Sometimes building web applications, you meet repetitive tasks that should be run at a specific time every day, month, or year. Some of these tasks may include:

  • Sending periodic emails to customers;
  • Backing up the organization’s data;
  • Clearing logs from databases;

Here is where Scheduler comes in. It allows you to schedule jobs (arbitrary functions) for execution at specific dates, with optional recurrence rules. Scheduler service shares the same code base with API and located in the scheduler folder.

It runs and deploys as a standalone service with its own Dockerfile. This workflow allows time-consuming tasks not to interfere with the server processing requests.

Scheduler consists of two parts: Cron jobs and Handlers.

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Cron jobs

import schedule from 'node-schedule';
import { EventEmitter } from 'events';

const eventEmitter = new EventEmitter();

schedule.scheduleJob('* * * * *', () => {
  eventEmitter.emit('cron:every-hour');
});

export default eventEmitter;

The cron jobs part is responsible for waiting some moment and emitting more specific events. It uses a node-schedule library. In short, you should create a cron job in the appropriate format and emit an event using EventEmitter that will be handled in the Handlers part forward.

Cron format

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Handlers

import cron from 'scheduler/cron';

cron.on('cron:every-hour', async () => {
  try {
    const users = await userService.find({
      'scheduledEmail.deliveryDate': { $lte: new Date() },
    });

    users.map((u) => emailService.sendSignupWelcome(u.email))
  } catch (err) {
    console.log(`cron:every-hour error: ${err}`);
  }
});

The handlers part is responsible for listening to events from Cron jobs and running some handlers. Wrap handlers with try/catch to prevent a server from stopping or rebooting and create error logs.

MigratorMailer