In this example we will look at how to create a cron job in our serverless app using Serverless Stack (SST). We’ll be creating a simple task that runs every minute and prints the weather forecast.

Requirements

Create an SST app

Let’s start by creating an SST app.

$ npx create-serverless-stack@latest cron-job
$ cd cron-job

By default our app will be deployed to an environment (or stage) called dev and the us-east-1 AWS region. This can be changed in the sst.json in your project root.

{
  "name": "cron-job",
  "stage": "dev",
  "region": "us-east-1"
}

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. src/ — App Code

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

Creating Cron Job

Let’s start by creating a cron job.

Replace the stacks/MyStack.js with the following.

import * as sst from "@serverless-stack/resources";

export default class MyStack extends sst.Stack {
  constructor(scope, id, props) {
    super(scope, id, props);

    // Create Cron Job
    new sst.Cron(this, "Cron", {
      schedule: "rate(1 minute)",
      job: "src/lambda.main",
    });
  }
}

This creates a serverless cron job using sst.Cron. We’ve configured the cron job to run every minute.

Adding function code

Now in our function, we’ll print out a message every time the function is run.

Replace src/lambda.js with the following.

export async function main() {
  console.log("Hi!");
  return {};
}

And let’s test what we have so far.

Starting your dev environment

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

$ npx sst start

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.

===============
 Deploying app
===============

Preparing your SST app
Transpiling source
Linting source
Deploying stacks
dev-cron-job-my-stack: deploying...

 ✅  dev-cron-job-my-stack


Stack dev-cron-job-my-stack
  Status: deployed

Wait for a couple of minutes and you should see Hi! gets printed out every minute in your terminal.

Checking weather forecast

Now let’s make a call to MetaWeather’s API and print out the weather in San Francisco.

Let’s install the node-fetch.

$ npm install node-fetch

Replace src/lambda.js with the following.

import fetch from "node-fetch";

export async function main() {
  const weather = await checkSFWeather();
  console.log(weather.consolidated_weather[0]);
  return {};
}

function checkSFWeather() {
  return fetch(
    "https://www.metaweather.com/api/location/2487956/"
  ).then((res) => res.json());
}

Now if you head over to your terminal and wait for the function to get invoked in the next minute, you’ll notice the weather data is printed out in the terminal!

{
  id: 5251329426456576,
  weather_state_name: 'Heavy Cloud',
  weather_state_abbr: 'hc',
  wind_direction_compass: 'WNW',
  created: '2021-02-08T09:20:16.839517Z',
  applicable_date: '2021-02-08',
  min_temp: 8.15,
  max_temp: 12.76,
  the_temp: 12.629999999999999,
  wind_speed: 4.307475905607254,
  wind_direction: 293.69935054043833,
  air_pressure: 1016,
  humidity: 71,
  visibility: 13.503017378509504,
  predictability: 71
}

Deploying to prod

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 cron job that checks the weather every minute. You can change this to run a job that you want. Check out the repo below for the code we used in this example. And leave a comment if you have any questions!