Sending time-sensitive notifications

Background executions, such as real-time updating, are central to online applications. They free up front-end interactions (and users) to continue with no pause.

However, there are times when you need to cancel a background execution. In this guide, you’ll see how to cancel a process after a short period of time.

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Real-world use cases

• Sending a 2FA code over email when signing in to a web application
• Sending chat notifications of live messages
• Communicating an available slot in the queue of a concert ticket-selling platform

In all cases, there’s a reason to time out the process: For the 2FA, it’s a security issue; for the live messages, the user has perhaps already received a notification; for the concert, someone’s just bought your ticket.

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How Defers sets a time limit

The code below uses Defer to execute the well-known “Sign-in magic link” use case.

Slack uses this magical sign-in technique. Instead of asking a user to type a password, it asks only for the user’s email. The user receives an email with a unique “magic link”, which they click to sign in to the application. Users are thus not required to remember or type in their passwords.

Useful, but as you can imagine, a magic link can be a security issue. So it’s best to time it out. Here, Defer ensures that this link no longer works after 5 minutes.

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The code

First, wrap the code that sends the email (sgMail.send) in the async function sendSignInEmail(), which Defer uses to time out the message.

import { defer } from "@defer/client";
import sgMail from "@sendgrid/mail";

import { getMagicMailFromUser } from "./lib/user";

async function sendSignInEmail(email: string) {
  await sgMail.send(getMagicMailFromUser(email));
}

export default defer(sendSignInEmail, {
  retry: false, // It is important to disable the retries
});

Second, set up Defer’s discardAfter() helper, which allows you to cancel the background execution. In this case, Defer will cancel the above sendSignInEmail after 5 min.

import express from "express";
import bodyParser from "body-parser";
import { discardAfter } from "@defer/client";

import sendSignInEmail from "./defer/sendSignInEmail";

const app = express();

app.use(bodyParser.json());

// discard any `sendSignInEmail()` execution if not
//  sent in the next 5 minutes after being requested
const sendExpirableSignInEmail = discardAfter(sendSignInEmail, "5min");

app.post("/sign-in", async function (req, res) {
  const email = req.params.email;

  await sendExpirableSignInEmail(email);

  res.render("sign-in_email_sent", { email });
});

app.listen(3000);

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Related Functionality — From cancelling to rescheduling a job

Being able to cancel a job plays an important role in managing your background executions. So too with rescheduling a job, which Defer provides with its reschedule() option. You may also want to look up Defer’s addMetadata() helper. With Metadata, you can add conditions to control the flow of your background jobs.