Scaling an Express.js Application with Memcache on DigitalOcean

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.

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:

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:

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

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:

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

Now you can login to your droplet via

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:

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

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

Set up your application on DigitalOcean

  1. 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.

  2. Install pm2 to handle app initiation on startup.

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

  3. Install Nginx and configure to listen to port 3000.

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

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

    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.

    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:

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

Commit and deploy your changes:

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.

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.

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

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

and configure it in app.js:

Caching expensive computations

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

  1. Pulling the results from the cache is much faster, resulting in a better user experience.
  2. 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:

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

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.

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:

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

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.

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

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

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:

Deploy again to DigitalOcean using Github:

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:

The configuration in app.js is easy enough:

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:

This will clean up all of the DigitalOcean resources.

Further reading & resources