Ever wondered how the application server back up the files periodically without any physical interruption. This is where Cron jobs come in.

Cron Jobs schedules a job periodically to do the actions that are configured to do.

there are few use cases where cron jobs play a vital role. they are,

• Deleting Log files - Application generates a lot of logs.clearing old logs will save lots of space in the server. it can be done using cron jobs.
• DB Backup - Database backup saves the application from disasters. Cron job will be helpful to do that.
• Application Logic - we can use cron jobs to do some application logic on a time basis.

How cron job works

we will write a cron job to archive the old records in the database in a production application.

Firstly, create a project and install the following dependencies,

npm init --yes
npm install express node-cron mongoose faker

• express - web server library in nodejs
• node-cron - cron job scheduler library in nodejs
• mongoose - ORM for MongoDB

After that, create a file called Model.js and add the following code

const mongoose = require("mongoose")
const weatherSchema = new mongoose.Schema({
  minTemp: {
    type: Number,
  },
  maxTemp: {
    type: Number,
  },
  recordedDate: {
    type: Date,
  },
  isArchived: {
    type: Boolean,
    default: false,
  },
})
class Weather {
  static getRec(date) {
    return this.find({
      recordedDate: {
        $lte: new Date(date),
      },
    }).exec()
  }
  static insertBulkData(data) {
    return this.insertMany(data)
  }
  static archiveData(date) {
    return this.updateMany(
      {
        recordedDate: {
          $lte: new Date(date),
        },
      },
      {
        $set: {
          isArchived: true,
        },
      }
    ).exec()
  }
  static getArchivedData() {
    return this.find({
      isArchived: true,
    }).exec()
  }
}
weatherSchema.loadClass(Weather)
module.exports = mongoose.model("Weather", weatherSchema)

Mainly, Model.js creates a mongoose schema for a DB table which stores the weather data in the database.

After that, create a file called scheduler.js and add the code for job scheduler.

cron.schedule("* * * * * *", () => {
  console.log("Running every minute")
})

cron schedule schedules the job for time format that is mentioned.

To learn more about cron job format, there is a great site crontab-guru which explains in detail

Connect the mongoose with the Express to insert some dummy data to database.

const cron = require("node-cron")
const express = require("express")
const mongoose = require("mongoose")
const app = express()
const faker = require("faker")
const model = require("./Model")
mongoose
  .connect("mongodb://localhost:27017/nodescheduler")
  .then(res => {
    console.log("mongoose connected successfully")
    app.get("/insertdata", async (req, res) => {
      let data = []
      for (let i = 0; i < 100; i++) {
        let record = {
          minTemp: faker.random.number(),
          maxTemp: faker.random.number(),
          recordedDate: faker.date.past(),
        }
        data.push(record)
      }
      await model.insertBulkData(data)
      res.send("Data is inserted")
    })
    app.listen(4000, () => {
      console.log("Server is running port 4000")
    })
  })
  .catch(err => {
    console.error(err)
  })

To insert some dummy data using fakerjs. run the script with the command, and visit the URL http://localhost:4000/insertdata

node scheduler.js

it will create some bulk dummy data to test the job scheduler. Now it is time to add the job scheduler.

cron.schedule("* * * * */3 *", async () => {
  var d = new Date()
  d.setMonth(d.getMonth() - 2) //1 month ago
  await model.archiveData(d)
  console.log("scheduler => archived")
})

Above cron job will run every 3 months, to mark the data as archived in the database.

Likewise, we can use cron jobs to schedule a job for our application logics.

Summary

Above all, cron jobs play a vital role in some application development use cases. it is always good to know how cron jobs work in application development.

#node-js #web-development