Weekly recommendations

​

Introduction

Defer makes it easy to turn a function into a CRON.

In this example, a marketplace wants to send product recommendations to users on a weekly basis.

For this, we will create a scheduled function (weeklyProductRecommendations()) that performs the following steps:

1. Gather all the users that opt-in for the weekly product recommendation
2. For each user, in parallel:
   • invoke a background function (computeProductRecommendation(user)), that will compute the product recommendations and send an email

​

CRON background function

A background function can be configured to run at a given frequency using defer.cron(fn, frequencyStr):

import { defer } from "@defer/client";

const weeklyProductRecommendations = async () => {
  // ...
};

export default defer.cron(weeklyProductRecommendations, "0 8 * * *");

A scheduled background function is similar to a regular background function:

• a scheduled background function should be async
• a scheduled background function file must be named following the function’s name and export as default

With some specificities:

• a scheduled background function should not take arguments
• a scheduled background function should not be invoked directly from your App (will result in errors)

Let’s now see how to implement our product recommendation scheduled function.

​

weeklyProductRecommendations() scheduled function

Our weekly product recommendation mechanism is implemented with 2 background functions:

- src/
  - defer/
    - weeklyProductRecommendations.ts ⬅
    - computeProductRecommendation.ts ⬅
  - ...
- prisma/
- package.json
- .env
- ...

import { defer } from "@defer/client";
import computeProductRecommendation from "./computeProductRecommendation";

const weeklyProductRecommendations = async () => {
  const users = await prisma.user.find({
    where: {
      weeklyProductRecommendations: true,
    },
  });

  users.forEach((user) => await computeProductRecommendation(user));
};

export default defer.cron(weeklyProductRecommendations, "0 8 * * *");

Which translates to the following executions, on each Monday, at 8am:

By leveraging the Defer background functions, our weekly product recommendation is:

• Scalable: by parallelizing the computation and sending of recommendations, the overall time to send the recommendations won’t be exponential over time.
• Reliable: any error during a product recommendation computing or email sending will trigger a Defer Smart Retry.
• Observable: get an overview of the status of the reporting from the Defer dashboard.
• Monitored: get notified on Slack if the app couldn’t deliver the product recommendation to a user.