Introduction to Job Scheduling in Node.js

When building web applications and APIs in Node.js, we sometimes come across tasks that need to be done repeatedly. This can be at a specific time everyday, month, or even year depending on the task. Some of these tasks may include: <!--more-->

1. Sending periodic emails to customers.
2. Backing up organization's data.
3. Clearing logs from databases etc.

Introduction #

This article goes through the basic syntax of node-cron. We will build a simple application that sends periodic emails.

Without further ado, let's dive in.

node-cron syntax #

It is a package used to schedule tasks (functions or commands) in Node.js. Its name is derived from the Greek word 'Chronos' meaning time.

These tasks can be scheduled to either run once or repeatedly. node-cron uses the crontab(cron table) syntax to represent different units of time.

This is what the crontab syntax looks like.

 # ┌────────────── second (optional) (0-59)
 # │ ┌──────────── minute (0-59)
 # │ │ ┌────────── hour (0-23)
 # │ │ │ ┌──────── day of month (1-31)
 # │ │ │ │ ┌────── month (1-12 (or names))
 # │ │ │ │ │ ┌──── day of week (0-7 (or names, 0 or 7 are Sunday))
 # │ │ │ │ │ │
 # │ │ │ │ │ │
 # * * * * * *

Each asterisk acts as a place holder for the specified time units. By default the schedule method runs every minute when no values are passed. Check out node-cron package at npm for more syntax details.

It's now time to build our application.

Prerequisites #

To follow through this tutorial, you will need to:

1. Have Node.js installed.
2. Have a basic knowledge of Node.js. Here is an article to get you started.
3. Have a basic knowledge of the Express framework. Here is an article to get you started.
4. Be familiar with sending emails through nodemailer. Refer to this article for a deep dive into node-mailer.

Let's get started.

Step 1 — Creating a Node.js Application #

Create a directory and give it a name of your choice. I'll name mine (node-cron-example). Then open the directory you have created using your favorite code editor. Now open the terminal and type:

npm init

Fill the required fields to create a package.json file that will be used to track dependencies. Add the express framework, node-cron module, and node-mailer module by typing

npm install express node-cron nodemailer

Step 2 — Creating The Server #

Now that we have created our Node.js application and initialized it. Let's go ahead and create the server that we will use for scheduling tasks.

First, type the code below to import express module and initialize it:

const express = require('express');
const app = express();

Next up, set the server to listen to port 2400. Note: You can use a port number of your choice.

app.listen(2400, () => {console.log("Server started at port 2400")});

That's all we need for our server now. To start the server type node <entry point file> in the terminal as shown below:

node index

This is the expected output from the console:

Server started at port 2400

Step 3 — Scheduling a Task #

Now that we have our server up and running. Let's see how to schedule a simple task before we get into sending our emails.

Let's start by importing the node-cron module:

const cron = require('node-cron');

To schedule a task, we pass in our crontab expression and the function we want to schedule on a schedule method.

In this example, we pass the default cron expression which will schedule the task to run every minute.

cron.schedule('* * * * *', () => {console.log("Task is running every minute " + new Date())});

The console output is as shown:

brain@Brain MINGW64 /c/Projects/node-cron example (master)
$ node index
Server started at port 2400
Every minute task ran on Mon Oct 12 2020 14:36:00 GMT+0300 (East Africa Time)
Every minute task ran on Mon Oct 12 2020 14:37:00 GMT+0300 (East Africa Time)
Every minute task ran on Mon Oct 12 2020 14:38:00 GMT+0300 (East Africa Time)
...

Step 4 — Creating The Email Function #

Let's create the function that will send emails. First, import the node-mailer module

const mailer = require('nodemailer');

Then create a transporter through nodemailer's createTransport method.

// Creating a transporter
const transporter = mailer.createTransport({
    host: 'smtp.ethereal.email',
    port: 587,
    auth: {
        user: 'your-username',
        pass: 'your-password'
    }
});

Now create a function and give it a name of your choice. In the function, we will use the transporter's sendMail method to send the email.

function sendEmail(message){
    //sending the email
    transporter.sendMail({
        from: '"Peter" <peter@kayere.com>',
        to: '"You there" <you@there.com>',
        subject: 'Scheduled Email',
        text: message
    })
        .then(_ => {console.log("Email sent on " + new Date())})
        .catch(error => {console.log(error)});
}

Step 5 — Scheduling The Email #

In this step, we are going to use the schedule method we created above to schedule the sendEmail function. We will schedule the function to run after every 10 minutes. To represent this using the crontab syntax, we will pass */10 for the minutes asterisk.

This is how.

cron.schedule('*/10 * * * *', sendEmail("Hey there, this email was sent to you automatically"));

And that's it!

This is the expected log message:

$ node index
Server started at port 2400
Email sent on Mon Oct 12 2020 14:40:00 GMT+0300 (East Africa Time)
Email sent on Mon Oct 12 2020 14:50:00 GMT+0300 (East Africa Time)

Conclusion #

In this article, we have gone through some of the basic syntax of the node-cron module. We have also built an application that sends an email every Sunday. This is helpful when we want to automate tasks. It helps the development process and scaling of the application. One does not need to perform the tasks manually.

The full code example of the application can be found on GitHub.

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Peer review contribution by: Linus Muema