Finding And Fixing Node.js Memory Leaks: A Practical Guide

Finding And Fixing Node.js Memory Leaks: A Practical Guide

Finding And Fixing Node.js Memory Leaks: A Practical Guide. Fixing memory leaks may not be not the shiniest skill on a CV, but when things ...

Finding And Fixing Node.js Memory Leaks: A Practical Guide. Fixing memory leaks may not be not the shiniest skill on a CV, but when things ...

Fixing memory leaks may not be not the shiniest skill on a CV, but when things go wrong on production, it’s better to be prepared!

After reading this article, you’ll be able to monitor, understand, and debug the memory consumption of a Node.js application.

When Memory Leaks Become A Problem

Memory leaks often go unnoticed. They become a problem when someone pays extra attention to the production performance metrics.

The first symptom of a memory leak on a production application is that memory, CPU usage, and the load average of the host machine increase over time, without any apparent reason.

Insidiously, the response time becomes higher and higher, until a point when the CPU usage reaches 100%, and the application stops responding. When the memory is full, and there is not enough swap left, the server can even fail to accept SSH connections.

But when the application is restarted, all the issues magically vanish! And nobody understands what happened, so they move on other priorities, but the problem repeats itself periodically.

Memory leaks aren’t always that obvious, but when this pattern appears, it’s time to look for a correlation between the memory usage and the response time.

Congratulations! You’ve found a memory leak. Now the fun begins for you.

Needless to say, I assumed that you monitor your server. Otherwise, I highly recommend taking a look at New Relic, Elastic APM, or any monitoring solution. What can’t be measured can’t be fixed.

Restart Before It’s Too Late

Finding and fixing a memory leak in Node.js takes time - usually a day or more. If your backlog can’t accomodate some time to investigate the leak in the near future, I advise to look for a temporary solution, and deal with the root cause later. A rational way (in the short term) to postpone the problem is to restart the application before it reaches the critical bloat.

For PM2 users, the [max_memory_restart](http://pm2.keymetrics.io/docs/usage/process-management/#max-memory-restart "max_memory_restart") option is available to automatically restart node processes when they reach a certain amount of memory.

Now that we’re comfortably seated, with a cup of tea and a few hours ahead, let’s dig into the tools that’ll help you find these little RAM squatters.

Creating An Effective Test Environment

Before measuring anything, do yourself a favor, and take the time to set up a proper test environment. It can be a Virtual Machine, or an AWS EC2 instance, but it needs to repeat the exact same conditions as in production.

The code should be built, optimized, and configured the exact same way as when it runs on production in order to reproduce the leak identically. Ideally, it’s better to use the same deployment artifact, so you can be certain that there is no difference between the production and the new test environment.

A duly configured test environment is not enough: it should run the same load as the production, too. To this end, feel free to grab production logs, and send the same requests to the test environment. During my debugging quest, I discovered siege an HTTP/FTP load tester and benchmarking utility, pretty useful when it comes to measuring memory under heavy load.

Also, resist the urge to enable developer tools or verbose loggers if they are not necessary, otherwise you’ll end up debugging these dev tools!

Accessing Node.js Memory Using V8 Inspector & Chrome Dev Tools

I love the Chrome Dev Tools. F12 is the key that I type the most after Ctrl+C and Ctrl+V (because I mostly do Stack Overflow-Driven Development - just kidding).

Did you know that you can use the same Dev Tools to inspect Node.js applications? Node.js and Chrome run the same engine, [Chrome V8](https://developers.google.com/v8/ "Chrome V8"), which contains the inspector used by the Dev Tools.

For educational purposes, let’s say that we have the simplest HTTP server ever, with the only purpose to display all the requests that it has ever received:

const http = require('http');

const requestLogs = [];
const server = http.createServer((req, res) => {
    requestLogs.push({ url: req.url, date: new Date() });
    res.end(JSON.stringify(requestLogs));
});

server.listen(3000);
console.log('Server listening to port 3000\. Press Ctrl+C to stop it.');


In order to expose the inspector, let’s run Node.js with the --inspect flag.

$ node --inspect index.js 
Debugger listening on ws://127.0.0.1:9229/655aa7fe-a557-457c-9204-fb9abfe26b0f
For help see https://nodejs.org/en/docs/inspector
Server listening to port 3000\. Press Ctrl+C to stop it.


Now, run Chrome (or Chromium), and go to the following URI: chrome://inspect. Voila! A full-featured debugger for your Node.js application.

Taking Snapshots Of The V8 Memory

Let’s play with the Memory tab a bit. The simplest option available is Take heap snapshot. It does what you expect: it creates a dump of the heap memory for the inspected application, with a lot of details about the memory usage.

Memory snapshots are useful to track memory leaks. A usual technique consists of comparing multiple snapshots at different key points to see if the memory size grows, when it does, and how.

For example, we’ll take three snapshots: one after the server start, one after 30 seconds of load, and the last one after another session of load.

To simulate the load, I’ll use the siege utility introduced above:

$ timeout 30s siege http://localhost:3000

** SIEGE 4.0.2          
** Preparing 25 concurrent users for battle.
The server is now under siege...
Lifting the server siege...
Transactions:               2682 hits
Availability:             100.00 %
Elapsed time:              30.00 secs
Data transferred:         192.18 MB
Response time:              0.01 secs
Transaction rate:          89.40 trans/sec
Throughput:             6.41 MB/sec
Concurrency:                0.71
Successful transactions:        2682
Failed transactions:               0
Longest transaction:            0.03
Shortest transaction:           0.00


Here is the result of my simulation (click to see the full size):

A lot to see!

On the first snapshot, there are already 5MB allocated before any request is processed. It’s totally expected: each variable or imported module is injected into memory. Analysing the first snapshot allows optimizing the server start for example - but that’s not our current task.

What interests me here is to know if the server memory grows over time while it’s used. As you can see, the third snapshot has 6.7MB while the second has 6.2MB: in the interval, some memory has been allocated. But which function did?

I can compare the difference of allocated objects by clicking on the latest snapshot (1), change the mode for Comparison (2), and select the Snapshot to compare with (3). This is the state of the current image.

Exactly 2,682 Date objects and 2,682 Objects have been allocated between the two load sessions. Unsurprisingly, 2,682 requests have been made by siege to the server: it’s a huge indicator that we have one allocation per request. But all “leaks” aren’t that obvious so the inspector shows you where it was allocated: in the requestLogs variable in the system Context (it’s the root scope of the app).

Tip: It’s normal that V8 allocates memory for new objects. JavaScript is a garbage-collected runtime, so the V8 engine frees up memory at regular intervals. What’s not normal is when it doesn’t collect the allocated memory after a few seconds.

Watching Memory Allocation In Real Time

Another method to measure the memory allocation is to see it live instead of taking multiple snapshots. To do so, click on Record allocation timeline while the siege simulation is in progress.

For the following example, I started siege after 5 seconds, and during 10 seconds.

For the firsts requests, you can see a visible spike of allocation. It’s related to the HTTP module initialization. But if you zoom in to the more common allocation (such as on the image above) you’ll notice that, again, it’s the dates and objects that take the most memory.

Using The Heap Dump Npm Package

An alternative method to get a heap snapshot is to use the heapdump module. Its usage is pretty simple: once the module is imported, you can either call the writeSnapshot method, or send a SIGUSR2 signal to the Node process.

Just update the app:

const http = require('http');
const heapdump = require('heapdump');

const requestLogs = [];
const server = http.createServer((req, res) => {
    if (req.url === '/heapdump') {
        heapdump.writeSnapshot((err, filename) => {
            console.log('Heap dump written to', filename)
        });
    }
    requestLogs.push({ url: req.url, date: new Date() });
    res.end(JSON.stringify(requestLogs));
});

server.listen(3000);
console.log('Server listening to port 3000\. Press Ctrl+C to stop it.');
console.log(`Heapdump enabled. Run "kill -USR2 ${process.pid}" or send a request to "/heapdump" to generate a heapdump.`);


And trigger a dump:

$ node index.js
Server listening to port 3000\. Press Ctrl+C to stop it.
Heapdump enabled. Run "kill -USR2 29431" or send a request to "/heapdump" to generate a heapdump.

$ kill -USR2 29431
$ curl http://localhost:3000/heapdump
$ ls
heapdump-31208326.300922.heapsnapshot
heapdump-31216569.978846.heapsnapshot


You’ll note that running kill -USR2 doesn’t actually kill the process. The kill command, despite its scary name, is just a tool to send signals to processes, by default a SIGTERM. With the argument -USR2, I choose to send a SIGUSR2 signal instead, which is a user-defined signal.

In last resort, you can use the signal method to generate a heapdump on the production instance. But you need to know that creating a heap snapshot requires twice the size of the heap at the time of the snapshot.

Once the snapshot is available, you can read it with the Chrome DevTools. Just open the Memory tab, right-click on the side and select Load.

Fixing the Leak

Now that I have identified what grows the memory heap, I have to find a solution. For my example, the solution is to store the logs not in memory, but on the filesystem. On a real project, it’s better to delegate log storage to another service like syslog, or use an appropriate storage like a database, a Redis instance, or whatever.

Here is the modified web server with no more memory leak:

// Not the best implementation. Do not try this at home.
const fs = require('fs');
const http = require('http');

const filename = './requests.json';

const readRequests = () => {
    try {
        return fs.readFileSync(filename);
    } catch (e) {
        return '[]';
    }
};

const writeRequest = (req) => {
    const requests = JSON.parse(readRequests());
    requests.push({ url: req.url, date: new Date() });
    fs.writeFileSync(filename, JSON.stringify(requests));
};

const server = http.createServer((req, res) => {
    writeRequest(req);
    res.end(readRequests());
});

server.listen(3000);
console.log('Server listening to port 3000\. Press Ctrl+C to stop it.');


Now, let’s run the same test scenario as before, and measure the outcome:

$ timeout 30s siege http://localhost:3000

** SIEGE 4.0.2
** Preparing 25 concurrent users for battle.
The server is now under siege...
Lifting the server siege...
Transactions:               1931 hits
Availability:             100.00 %
Elapsed time:              30.00 secs
Data transferred:        1065.68 MB
Response time:              0.14 secs
Transaction rate:          64.37 trans/sec
Throughput:            35.52 MB/sec
Concurrency:                9.10
Successful transactions:        1931
Failed transactions:               0
Longest transaction:            0.38
Shortest transaction:           0.01


As you can see, the memory growth is far slower! This is because we no longer store the request logs in memory (inside the [requestLogs](https://codequs.com/p/ry58OgNBV/finding-and-fixing-node-js-memory-leaks-a-practical-guide#first-code-example "requestLogs") variable) for each request.

This said, the API takes more time to respond: I had 89.40 transactions per second, now we have 64.37.

Reading and writing to the disk comes with a cost, so do other API calls or database requests.

Note that it’s important to measure memory consumption before and after a potential fix, in order to confirm (and prove) that the memory issue is fixed.

Conclusion

Actually, fixing a memory leak once it’s been identified is somewhat easy: use well known and tested libraries, don’t copy or store heavy objects for too long, and so on.

The hardest part is to find them. Fortunately, and despite few bugs, the current Node.js tools are neat. And now you know how to use them!

To keep this article short and understandable, I didn’t mention some other tools like the memwatch module (easy) or Core Dump analysis with llnode or mdb (advanced) but I let you with more detailed readings about them:

Further reading:

  • Debugging Memory Leaks in Node.js Applications
  • Understanding Garbage Collection and Hunting Memory Leaks in Node.js
  • llnode for Node.js Memory Leak Analysis
  • Debugging Node.js applications using core dumps

=============================

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How to Use Express.js, Node.js and MongoDB.js

How to Use Express.js, Node.js and MongoDB.js

In this post, I will show you how to use Express.js, Node.js and MongoDB.js. We will be creating a very simple Node application, that will allow users to input data that they want to store in a MongoDB database. It will also show all items that have been entered into the database.

In this post, I will show you how to use Express.js, Node.js and MongoDB.js. We will be creating a very simple Node application, that will allow users to input data that they want to store in a MongoDB database. It will also show all items that have been entered into the database.

Creating a Node Application

To get started I would recommend creating a new database that will contain our application. For this demo I am creating a directory called node-demo. After creating the directory you will need to change into that directory.

mkdir node-demo
cd node-demo

Once we are in the directory we will need to create an application and we can do this by running the command
npm init

This will ask you a series of questions. Here are the answers I gave to the prompts.

The first step is to create a file that will contain our code for our Node.js server.

touch app.js

In our app.js we are going to add the following code to build a very simple Node.js Application.

var express = require("express");
var app = express();
var port = 3000;
 
app.get("/", (req, res) => {
  res.send("Hello World");
});
 
app.listen(port, () => {
  console.log("Server listening on port " + port);
});

What the code does is require the express.js application. It then creates app by calling express. We define our port to be 3000.

The app.use line will listen to requests from the browser and will return the text “Hello World” back to the browser.

The last line actually starts the server and tells it to listen on port 3000.

Installing Express

Our app.js required the Express.js module. We need to install express in order for this to work properly. Go to your terminal and enter this command.

npm install express --save

This command will install the express module into our package.json. The module is installed as a dependency in our package.json as shown below.

To test our application you can go to the terminal and enter the command

node app.js

Open up a browser and navigate to the url http://localhost:3000

You will see the following in your browser

Creating Website to Save Data to MongoDB Database

Instead of showing the text “Hello World” when people view your application, what we want to do is to show a place for user to save data to the database.

We are going to allow users to enter a first name and a last name that we will be saving in the database.

To do this we will need to create a basic HTML file. In your terminal enter the following command to create an index.html file.

touch index.html

In our index.html file we will be creating an input filed where users can input data that they want to have stored in the database. We will also need a button for users to click on that will add the data to the database.

Here is what our index.html file looks like.

<!DOCTYPE html>
<html>
  <head>
    <title>Intro to Node and MongoDB<title>
  <head>

  <body>
    <h1>Into to Node and MongoDB<&#47;h1>
    <form method="post" action="/addname">
      <label>Enter Your Name<&#47;label><br>
      <input type="text" name="firstName" placeholder="Enter first name..." required>
      <input type="text" name="lastName" placeholder="Enter last name..." required>
      <input type="submit" value="Add Name">
    </form>
  <body>
<html>

If you are familiar with HTML, you will not find anything unusual in our code for our index.html file. We are creating a form where users can input their first name and last name and then click an “Add Name” button.

The form will do a post call to the /addname endpoint. We will be talking about endpoints and post later in this tutorial.

Displaying our Website to Users

We were previously displaying the text “Hello World” to users when they visited our website. Now we want to display our html file that we created. To do this we will need to change the app.use line our our app.js file.

We will be using the sendFile command to show the index.html file. We will need to tell the server exactly where to find the index.html file. We can do that by using a node global call __dirname. The __dirname will provide the current directly where the command was run. We will then append the path to our index.html file.

The app.use lines will need to be changed to
app.use("/", (req, res) => {   res.sendFile(__dirname + "/index.html"); });

Once you have saved your app.js file, we can test it by going to terminal and running node app.js

Open your browser and navigate to “http://localhost:3000”. You will see the following

Connecting to the Database

Now we need to add our database to the application. We will be connecting to a MongoDB database. I am assuming that you already have MongoDB installed and running on your computer.

To connect to the MongoDB database we are going to use a module called Mongoose. We will need to install mongoose module just like we did with express. Go to your terminal and enter the following command.
npm install mongoose --save

This will install the mongoose model and add it as a dependency in our package.json.

Connecting to the Database

Now that we have the mongoose module installed, we need to connect to the database in our app.js file. MongoDB, by default, runs on port 27017. You connect to the database by telling it the location of the database and the name of the database.

In our app.js file after the line for the port and before the app.use line, enter the following two lines to get access to mongoose and to connect to the database. For the database, I am going to use “node-demo”.

var mongoose = require("mongoose"); mongoose.Promise = global.Promise; mongoose.connect("mongodb://localhost:27017/node-demo");

Creating a Database Schema

Once the user enters data in the input field and clicks the add button, we want the contents of the input field to be stored in the database. In order to know the format of the data in the database, we need to have a Schema.

For this tutorial, we will need a very simple Schema that has only two fields. I am going to call the field firstName and lastName. The data stored in both fields will be a String.

After connecting to the database in our app.js we need to define our Schema. Here are the lines you need to add to the app.js.
var nameSchema = new mongoose.Schema({   firstName: String,   lastNameName: String });

Once we have built our Schema, we need to create a model from it. I am going to call my model “DataInput”. Here is the line you will add next to create our mode.
var User = mongoose.model("User", nameSchema);

Creating RESTful API

Now that we have a connection to our database, we need to create the mechanism by which data will be added to the database. This is done through our REST API. We will need to create an endpoint that will be used to send data to our server. Once the server receives this data then it will store the data in the database.

An endpoint is a route that our server will be listening to to get data from the browser. We already have one route that we have created already in the application and that is the route that is listening at the endpoint “/” which is the homepage of our application.

HTTP Verbs in a REST API

The communication between the client(the browser) and the server is done through an HTTP verb. The most common HTTP verbs are
GET, PUT, POST, and DELETE.

The following table explains what each HTTP verb does.

HTTP Verb Operation
GET Read
POST Create
PUT Update
DELETE Delete

As you can see from these verbs, they form the basis of CRUD operations that I talked about previously.

Building a CRUD endpoint

If you remember, the form in our index.html file used a post method to call this endpoint. We will now create this endpoint.

In our previous endpoint we used a “GET” http verb to display the index.html file. We are going to do something very similar but instead of using “GET”, we are going to use “POST”. To get started this is what the framework of our endpoint will look like.

app.post("/addname", (req, res) => {
 
});
Express Middleware

To fill out the contents of our endpoint, we want to store the firstName and lastName entered by the user into the database. The values for firstName and lastName are in the body of the request that we send to the server. We want to capture that data, convert it to JSON and store it into the database.

Express.js version 4 removed all middleware. To parse the data in the body we will need to add middleware into our application to provide this functionality. We will be using the body-parser module. We need to install it, so in your terminal window enter the following command.

npm install body-parser --save

Once it is installed, we will need to require this module and configure it. The configuration will allow us to pass the data for firstName and lastName in the body to the server. It can also convert that data into JSON format. This will be handy because we can take this formatted data and save it directly into our database.

To add the body-parser middleware to our application and configure it, we can add the following lines directly after the line that sets our port.

var bodyParser = require('body-parser');
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: true }));
Saving data to database

Mongoose provides a save function that will take a JSON object and store it in the database. Our body-parser middleware, will convert the user’s input into the JSON format for us.

To save the data into the database, we need to create a new instance of our model that we created early. We will pass into this instance the user’s input. Once we have it then we just need to enter the command “save”.

Mongoose will return a promise on a save to the database. A promise is what is returned when the save to the database completes. This save will either finish successfully or it will fail. A promise provides two methods that will handle both of these scenarios.

If this save to the database was successful it will return to the .then segment of the promise. In this case we want to send text back the user to let them know the data was saved to the database.

If it fails it will return to the .catch segment of the promise. In this case, we want to send text back to the user telling them the data was not saved to the database. It is best practice to also change the statusCode that is returned from the default 200 to a 400. A 400 statusCode signifies that the operation failed.

Now putting all of this together here is what our final endpoint will look like.

app.post("/addname", (req, res) => {
  var myData = new User(req.body);
  myData.save()
    .then(item => {
      res.send("item saved to database");
    })
    .catch(err => {
      res.status(400).send("unable to save to database");
    });
});
Testing our code

Save your code. Go to your terminal and enter the command node app.js to start our server. Open up your browser and navigate to the URL “http://localhost:3000”. You will see our index.html file displayed to you.

Make sure you have mongo running.

Enter your first name and last name in the input fields and then click the “Add Name” button. You should get back text that says the name has been saved to the database like below.

Access to Code

The final version of the code is available in my Github repo. To access the code click here. Thank you for reading !

Node.js for Beginners - Learn Node.js from Scratch (Step by Step)

Node.js for Beginners - Learn Node.js from Scratch (Step by Step)

Node.js for Beginners - Learn Node.js from Scratch (Step by Step) - Learn the basics of Node.js. This Node.js tutorial will guide you step by step so that you will learn basics and theory of every part. Learn to use Node.js like a professional. You’ll learn: Basic Of Node, Modules, NPM In Node, Event, Email, Uploading File, Advance Of Node.

Node.js for Beginners

Learn Node.js from Scratch (Step by Step)

Welcome to my course "Node.js for Beginners - Learn Node.js from Scratch". This course will guide you step by step so that you will learn basics and theory of every part. This course contain hands on example so that you can understand coding in Node.js better. If you have no previous knowledge or experience in Node.js, you will like that the course begins with Node.js basics. otherwise if you have few experience in programming in Node.js, this course can help you learn some new information . This course contain hands on practical examples without neglecting theory and basics. Learn to use Node.js like a professional. This comprehensive course will allow to work on the real world as an expert!
What you’ll learn:

  • Basic Of Node
  • Modules
  • NPM In Node
  • Event
  • Email
  • Uploading File
  • Advance Of Node

Top 7 Most Popular Node.js Frameworks You Should Know

Top 7 Most Popular Node.js Frameworks You Should Know

Node.js is an open-source, cross-platform, runtime environment that allows developers to run JavaScript outside of a browser. In this post, you'll see top 7 of the most popular Node frameworks at this point in time (ranked from high to low by GitHub stars).

Node.js is an open-source, cross-platform, runtime environment that allows developers to run JavaScript outside of a browser.

One of the main advantages of Node is that it enables developers to use JavaScript on both the front-end and the back-end of an application. This not only makes the source code of any app cleaner and more consistent, but it significantly speeds up app development too, as developers only need to use one language.

Node is fast, scalable, and easy to get started with. Its default package manager is npm, which means it also sports the largest ecosystem of open-source libraries. Node is used by companies such as NASA, Uber, Netflix, and Walmart.

But Node doesn't come alone. It comes with a plethora of frameworks. A Node framework can be pictured as the external scaffolding that you can build your app in. These frameworks are built on top of Node and extend the technology's functionality, mostly by making apps easier to prototype and develop, while also making them faster and more scalable.

Below are 7of the most popular Node frameworks at this point in time (ranked from high to low by GitHub stars).

Express

With over 43,000 GitHub stars, Express is the most popular Node framework. It brands itself as a fast, unopinionated, and minimalist framework. Express acts as middleware: it helps set up and configure routes to send and receive requests between the front-end and the database of an app.

Express provides lightweight, powerful tools for HTTP servers. It's a great framework for single-page apps, websites, hybrids, or public HTTP APIs. It supports over fourteen different template engines, so developers aren't forced into any specific ORM.

Meteor

Meteor is a full-stack JavaScript platform. It allows developers to build real-time web apps, i.e. apps where code changes are pushed to all browsers and devices in real-time. Additionally, servers send data over the wire, instead of HTML. The client renders the data.

The project has over 41,000 GitHub stars and is built to power large projects. Meteor is used by companies such as Mazda, Honeywell, Qualcomm, and IKEA. It has excellent documentation and a strong community behind it.

Koa

Koa is built by the same team that built Express. It uses ES6 methods that allow developers to work without callbacks. Developers also have more control over error-handling. Koa has no middleware within its core, which means that developers have more control over configuration, but which means that traditional Node middleware (e.g. req, res, next) won't work with Koa.

Koa already has over 26,000 GitHub stars. The Express developers built Koa because they wanted a lighter framework that was more expressive and more robust than Express. You can find out more about the differences between Koa and Express here.

Sails

Sails is a real-time, MVC framework for Node that's built on Express. It supports auto-generated REST APIs and comes with an easy WebSocket integration.

The project has over 20,000 stars on GitHub and is compatible with almost all databases (MySQL, MongoDB, PostgreSQL, Redis). It's also compatible with most front-end technologies (Angular, iOS, Android, React, and even Windows Phone).

Nest

Nest has over 15,000 GitHub stars. It uses progressive JavaScript and is built with TypeScript, which means it comes with strong typing. It combines elements of object-oriented programming, functional programming, and functional reactive programming.

Nest is packaged in such a way it serves as a complete development kit for writing enterprise-level apps. The framework uses Express, but is compatible with a wide range of other libraries.

LoopBack

LoopBack is a framework that allows developers to quickly create REST APIs. It has an easy-to-use CLI wizard and allows developers to create models either on their schema or dynamically. It also has a built-in API explorer.

LoopBack has over 12,000 GitHub stars and is used by companies such as GoDaddy, Symantec, and the Bank of America. It's compatible with many REST services and a wide variety of databases (MongoDB, Oracle, MySQL, PostgreSQL).

Hapi

Similar to Express, hapi serves data by intermediating between server-side and client-side. As such, it's can serve as a substitute for Express. Hapi allows developers to focus on writing reusable app logic in a modular and prescriptive fashion.

The project has over 11,000 GitHub stars. It has built-in support for input validation, caching, authentication, and more. Hapi was originally developed to handle all of Walmart's mobile traffic during Black Friday.