How to Install “Varnish” (HTTP Accelerator) and Perform Load Testing Using Apache Benchmark

Varnish is an HTTP accelerator that improves the performance of web applications by caching frequently requested content in memory. It is a powerful tool that can speed up your website and reduce server load by serving cached content directly to users instead of processing requests every time they visit. In this article, we'll take a look at how to install Varnish and perform load testing using Apache Benchmark.


Before we begin, make sure you have the following software installed on your server −

  • Apache web server

  • Varnish

  • Apache Benchmark (ab)

If you don't have these installed, you can use the following commands to install them −

sudo apt-get update
sudo apt-get install apache2
sudo apt-get install varnish
sudo apt-get install apache2-utils

Once you have these installed, you can proceed with the installation and configuration of Varnish.

Installing Varnish

To install Varnish on your server, follow these steps −

Step 1: Install Varnish

sudo apt-get update
sudo apt-get install varnish

Step 2: Configure Varnish

By default, Varnish listens on port 6081. You can change this by editing the default Varnish configuration file −

sudo nano /etc/default/varnish

In this file, find the following line −

DAEMON_OPTS="-a :6081
-T localhost:6082
-f /etc/varnish/default.vcl
-S /etc/varnish/secret
-s malloc,256m"

Change the "-a" flag to the port you want Varnish to listen on. For example, if you want Varnish to listen on port 80, change the line to −

-T localhost:6082
-f /etc/varnish/default.vcl
-S /etc/varnish/secret
-s malloc,256m"

Save and close the file.

Step 3: Configure Apache

Next, you need to configure Apache to work with Varnish. Edit the default Apache virtual host configuration file −

sudo nano /etc/apache2/sites-available/000-default.conf

Add the following lines to the file, before the closing "</VirtualHost>" tag −

<IfModule mod_headers.c>
    Header set Cache-Control "public, max-age=120"

This tells Apache to set the "Cache-Control" header for all responses to "public" and set a maximum cache age of 120 seconds.

Save and close the file.

Step 4: Restart Services

Finally, restart both Varnish and Apache to apply the changes −

sudo service varnish restart
sudo service apache2 restart

You should now have Varnish installed and configured to work with Apache.

Performing Load Testing Using Apache Benchmark

Now that Varnish is installed and configured, you can perform load testing to see how it affects your website's performance. Apache Benchmark (ab) is a tool that allows you to simulate a large number of requests to your web server.

To perform load testing using Apache Benchmark, follow these steps −

Step 1: Run Apache Benchmark

Open a terminal window and run the following command −

ab -n 1000 -c 100 http://localhost/

This will simulate 1000 requests to your server with a concurrency level of 100.

Step 2: View Results

After Apache Benchmark has finished running, it will display the results in the terminal. The most important metrics to look at are the "Requests per second" and "Time per request" values.

"Requests per second" tells you how many requests your server can handle per second, while "Time per request" tells you how long it takes for your server to process each request.

Step 3: Test with Varnish

Now, repeat the same test with Varnish enabled by running the following command −

ab -n 1000 -c 100 http://localhost:80/

This time, the requests are sent to Varnish instead of Apache. You should see a significant increase in the "Requests per second" value and a decrease in the "Time per request" value, indicating that Varnish is serving cached content much faster than Apache can process new requests.

While Varnish is an excellent tool for improving website performance, it's essential to keep in mind that it's not a one-size-fits-all solution. The effectiveness of Varnish depends on the type of content being served, the frequency of updates, and the traffic patterns of your website. For websites with highly dynamic content or infrequent visitors, the benefits of Varnish may be limited.

Additionally, it's crucial to monitor the cache hit rate and adjust Varnish's caching policies accordingly. If Varnish is serving a high percentage of cached content, you may want to increase the cache size or cache longer. Conversely, if the cache hit rate is low, you may want to decrease the cache size or cache for a shorter period.

Varnish also supports several advanced features, such as load balancing, SSL termination, and ESI (Edge Side Includes), which allow for more complex caching strategies. These features require additional configuration and may not be necessary for all use cases.

When installing Varnish, it's essential to ensure that it's compatible with the other components of your web server stack. For example, if you're using a content management system (CMS) like WordPress, you may need to install a plugin or modify the Varnish configuration to ensure that it caches dynamic content correctly. Similarly, if you're using a reverse proxy or a load balancer, you may need to adjust the configuration to ensure that Varnish works correctly.

It's also important to consider security when installing Varnish. By default, Varnish listens on a public-facing port, which can be a security risk. You can mitigate this risk by restricting access to the Varnish port to trusted IP addresses or by using a firewall to block access from untrusted sources.

Another consideration when using Varnish is the impact on website analytics. Because Varnish serves cached content directly to users, the web server logs may not accurately reflect the number of requests and traffic to your website. You can mitigate this issue by configuring Varnish to include custom headers or by using a specialized analytics tool that works with Varnish.

Finally, it's worth noting that Varnish is just one tool in a broader toolkit for improving website performance. Other tools, such as a content delivery network (CDN), image optimization, and minification, can also help to speed up your website and reduce server load. It's essential to evaluate your website's performance holistically and use a combination of tools to achieve optimal results.


Installing and configuring Varnish is a straightforward process that can significantly improve the performance of your web application. By caching frequently requested content in memory, Varnish can reduce the load on your server and speed up your website for users. Load testing with Apache Benchmark can help you evaluate the impact of Varnish on your website's performance and determine if it is a useful tool for your specific use case.

Updated on: 28-Apr-2023


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