a sst.config.ts file in your project root.

Start dev mode

Run the following to start dev mode. This’ll start SST and your Next.js app.

npx sst dev

Once complete, click on MyWeb in the sidebar and open your Next.js app in your browser.

2. Add Realtime

Let’s add the Realtime component and link it to the Next.js app. Update your sst.config.ts.

async run() {
  const realtime = new sst.aws.Realtime("MyRealtime", {
    authorizer: "authorizer.handler",
  });

  new sst.aws.Nextjs("MyWeb", {
    link: [realtime],
  });
},

This component allows us to set up topics that can be subscribed to. The authorizer function can be used control who has access to these.

Add an authorizer

Add the following to a new authorizer.ts file in your project root.

import { Resource } from "sst";
import { realtime } from "sst/aws/realtime";

export const handler = realtime.authorizer(async (token) => {
  const prefix = `${Resource.App.name}/${Resource.App.stage}`;

  const isValid = token === "PLACEHOLDER_TOKEN";

  return isValid
    ? {
      publish: [`${prefix}/*`],
      subscribe: [`${prefix}/*`],
    }
    : {
      publish: [],
      subscribe: [],
    };
});

Here we are saying that a user with a valid token has access to publish and subscribe to any topic namespaced user the app and stage name.

In production, we would validate the given token against our database or auth provider.

3. Create the chat UI

Now let’s create a chat interface in our app. Create a new component in components/chat.tsx with the following.

"use client";

import mqtt from "mqtt";
import { useState, useEffect } from "react";
import styles from "./chat.module.css";

export default function Chat(
  { topic, endpoint, authorizer }: { topic: string, endpoint: string, authorizer: string }
) {
  const [messages, setMessages] = useState<string[]>([]);
  const [connection, setConnection] = useState<mqtt.MqttClient | null>(null);

  return (
    <div className={styles.chat}>
      {connection && messages.length > 0 &&
        <div className={styles.messages}>
          {messages.map((msg, i) => (
            <div key={i}>{msg}</div>
          ))}
        </div>
      }
      <form
        className={styles.form}
        onSubmit={async (e) => {
          e.preventDefault();

          const input = (e.target as HTMLFormElement).message;

          connection!.publish(topic, input.value, { qos: 1 });
          input.value = "";
        }}
      >
        <input
          required
          autoFocus
          type="text"
          name="message"
          placeholder={
            connection ? "Ready! Say hello..." : "Connecting..."
          }
        />
        <button type="submit" disabled={connection === null}>Send</button>
      </form>
    </div>
  );
}

Here we are going to publish a message that’s submitted to the given topic. We’ll create the realtime connection below.

Add some styles.

.chat {
  gap: 1rem;
  width: 30rem;
  display: flex;
  padding: 1rem;
  flex-direction: column;
  border-radius: var(--border-radius);
  background-color: rgba(var(--callout-rgb), 0.5);
  border: 1px solid rgba(var(--callout-border-rgb), 0.3);
}

.messages {
  padding-bottom: 0.125rem;
  border-bottom: 1px solid rgba(var(--callout-border-rgb), 0.3);
}
.messages > div {
  line-height: 1.1;
  padding-bottom: 0.625rem;
}

.form {
  display: flex;
  gap: 0.625rem;
}
.form input {
  flex: 1;
  font-size: 0.875rem;
  padding: 0.5rem 0.75rem;
  border-radius: calc(1rem - var(--border-radius));
  border: 1px solid rgba(var(--callout-border-rgb), 1);
}
.form button {
  font-weight: 500;
  font-size: 0.875rem;
  padding: 0.5rem 0.75rem;
  border-radius: calc(1rem - var(--border-radius));
  background: linear-gradient(
    to bottom right,
    rgba(var(--tile-start-rgb), 1),
    rgba(var(--tile-end-rgb), 1)
  );
  border: 1px solid rgba(var(--callout-border-rgb), 1);
}
.form button:active:enabled {
  background: linear-gradient(
    to top left,
    rgba(var(--tile-start-rgb), 1),
    rgba(var(--tile-end-rgb), 1)
  );
}

Install the npm package.

npm install mqtt

Add to the page

Let’s use the component in our page. Replace the Home component in app/page.tsx.

import { Resource } from "sst";
import Chat from "@/components/chat";

const topic = "sst-chat";

export default function Home() {
  return (
    <main className={styles.main}>

      <div className={styles.center}>
        <Image
          className={styles.logo}
          src="/next.svg"
          alt="Next.js Logo"
          width={180}
          height={37}
          priority
        />
      </div>

      <div>
        <Chat
          endpoint={Resource.MyRealtime.endpoint}
          authorizer={Resource.MyRealtime.authorizer}
          topic={`${Resource.App.name}/${Resource.App.stage}/${topic}`}
        />
      </div>

    </main>
  );
}

Here we are going to publish and subscribe to a topic called sst-chat, namespaced under the name of the app and the stage our app is deployed to.

4. Create a connection

When our chat component loads, it’ll create a new connection to our realtime service. Add the following below the imports in components/chat.tsx.

function createConnection(endpoint: string, authorizer: string) {
  return mqtt.connect(`wss://${endpoint}/mqtt?x-amz-customauthorizer-name=${authorizer}`, {
    protocolVersion: 5,
    manualConnect: true,
    username: "", // Must be empty for the authorizer
    password: "PLACEHOLDER_TOKEN", // Passed as the token to the authorizer
    clientId: `client_${window.crypto.randomUUID()}`,
  });
}

We are using a placeholder token here. In production this might be a user’s session token.

Now let’s subscribe to it and save the messages we receive. Add this to the Chat component.

useEffect(() => {
  const connection = createConnection(endpoint, authorizer);

  connection.on("connect", async () => {
    try {
      await connection.subscribeAsync(topic, { qos: 1 });
      setConnection(connection);
    } catch (e) { }
  });
  connection.on("message", (_fullTopic, payload) => {
    const message = new TextDecoder("utf8").decode(new Uint8Array(payload));
    setMessages((prev) => [...prev, message]);
  });
  connection.on("error", console.error);

  connection.connect();

  return () => {
    connection.end();
    setConnection(null);
  };
}, [topic, endpoint, authorizer]);

Head over to the local Next.js app in your browser, http://localhost:3000 and try sending a message, you should see it appear right away. You can also open a new browser window and see them appear in both places.