In this example we will look at how to use SNS to create a pub/sub system in our serverless app using SST. We’ll be creating a simple checkout flow.

Requirements

Create an SST app

Change indicator Let’s start by creating an SST app.

$ npx create-sst@latest --template=base/example pub-sub
$ cd pub-sub
$ npm install

By default, our app will be deployed to the us-east-1 AWS region. This can be changed in the sst.config.ts in your project root.

import { SSTConfig } from "sst";

export default {
  config(_input) {
    return {
      name: "pub-sub",
      region: "us-east-1",
    };
  },
} satisfies SSTConfig;

Project layout

An SST app is made up of two parts.

  1. stacks/ — App Infrastructure

    The code that describes the infrastructure of your serverless app is placed in the stacks/ directory of your project. SST uses AWS CDK, to create the infrastructure.

  2. packages/functions/ — App Code

    The code that’s run when your API is invoked is placed in the packages/functions/ directory of your project.

Adding SNS Topic

Amazon SNS is a reliable and high-throughput messaging service. You are charged based on the number of API requests made to SNS. And you won’t get charged if you are not using it.

Change indicator Replace the stacks/ExampleStack.ts with the following.

import { Api, StackContext, Topic } from "sst/constructs";

export function ExampleStack({ stack }: StackContext) {
  // Create Topic
  const topic = new Topic(stack, "Ordered", {
    subscribers: {
      receipt: "packages/functions/src/receipt.main",
      shipping: "packages/functions/src/shipping.main",
    },
  });
}

This creates an SNS topic using Topic. And it has two subscribers. Meaning when the topic is published, both the functions will get run.

Setting up the API

Now let’s add the API.

Change indicator Add this below the Topic definition in stacks/ExampleStack.ts.

// Create the HTTP API
const api = new Api(stack, "Api", {
  defaults: {
    function: {
      // Bind the SNS topic name to our API
      bind: [topic],
    },
  },
  routes: {
    "POST /order": "packages/functions/src/order.main",
  },
});

// Show the API endpoint in the output
stack.addOutputs({
  ApiEndpoint: api.url,
});

Our API simply has one endpoint (/order). When we make a POST request to this endpoint the Lambda function called main in packages/functions/src/order.ts will get invoked.

We’ll also bind our topic to our API.

Adding function code

We will create three functions, one handling the /order API request, and two for the topic subscribers.

Change indicator Add a packages/functions/src/order.ts.

export async function main() {
  console.log("Order confirmed!");
  return {
    statusCode: 200,
    body: JSON.stringify({ status: "successful" }),
  };
}

Change indicator Add a packages/functions/src/receipt.ts.

import { SNSEvent } from "aws-lambda";

export async function main(event: SNSEvent) {
  const records: any[] = event.Records;
  console.log(`Receipt sent: "${records[0].Sns.Message}"`);

  return {};
}

Change indicator Add a packages/functions/src/shipping.ts.

import { SNSEvent } from "aws-lambda";

export async function main(event: SNSEvent) {
  const records: any[] = event.Records;
  console.log(`Item shipped: "${records[0].Sns.Message}"`);

  return {};
}

Now let’s test our new API.

Starting your dev environment

Change indicator SST features a Live Lambda Development environment that allows you to work on your serverless apps live.

$ npm run dev

The first time you run this command it’ll take a couple of minutes to deploy your app and a debug stack to power the Live Lambda Development environment.

Deployed:
ExampleStack
ApiEndpoint: https://gevkgi575a.execute-api.us-east-1.amazonaws.com

The ApiEndpoint is the API we just created.

Let’s test our endpoint. Run the following in a new terminal.

$ curl -X POST https://gevkgi575a.execute-api.us-east-1.amazonaws.com/order

This makes a POST request to our API. You should see Order confirmed! in the sst dev terminal.

Publishing to our topic

Now let’s publish a message to our topic.

Change indicator Replace the packages/functions/src/order.ts with the following.

import AWS from "aws-sdk";
import { Topic } from "sst/node/topic";

const sns = new AWS.SNS();

export async function main() {
  // Publish a message to topic
  await sns
    .publish({
      // Get the topic from the environment variable
      TopicArn: Topic.Ordered.topicArn,
      Message: JSON.stringify({ ordered: true }),
      MessageStructure: "string",
    })
    .promise();

  console.log("Order confirmed!");

  return {
    statusCode: 200,
    body: JSON.stringify({ status: "successful" }),
  };
}

Here we are getting the topic arn from the environment variable, and then publishing a message to it.

Change indicator Let’s install the aws-sdk package in the packages/functions/ folder.

$ npm install aws-sdk

Now if you hit our API again.

$ curl -X POST https://gevkgi575a.execute-api.us-east-1.amazonaws.com/order

You should see the following in your sst dev terminal.

Item shipped: "{"ordered":true}"
Receipt sent: "{"ordered":true}"

Deploying to prod

Change indicator To wrap things up we’ll deploy our app to prod.

$ npx sst deploy --stage prod

This allows us to separate our environments, so when we are working in dev, it doesn’t break the API for our users.

Cleaning up

Finally, you can remove the resources created in this example using the following commands.

$ npx sst remove
$ npx sst remove --stage prod

Conclusion

And that’s it! We’ve got a completely serverless checkout system, powered by SNS. Check out the repo below for the code we used in this example. And leave a comment if you have any questions!