What is Varnish

Varnish is an open-source HTTP accelerator designed to cache and optimize content delivery. It acts as a reverse proxy, sitting between your web server and users, caching frequently accessed content to serve it quickly. This reduces the load on your backend server, speeds up response times, and improves the overall performance of your website. It’s especially beneficial for high-traffic websites that need to handle thousands of simultaneous requests efficiently.

Installing Varnish on an Ubuntu VPS

Here’s a step-by-step guide to installing Varnish on an Ubuntu server. This guide assumes you have root or sudo privileges.

Before installing any new packages, ensure your system is up-to-date by running:

Varnish is available in the official Ubuntu repositories, so installation is straightforward. Use the following command:

After installation, Varnish will listen on port 6081 by default. You’ll need to configure it to work with your web server, like Apache or Nginx. The primary configuration file is located at /etc/varnish/default.vcl.

Example Configuration for Apache:

To configure Varnish to work with Apache, follow these steps:

1. Edit the default.vcl file:

2. Define your backend server (Apache) in the configuration. Replace 127.0.0.1 and 8080 with your web server's IP and port:

3. Save and exit (Ctrl + X, then Y and Enter).

By default, Varnish listens on port 6081, but we want it to serve HTTP traffic on port 80 (the default HTTP port). To do this, edit the Varnish service configuration file:

Find the ExecStart line and modify the listening port:

This tells Varnish to listen on port 80 and allocate 256MB of memory for caching.

Save and exit the file.

Since Varnish is now handling HTTP requests on port 80, you need to modify Apache or Nginx to run on a different port (usually port 8080).

- For Apache:

Edit the Apache ports configuration:

Change the Listen directive to port 8080:

Then, edit your virtual host configuration file (typically located in /etc/apache2/sites-available/000-default.conf):

Save the file and restart Apache:

- For Nginx:

If you are using Nginx, open the server block configuration (typically found in /etc/nginx/sites-available/default):

Change the listen directive to port 8080:

Save the file and restart Nginx:

Now start the Varnish service and enable it to run on boot:

You can check if Varnish is running correctly by using the following command:

It should display that Varnish is active and running. You can also check if it’s caching content properly by using curl to inspect headers:

If Varnish is working, you should see the X-Varnish header, which shows the cache status (HIT or MISS).

Example of Using Varnish

Once Varnish is up and running, you can test its caching capabilities by sending HTTP requests to your site and examining the headers.

The first time you make a request, the content will be fetched from the backend server, and Varnish will store a copy in its cache.

The response header will show a MISS:

This indicates that Varnish fetched the content from the backend (Apache or Nginx) and cached it.

If you request the same content again, Varnish will serve it from its cache, resulting in a HIT:

This time, you should see:

The X-Varnish header shows that the content was served from the cache, and the Age header indicates how long the content has been cached.

Purging Cached Content

If you want to purge specific content from the cache, you can use the varnishadm tool. For example, to purge the home page from the cache:

This forces Varnish to remove the cached version of the homepage, and the next request will be fetched from the backend.

Varnish is a powerful tool for optimizing web performance by caching frequently requested content. By following this guide, you can install Varnish on your Ubuntu VPS, configure it to work with Apache or Nginx, and start enjoying the benefits of faster load times and reduced server load. Whether you're running a high-traffic website or just looking to optimize your server, Varnish is a great solution to improve responsiveness and scalability.