Node js Race Conditions

One of the basic concepts for node js developers is understanding race conditions. Let's make a simple API that registers users for a newsletter to understand race conditions. The only requirement is user should be able to register only once.

We will use Node js, Express js, and MongoDB for development.

Step 1: Install Mongo DB

We will use docker to start the local MongoDB instance.

docker run -p 27017:27017 --name newsletter -d mongo:latest

Note: Please map volume if you want to persist data.

Step 2: version 1 API that will return ‘success’ if it's a new email or error for already enrolled

function wait(timeout) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve()
    }, timeout);
  })
}

app.post('/v1/register', async (req, res) => {
  try {
    // find if user is already registered
    let user = await client.db("newsletter").collection("usersV1").findOne({email:req.body.email})

    if (user) {
      return res.json({ message: `user already registered` })
    }

    // adding this line makes easy to test parallel requests from same user
    await wait(10000)

    const result = await client.db("newsletter").collection("usersV1").insertOne({email:req.body.email});

    res.json({ message: `user registered successfully` })
  } catch (error) {
    console.log(error)
    res.json({ message: error.message })
  }
})

step 3: test version 1 API with curl

curl --location --request POST 'localhost:3000/v1/register' \
--header 'Content-Type: application/json' \
--data-raw '{
    "email":"test1@gmail.com"

}'

When we hit this request from multiple terminals at the same time, same user is registered twice.

step 4: version 2API that will return ‘success’ if it's a new email or ‘error: for already enrolled

app.post('/v2/register', async (req, res) => {
  try {
    const result = await client.db("newsletter").collection("usersV2").insertOne(req.body);
    res.json({ message: `user registered successfully` })
  } catch (error) {
    console.error(error.message)
    res.json({ message: error.message })
  }
})

step 5: create a unique index for email field when we connect to DB

client.connect().then(async () => {
  console.log(`connected to DB`);
  await client.db("newsletter").collection("usersV2").createIndex({ email: 1 }, { unique: true })
  app.listen(port, () => {
    console.log(`Example app listening on port ${port}`)
  })
})

step 6: test v2 API

In the case of V2, never duplicate users will be registered, even when we get more than 1 request from the same user at the same time. We can also validate this using artillery or JMeter.

Complete code:

const express = require('express')
var bodyParser = require('body-parser')
const { MongoClient } = require('mongodb');

// const uri = "mongodb+srv://<username>:<password>@<your-cluster-url>/test?retryWrites=true&w=majority";
const uri = "mongodb://localhost:27017/newsletter";

const client = new MongoClient(uri);

const app = express()
const port = 3000

app.use(bodyParser.json())

app.get('/', (req, res) => {
  res.send('Hello World to Race Conditions')
})


app.post('/v1/register', async (req, res) => {
  try {
    // find if user is already registered
    let user = await client.db("newsletter").collection("usersV1").findOne({email:req.body.email})

    if (user) {
      return res.json({ message: `user already registered` })
    }

    // adding this line makes easy to test parallel requests from same user
    await wait(10000)

    const result = await client.db("newsletter").collection("usersV1").insertOne({email:req.body.email});

    res.json({ message: `user registered successfully` })
  } catch (error) {
    console.log(error)
    res.json({ message: error.message })
  }
})

app.post('/v2/register', async (req, res) => {
  try {
    const result = await client.db("newsletter").collection("usersV2").insertOne({email:req.body.email});
    res.json({ message: `user registered successfully` })
  } catch (error) {
    console.error(error.message)
    res.json({ message: error.message })
  }
})



function wait(timeout) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve()
    }, timeout);
  })
}

client.connect().then(async () => {
  console.log(`connected to DB`);

  await client.db("newsletter").collection("usersV2").createIndex({ email: 1 }, { unique: true })

  app.listen(port, () => {
    console.log(`Example app listening on port ${port}`)
  })
})

GitHub repo:

GitHub - riddheshganatra/nodejs-race-conditions

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