Scaling an Express.js Application with Memcache on DigitalOcean

Lindsey ~ February 24, 2019

Tags: DigitalOcean, Express.js, Tutorial

In this guide, we’ll explore how to create a simple Express 4 application, deploy it using DigitalOcean, then add Memcache to alleviate a performance bottleneck.

Memcache is a technology that improves the performance and scalability of web apps and mobile app backends. You should consider using Memcache when your pages are loading too slowly or your app is having scalability issues. Even for small sites, Memcache can make page loads snappy and help future-proof your app.

Prerequisites

Before you complete the steps in this guide, make sure you have all of the following:

Familiarity with Node.js (and ideally Express.js)
A DigitalOcean account.
If you like managing DigitalOcean resource via the CLI, you need the doctl installed and configured.
Node.js, npm, and git installed on your computer.

Deploying an Express.js application to DigitalOcean

Express.js is a minimalist framework that doesn’t require an application skeleton. To make things easier for you, we’ve got a basic example application set up here.

We’ll be using Github to track file changes in DigitalOcean, so if you’d like to follow along, go ahead and fork that repository to your personal account, and clone your fork to your local machine. Then cd into the examples-expressjs directory.

$ git clone git@github.com:memcachier/examples-expressjs.git
$ cd examples-expressjs
$ git checkout digital-ocean

The master branch in the repository is specifically for deploying to Heroku. You can find that
tutorial here.

Next, install all of the required packages using:

$ npm install

To simplify development, we’ve use a template engine, ejs, but you can use whichever engine you prefer, including mustache, pug, or nunjucks.

Now that we’ve installed all the packages we need, we can add our app code. We’ll create a page that calculates the largest prime number that’s smaller than a number a visitor submits.

Open up app.js and add the following code into the section labeled ADD THE DIY CODE HERE:

/* ADD THE DIY CODE HERE */

// Super simple algorithm to find largest prime <= n
var calculatePrime = function(n){
  var prime = 1;
  for (var i = n; i > 1; i--) {
    var is_prime = true;
    for (var j = 2; j < i; j++) {
      if (i % j == 0) {
        is_prime = false;
        break;
      }
    }
    if (is_prime) {
      prime = i;
      break;
    }
  }
  return prime;
}

// Set up the GET route
app.get('/', function (req, res) {
  if(req.query.n) {
    // Calculate prime and render view
    var prime = calculatePrime(req.query.n);
    res.render('index', { n: req.query.n, prime: prime});
  }
  else {
    // Render view without prime
    res.render('index', {});
  }
});

/* END DIY CODE */

Now let’s add a corresponding view. Open up the file views/index.ejs and copy the following ejs-enhanced HTML into it:

<!-- ADD DIY INDEX.HTML CODE HERE -->
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">

    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css">

    <title>Express.js caching example</title>
  </head>

  <body>
    <div class="container">
      <h1>
        Express.js caching example
      </h1>

      <p class="lead">For any number N (max 10000), we'll find the largest prime number
        less than or equal to N.
      </p>
      <!-- Form to submit a number -->
      <form class="form-inline" action="/">
        <input type="text" class="form-control" name="n" />
        <input type="submit" class="btn btn-primary" value="Find Prime" />
      </form>

      <hr>
      <!-- Show the result -->
      <% if (locals.prime) { %>
        <div class="alert alert-primary">
          <p class="lead">Largest prime less or equal than <%= n %> is <%= prime %></p>
        </div>
      <% } %>

      <!-- TODO: Error handling -->

    </div>
  </body>
</html>

You now have a working app that you can start locally by running npm start.

Create an Express One-Click application

Update: One-click-apps are now Marketplace images.

To build an app we first need a droplet. Either go to your DigitalOcean dashboard and create one or launch one via the CLI:

$ doctl compute droplet create express-memcache --image nodejs-18-04 --region nyc1 --size s-1vcpu-1gb --ssh-keys <KEY_FINGERPRINT>

Give the droplet a minute to come up and then look up its IP via the dashboard or by typing

$ doctl compute droplet list

Now you can login to your droplet via

$ ssh root@<DROPLET_IP>

Visiting the page at http://<DROPLET_IP>/ isn’t currently working. Let’s fix that.

Create a new user

For security reasons, we need to create another user besides root. Start by logging (ssh’ing) into your droplet and typing the following commands:

root@express-memcache:~\# adduser linzjax # or whatever username you'd like.
# We'll need to give our new user sudo permissions:
root@express-memcache:~\# usermod -aG sudo linzjax
# Then finally switch over to that user
root@express-memcache:~\# su linzjax

Next we’ll need to give our new user ssh permissions:

linzjax@express-memcache:~$ mkdir .ssh
linzjax@express-memcache:~$ sudo cp /root/.ssh/authorized_keys .ssh/
linzjax@express-memcache:~$ sudo chown linzjax:linzjax .ssh/authorized_key

We can now login to our droplet with our new user from our terminal:

$ ssh linzjax@<DROPLET_IP>

Set up your application on DigitalOcean

In order to get a copy of your application into the droplet, either clone your GitHub repo you forked at the beginning of the tutorial.
```
linzjax@express-memcache:~$ git clone https://github.com/<username>/examples-expressjs.git
```

Install pm2 to handle app initiation on startup.

linzjax@express-memcache:~/$ cd examples-express
linzjax@express-memcache:~/examples-express$ sudo npm install -g pm2
linzjax@express-memcache:~/examples-express$ pm2 start app.js
# You'll get a message indicating that the application has started.
# To test this you can run `curl http://localhost:3000` in the root user window.

pm2 can be set up to launch when ever the application starts or reboots.

linzjax@express-memcache:~/examples-express$ pm2 startup systemd
# follow the directions to copy and paste the `sudo env PATH=$PATH` line into
# your terminal.

Install Nginx and configure to listen to port 3000.

linzjax@express-memcache:~$ sudo apt-get update
linzjax@express-memcache:~$ sudo apt-get install nginx

Once Nginx is install, we’ll need to open the firewall to allow http requests.

linzjax@express-memcache:~$ sudo ufw allow 'Nginx HTTP'

We’ll configure Nginx to serve requests to port 3000.

linzjax@express-memcache:~$ sudo vim /etc/nginx/sites-available/default

Copy and paste the following settings into the location section.

location \{
  proxy_set_header X-Real-IP $remote_addr;
  proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
  proxy_set_header Host $http_host;
  proxy_set_header X-NginX-Proxy true;
  proxy_pass http://localhost:8888/;
  proxy_redirect off;
}

Type :wq to save and exit vim. Now all that’s left is to check that our Nginx configuration is correct, and restart it.

linzjax@express-memcache:~$ sudo nginx -t
# Should get the following message.
nginx: the configuration file /etc/nginx/nginx.conf syntax is ok
nginx: configuration file /etc/nginx/nginx.conf test is successful

linzjax@express-memcache:~$ sudo systemctl restart nginx

Now we can visit our DigitalOcean IP address in our browser and you should be able to see our app!

If you’re getting error messages, you can explore the logs by using pm2 log.

Learn to write Express.js middleware

Our prime-calculating app works, but it has one mayor flaw: a user can submit invalid input, such as a string of letters. To validate the input, we’ll create middleware in Express.

There are several validation middleware packages available for Express, and you should use one of those in most cases. In this tutorial, we create our own validation for demonstration purposes.

Express middleware typically consists of a chain of functions that inspect and potentially modify the details of a request and its corresponding response. Each function takes three parameters:

The request object
The response object
A next function that represents the next middleware function in the chain

Each middleware function can modify the request and response objects as necessary. After doing so, it can either call the next middleware function or return to terminate the chain prematurely.

For our app, we create a validation middleware function that parses the submitted query and checks whether it’s a number below 10000.

If it is, the function calls next.
If it isn’t, the function returns an error response.

Add this function to app.js and call it when processing the GET route:

// ...

var validate = function(req, res, next) {
  if(req.query.n) {
    number = parseInt(req.query.n, 10);
    if(isNaN(number) || number < 1 || number > 10000){
      res.render('index', {error: 'Please submit a valid number between 1 and 10000.'});
      return;
    }
    req.query.n = number;
  }
  next();
}

app.get('/', validate, function (req, res) {
  // ...
})
// ...

The validation middleware might return an error message, which we need to display in the index.ejs view:

<!-- Show the result -->
<!-- ... -->

<!-- Error handling -->
<% if (locals.error) { %>
  <div class="alert alert-danger">
    <p class="lead"><%= error %></p>
  </div>
<% } %>

Commit and deploy your changes:

$ git commit -am 'Add input validation'
# This is why you should make a fork of the repo, rather than just cloning ours ;)
$ git push -u origin master
$ ssh linzjax@<DROPLET_IP>
linzjax@express-memcache:~$ cd examples-express
linzjax@express-memcache:~/examples-express$ git pull origin master
linzjax@express-memcache:~/examples-express$ pm2 restart app.js

Open the app and submit some invalid queries to see the error message in action.

Adding caching to Express

Memcache is an in-memory, distributed cache. Its primary API consists of two operations: SET(key, value) and GET(key). Memcache is like a hashmap (or dictionary) that is spread across multiple servers, where operations are still performed in constant time.

The most common use for Memcache is to cache expensive database queries and HTML renders so that these expensive operations don’t need to happen over and over again.

Set up Memcache

To use Memcache in Express, you first need to provision an actual Memcache cache. MemCachier provides a fast and flexible multi-tenant cache system that’s compatible with the protocol used by the popular memcached software. When you create a cache with MemCachier, you’re provided with one or more endpoints that you can connect to using the memcached protocol, accessing your cache just as if you had set up your own memcached server. So head over to https://www.memcachier.com, sign up for an account, and create a free development cache. If you need help getting it set up, follow the directions here.

There are three config vars to you’ll need for your application to be able to connect to your cache: MEMCACHIER_SERVERS, MEMCACHIER_USERNAME, and MEMCACHIER_PASSWORD. Let’s add them to a .env file in your local files.

$ vim .env

Add the following variables to your .env file:

MEMCACHIER_USERNAME=<username>
MEMCACHIER_PASSWORD=<password>
MEMCACHIER_SERVERS=<servers>

We’ll need to install dotenv so our application will read our .env file.

$ npm install dotenv

and configure it at the very top of app.js:

require('dotenv').config();

Once your environment variables are set up, next step is to install memjs with npm so we can use caching in Express.

$ npm install memjs

and configure it in app.js:

// ...

var memjs = require('memjs')
var mc = memjs.Client.create(process.env.MEMCACHIER_SERVERS, {
  failover: true,  // default: false
  timeout: 1,      // default: 0.5 (seconds)
  keepAlive: true  // default: false
})

// ...

Caching expensive computations

There are two reasons why caching the results of expensive computations is a good idea:

Pulling the results from the cache is much faster, resulting in a better user experience.
Expensive computations use significant CPU resources, which can slow down the rest of your app.

Our prime number calculator doesn’t really have any expensive computations, because we limit the input value to 10000. For the sake of the tutorial, however, let’s assume that calculating the prime is an expensive computation we would like to cache.

To achieve this, let’s modify the GET route in app.js as follows:

// ...

app.get('/', validate, function (req, res) {
  if(req.query.n) {
    var prime;
    var prime_key = 'prime.' + req.query.n;
    // Look in cache
    mc.get(prime_key, function(err, val) {
      if(err == null && val != null) {
        // Found it!
        prime = parseInt(val)
      }
      else {
        // Prime not in cache (calculate and store)
        prime = calculatePrime(req.query.n)
        mc.set(prime_key, '' + prime, {expires:0}, function(err, val){/* handle error */})
      }
      // Render view with prime
      res.render('index', { n: req.query.n, prime: prime });
    })
  }
  else {
    // Render view without prime
    res.render('index', {});
  }
});

// ...

Push these changes to Github and pull them into your droplet. Then submit some numbers to find primes:

$ git commit -am 'Add caching'
$ git push
$ ssh linzjax@<DROPLET_IP>
linzjax@express-memcache:~$ cd examples-express
linzjax@express-memcache:~/examples-express$ git pull origin master
linzjax@express-memcache:~/examples-express$ pm2 restart app.js
linzjax@express-memcache:~/examples-express$ pm2 log

The page should work just as before. However, under the hood, already calculated primes are now cached. To see what’s going on in your cache, open the MemCachier dashboard (which is where you found your environment variables.)

On the dashboard you can refresh the stats each time you request a prime. The first time you enter a number, the get misses will increase. For any subsequent request of the same number, you should get an additional get hit.

If it’s not working, and you’re getting the following error:

  Error: connection ECONNREFUSED 12.0.0.1:11211.

Check and make sure that your MEMCACHIER_* ENV variables are set correctly. If it’s still not working, please note: Only MemCachier caches with DigitalOcean as the provider will work with DigitalOcean droplets.

Caching rendered views

Rendering HTML views is generally an expensive computation, and you should cache rendered views whenever possible. In Express, you can achieve this easily with middleware. Let’s add a cacheView middleware function to app.js that checks whether the view for a given URL (including query parameters) is in the cache.

If it is, the view is sent immediately from the cache.
If not, we wrap the send function in the response object to cache the rendered view and call the next function.

// ...

var cacheView = function(req, res, next) {
  var view_key = '_view_cache_' + req.originalUrl || req.url;
  mc.get(view_key, function(err, val) {
    if(err == null && val != null) {
      // Found the rendered view -> send it immediately
      res.send(val.toString('utf8'));
      return;
    }
    // Cache the rendered view for future requests
    res.sendRes = res.send
    res.send = function(body){
      mc.set(view_key, body, {expires:0}, function(err, val){/* handle error */})
      res.sendRes(body);
    }
    next();
  });
}

app.get('/', validate, cacheView, function (req, res) {
  // ...
});
//..

This is easy enough and works well. However, if the view ever changes, we need to be careful. To illustrate the case of a changing page, let’s add a “Like” button to each number and its calculated largest prime. Let’s put the button just below the calculated prime in the index.ejs file:

<!-- ... -->

<!-- Show the result -->
<% if (locals.prime) { %>
  <div class="alert alert-primary">
    <p class="lead">Largest prime less or equal than <%= n %> is <%= prime %></p>
    <p>Likes: <%= likes %></p>
    <form method='POST'>
      <input type="hidden" name="n" value="<%= n %>" />
      <input type="submit" class="btn btn-primary" value="Like!" />
    </form>
  </div>
<% } %>

<!-- ... -->

The like is submitted via POST request, and to parse its input we need the body-parser package:

$ npm install body-parser

We can now create a controller for the POST route in app.js and store the posted like in a variable.

Storing likes in a variable is a bad idea. Each time the app restarts, it wipes all likes. We do this here only for convenience. In a production application, you should store such information in a database.

// ...

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

// Like storage (in a serious app you should use a permanent storage like a database)
var likes = {}

app.post('/', function (req, res) {
  likes[req.query.n] = (likes[req.query.n] || 0) + 1
  res.redirect('/?n=' + req.query.n)
});

// ...

In addition, we also need to make sure the likes are passed to the render function in the GET controller:

// ...

// Render view with prime
res.render('index', { n: req.query.n, prime: prime, likes: likes[req.query.n] || 0 });

// ...

To illustrate the problem with changing pages, let’s commit our current implementation and test it:

$ git commit -am 'Add view caching'
$ git push
$ ssh linzjax@<DROPLET_IP>
linzjax@express-memcache:~$ cd examples-express
linzjax@express-memcache:~/examples-express$ git pull origin master
linzjax@express-memcache:~/examples-express$ pm2 restart app.js
linzjax@express-memcache:~/examples-express$ pm2 log

If you submit a number, you will now get the largest prime below it, together with a Like button. However, when you click Like!, the like count doesn’t increase. This is because the view is cached.

To resolve this, we need to invalidate the cached view whenever it is updated:

// ...

app.post('/', function (req, res) {
  mc.delete('_view_cache_/?n=' + req.body.n, function(err, val){/* handle error */});
  likes[req.query.n] = (likes[req.query.n] || 0) + 1
  res.redirect('/?n=' + req.query.n)
});

// ...

Deploy again to DigitalOcean using Github:

$ git commit -am 'Fix view caching'
$ git push
$ ssh linzjax@<DROPLET_IP>
linzjax@express-memcache:~$ cd examples-express
linzjax@express-memcache:~/examples-express$ git pull origin master
linzjax@express-memcache:~/examples-express$ pm2 restart app.js
linzjax@express-memcache:~/examples-express$ pm2 log

Now you can see the number of likes increase.

Session Caching

Memcache works well for storing information for short-lived sessions that time out. However, because Memcache is a cache and therefore not persistent, long-lived sessions are better suited to permanent storage options, such as your database.

To use sessions in Express, you need express-session. To store the sessions in Memcache, you need connect-memjs:

$ npm install express-session connect-memjs

The configuration in app.js is easy enough:

//...

var session = require('express-session');
var MemcachedStore = require('connect-memjs')(session);

// Session config
app.use(session({
  secret: 'ClydeIsASquirrel',
  resave: 'false',
  saveUninitialized: 'false',
  store: new MemcachedStore({
    servers: [process.env.MEMCACHIER_SERVERS],
    prefix: '_session_'
  })
}));

//...

Now you can now use sessions as you please. For more information about session usage in Express, check out the express-session documentation.

Clean up

Once you’re done with this tutorial and don’t want to use it anymore, you can clean up your DigitalOcean droplet instance by using:

$ doctl compute droplet delete express-memcache

This will clean up all of the DigitalOcean resources.