What is Varnish? Understanding its Proxy Mechanism, Features, Capabilities, and Advantages

Discover how Varnish, a high-performance HTTP accelerator, boosts web application speed, ensures efficient traffic management, and optimizes network resources. This guide also covers the installation, configuration, and ways to fully leverage the benefits of Varnish proxy.

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Dimitri Nek
Dimitri Nek
Web Hosting Geek

Varnish

In modern web hosting, speed and performance are paramount. Every millisecond counts, and this is where caching comes into play. Caching is a technique that stores copies of files in a temporary storage location so that they can be accessed more quickly. For web servers, this means storing web page resources that don’t change often, so they can be delivered to the user more rapidly. This results in faster page load times, reduced network costs, and a better user experience.

In this article we will talk about Varnish, a robust and efficient caching proxy for web servers. We will explore what Varnish is, its key features, how it works, and the benefits it offers. We will also compare it with other popular caching solutions and provide a basic guide on how to install and configure Varnish on CentOS and Ubuntu.

Whether you’re an experienced server administrator or a curious novice webmaster, this article aims to provide you with a deeper understanding of Varnish and its role in hosting optimization.

Let’s get started.

Key Takeaways

  • Varnish is a high-performance HTTP accelerator that significantly enhances web application speed and performance. Its advanced caching mechanism serves content directly from memory, reducing the load on backend servers.
  • The flexibility of Varnish Configuration Language allows for precise control over request and response handling, enabling custom caching strategies and behaviors. This adaptability makes Varnish suitable for a broad spectrum of scenarios.
  • With a rich feature set including load distribution, system health monitoring, data compression, content invalidation, and more, Varnish enhances the robustness and resilience of your web platform.
  • The benefits of implementing Varnish are manifold, encompassing enhanced user experience, SEO gains, cost-effectiveness, and in-depth analytics. Its capacity to manage high traffic volumes and swiftly deliver content can lead to heightened user engagement and improved conversion rates.
  • The process of installing and setting up Varnish involves several steps, but the outcome is a more responsive and scalable web platform. Given its extensive compatibility and versatile deployment options, Varnish proves to be an invaluable asset for any web server administrator or webmaster.

What is Varnish?

Varnish is a high-performance HTTP accelerator and caching reverse proxy designed to speed up web applications by caching content in memory. It sits between the client and the backend server, handling requests from clients and serving cached content when available, thereby reducing the load on the backend server and improving the overall performance and speed of the web application.

Varnish was first released in 2006 by a Norwegian newspaper, Verdens Gang, to handle their high web traffic load. The project was led by Poul-Henning Kamp, a renowned FreeBSD developer. The goal was to create a caching HTTP reverse proxy that could handle high volumes of concurrent connections and deliver high-performance content serving. Over the years, Varnish has evolved and improved, with contributions from developers worldwide, and it’s now used by high-traffic websites, including Wikipedia, The New York Times, and Facebook.

At its core, Varnish works by receiving HTTP requests from clients and checking if the requested content is in its cache. If the content is cached (a cache hit), Varnish immediately serves it to the client. If the content is not in the cache (a cache miss), Varnish forwards the request to the backend server, caches the response, and then serves it to the client. This process significantly reduces the load on the backend server and speeds up content delivery.

Varnish’s flexibility allows it to be configured for various caching strategies using the Varnish Configuration Language (i.e. VCL), making it a powerful tool for web server optimization.

Features of Varnish

Varnish boasts a multitude of features designed to optimize web performance, enhance scalability, and provide granular control over how content is served.

High-performance HTTP Accelerator

Varnish is designed to handle high traffic loads and deliver content at lightning-fast speeds. It achieves this by storing cached content in virtual memory, allowing for rapid access and delivery. This makes Varnish an excellent choice for high-traffic websites and applications that require fast content delivery to provide a smooth user experience.

Flexibility and Configurability

One of the standout features of Varnish is its flexibility and configurability. Varnish uses a domain-specific language called Varnish Configuration Language that allows administrators to define how incoming requests should be handled. This includes defining caching policies, modifying requests or responses, and implementing various other custom behaviors. This level of control makes Varnish adaptable to a wide range of use cases.

Edge Side Includes Support

Varnish supports Edge Side Includes (i.e. ESI), a markup language that allows different parts of a web page to be cached and served separately. This is particularly useful for dynamic websites where certain parts of a page change frequently while others remain static. With ESI, Varnish can serve the static parts from cache and the dynamic parts from the backend, improving efficiency and load times.

Load Balancing Capabilities

Varnish can also act as a load balancer, distributing client requests across multiple backend servers to balance the load and ensure high availability. This can be particularly useful for websites and applications that experience high levels of traffic, as it helps to prevent any single server from becoming a bottleneck and potentially impacting performance.

SSL/TLS Support

While Varnish itself does not natively support SSL/TLS, it can be easily configured to work with other software that does, such as Nginx or Apache, to handle encrypted traffic. This allows Varnish to be used in environments where secure connections are a requirement, ensuring that the benefits of Varnish can be enjoyed without compromising on security.

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Hot Content Detection

Varnish has a built-in mechanism for detecting “hot” content, i.e., content that is requested frequently. When such content is detected, Varnish keeps it in the cache and serves it directly from memory, bypassing the need for disk I/O. This feature can significantly improve performance for websites with certain pages or resources that are particularly popular.

Request Coalescing

When Varnish receives multiple requests for the same uncached content, instead of sending all these requests to the backend, it sends just one and then uses the response to answer all the waiting requests. This feature, known as request coalescing, reduces the load on the backend server and helps to prevent cache stampedes, where a sudden influx of requests for the same content could overwhelm the server.

Health Checks

Varnish can perform health checks on your backend servers. This means it can automatically detect if a server is down or not responding and stop sending requests to it until it’s back online. This feature helps maintain the availability and reliability of your web application by ensuring that traffic is only directed to healthy servers.

Grace Mode and Saint Mode

Grace mode allows Varnish to serve stale content if the backend server is slow or not responding, ensuring that users still receive a response. Saint mode, on the other hand, allows Varnish to temporarily blacklist a backend server if it’s consistently returning errors, directing traffic to other, healthier servers instead. These features contribute to the robustness and resilience of your web application.

Gzip Compression

Varnish supports Gzip compression, which can significantly reduce the size of the data that’s sent to clients. This can lead to faster data transmission and quicker page load times, particularly for users with slower internet connections. It also reduces the amount of bandwidth used, which can be a significant cost saving.

Content Purging

Varnish allows for content purging, which is the ability to remove content from the cache before it naturally expires. This is particularly useful for websites where content updates are frequent and you want to ensure that users are always served the most up-to-date content. With Varnish, you can purge individual URLs, content types, or use wildcards to purge multiple pages at once.

Streaming Support

Varnish supports streaming, which is particularly useful for delivering large files or media content. When Varnish receives a request for a large file that isn’t in the cache, it can start streaming the content from the backend to the client while simultaneously storing it in the cache. This means the client doesn’t have to wait for the entire file to be cached before it starts receiving data, leading to faster response times.

Statistics and Logging

Varnish provides comprehensive statistics and logging capabilities, which can be invaluable for monitoring performance and troubleshooting issues. Varnish’s shared memory log (i.e. VSL) provides detailed, real-time information about every request and response, while the Varnish Statistic Collection (i.e. VSC) provides high-level statistics about cache hits, misses, and other key metrics.

How Varnish Works

Varnish operates as a caching HTTP reverse proxy, sitting between the client and the backend server. When a client sends a request, Varnish first checks its cache to see if the requested content is available.

If a suitable response is found in the cache, it is served to the client, reducing the load on the backend server. If there is no suitable response in the cache, Varnish sends the request to the backend server, caches the response, and then serves it to the client. Varnish understands the Cache-Control HTTP header if your backend server sends one, but ultimately the VCL program makes the decision to cache and for how long.

Varnish Configuration Language

VCL is a powerful tool that allows you to control how Varnish handles each request. For each request, Varnish runs through the VCL program to decide what should happen: Which backend has this content, how long can we cache it, is it accessible for this request, should it be redirected elsewhere, and so on. When you load the VCL program into Varnish, it is compiled into a C-program which is compiled into a shared library, which Varnish then loads and calls into. This makes VCL code fast and efficient.

How Varnish Handles HTTP Requests and Responses

Varnish sits between the client and the backend server, handling requests and responses. When a request comes in, Varnish checks its cache to see if it has a stored response that can be used. If it does, it serves that response to the client. If it doesn’t, it forwards the request to the backend server, then stores the response in its cache before serving it to the client. This process is controlled by the rules defined in the VCL. Varnish also supports content composition with Edge Side Includes (ESI), allowing responses to be composed of different bits from different backends, each with its own caching policy.

Compatibility and Deployment Options of Varnish

Varnish is a highly versatile HTTP accelerator, designed with broad compatibility and flexible deployment options. It is capable of running on a wide array of modern UNIX-like operating systems, including but not limited to Linux, OS/X, FreeBSD, OpenBSD, NetBSD, Solaris, OmniOs, and SmartOS. This extensive compatibility allows for seamless integration into various server environments.

In terms of hardware, Varnish is not limited by CPU architecture and can run on systems based on i386, amd64, arm32, arm64, mips, power, riscV, s390, and more. As for deployment, Varnish offers a range of options to suit different needs. It can be installed on dedicated hardware for optimal performance, deployed in virtual machines (VMs) for scalability, or run in containers for efficient resource usage and portability.

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It’s also cloud-compatible, allowing for deployment on platforms like AWS, Google Cloud, or Azure.

Benefits of Using Varnish

Varnish offers numerous benefits, including significant speed and performance improvements, enhanced scalability, improved user experience, SEO advantages, and more.

Speed and Performance Improvements

One of the primary benefits of using Varnish is the significant improvement in speed and performance it brings to web applications. By caching content in memory and serving it directly to clients, Varnish reduces the load on backend servers and delivers content at lightning-fast speeds. This can lead to a more responsive and efficient web application, capable of handling high traffic loads without compromising on performance.

Scalability

Varnish enhances the scalability of web applications. As traffic to a website grows, Varnish can effectively manage increased load by serving cached content, reducing the need for additional backend server resources. Its load balancing capabilities also allow it to distribute traffic evenly across multiple backend servers, ensuring that no single server becomes a bottleneck.

Improved User Experience

By accelerating content delivery, Varnish contributes to a smoother and more enjoyable user experience. Faster page load times mean users can navigate a website more quickly and efficiently, leading to increased user satisfaction and potentially higher engagement and conversion rates.

SEO Benefits

Speed is a critical factor in search engine optimization. Search engines favor websites that load quickly, as they provide a better user experience. By improving page load times, Varnish can contribute to higher search engine rankings, leading to increased visibility, more organic traffic, and ultimately, better SEO performance.

Robustness and Resilience

Varnish’s features like health checks, grace mode, and saint mode contribute to the robustness and resilience of your web application. It can automatically detect if a server is down or not responding and stop sending requests to it until it’s back online. It can also serve stale content if the backend server is slow or not responding, ensuring that users still receive a response.

Cost Efficiency

By reducing the load on your backend servers and efficiently managing network resources, Varnish can lead to significant cost savings. It can help you get more out of your existing infrastructure, reducing the need for additional servers or bandwidth as your traffic grows.

Flexibility and Control

With its Varnish Configuration Language, Varnish gives you a high degree of control over how requests are handled. This allows you to tailor Varnish’s behavior to your specific needs, making it a flexible solution that can be adapted to a wide range of use cases.

Detailed Analytics and Logging

Varnish provides comprehensive statistics and logging capabilities, which can be invaluable for monitoring performance and troubleshooting issues. This can give you deep insights into your web traffic and help you make informed decisions to optimize your web application.

Who is Varnish For

Pretty much any industry or website that needs to deliver content quickly to a large number of users can benefit from using Varnish. Here are few examples:

  • Media and Publishing: High-traffic news sites and online magazines, like The New York Times and The Guardian, use Varnish to handle large volumes of traffic and deliver content quickly to users. Varnish helps these sites manage sudden traffic spikes, like those that occur during breaking news events.
  • E-commerce: Online retailers can benefit from Varnish’s ability to speed up page load times, improving the shopping experience and potentially increasing conversion rates. Companies like eBay and Magento use Varnish to ensure their product pages load quickly and can handle high traffic during peak shopping periods.
  • Streaming Services: Varnish can be used by video streaming platforms to deliver content quickly and efficiently, ensuring smooth playback and reducing buffering. It’s used by companies like Vimeo to provide a seamless viewing experience.
  • Social Media: Platforms like Facebook and Twitter, which have to handle massive amounts of dynamic content and user traffic, use Varnish to improve site performance and scalability.
  • Education: Educational institutions and e-learning platforms can use Varnish to ensure that their online resources are accessible and fast-loading, providing a better learning experience for students.
  • Government: Government websites, which often have to provide important information to a large number of people, can use Varnish to ensure their sites are reliable and fast, even under heavy traffic.

Comparing Varnish with Other Caching Solutions

While Varnish, Squid, Nginx, and Apache Traffic Server have its strengths and weaknesses, Varnish stands out for its advanced caching capabilities, high performance, and the flexibility provided by the VCL. Below is a basic comparison, highlighting what each is best used for, as well as their pros and cons.

Name Best Used For Advantages Disadvantages
Varnish High-traffic websites requiring advanced caching capabilities Superior performance, advanced caching features, highly configurable via VCL Lacks native SSL/TLS support
Squid General web caching for a variety of protocols Supports a wide array of protocols, mature and stable Less performant for high-traffic sites, less modern, configuration can be intricate
Nginx Multi-purpose web server needs, including reverse proxying, load balancing, and basic HTTP caching Multi-functional, straightforward to configure, built-in SSL/TLS support Less advanced caching capabilities, no support for ESI
Apache Traffic Server Improving network efficiency and performance with web proxy caching Robust and adaptable, good performance, built-in SSL/TLS support Lacks some of Varnish’s advanced features, configuration can be intricate

Varnish vs Squid

Squid is a caching proxy for the Web supporting HTTP, HTTPS, FTP, and more. It reduces bandwidth and improves response times by caching and reusing frequently-requested web pages. However, Squid is generally considered less modern and less performant than Varnish, especially for high-traffic sites.

Varnish vs Nginx

Nginx is a web server that can also be used as a reverse proxy, load balancer, and HTTP cache. While Nginx is a powerful tool that can handle a wide range of tasks, its caching capabilities are less advanced than Varnish’s. For example, Nginx does not support advanced caching features like ESI.

Varnish vs Apache Traffic Server

Apache Traffic Server is a high-performance web proxy cache that improves network efficiency and performance by caching frequently-accessed information at the edge of the network. While ATS is a robust and flexible solution, it lacks some of the advanced features of Varnish, such as the VCL, which provides granular control over caching behavior.

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How to Install and Configure Varnish

Installing and configuring Varnish involves a series of easy steps that ensure it is properly set up to accelerate your web application’s performance.

Here’s a basic guide on how to install Varnish on Ubuntu and CentOS:

Ubuntu:

Update your system packages: sudo apt-get update.

Install Varnish: sudo apt-get install varnish.

CentOS:

First, you need to add the EPEL repository to your system: sudo yum install epel-release.

Then, install Varnish: sudo yum install varnish.

For other systems, please refer to the official Varnish documentation or your system’s package manager.

Default Configuration

After installation, Varnish’s default configuration file can be found at /etc/varnish/default.vcl on most systems. This file contains the default VCL, which specifies how Varnish should handle requests and responses. It is divided into various subroutines, each responsible for a specific part of the request-response cycle.

These subroutines include:

  • vcl_recv: This subroutine is executed when a request is received. Here, you can define actions like rewriting the request, performing redirects, or deciding whether to look up the request in the cache.
  • vcl_backend_response: This subroutine is executed when a response is received from the backend. Here, you can manipulate the response before it’s cached, such as setting the Time-To-Live (TTL) for the object in cache.
  • vcl_deliver: This subroutine is executed before the response is delivered to the client. Here, you can modify the response headers.

Here’s an example of VCL file:


backend default {
    .host = "127.0.0.1";
    .port = "8080";
}

sub vcl_recv {
    # Remove cookies for static files and set a long TTL to increase cache hit rate
    if (req.url ~ "(jpg|jpeg|png|gif|ico|swf|css|js|html|htm)$") {
        unset req.http.Cookie;
        return (hash);
    }

    # Normalize the Accept-Encoding header to reduce cache fragmentation
    if (req.http.Accept-Encoding) {
        if (req.http.Accept-Encoding ~ "gzip") {
            set req.http.Accept-Encoding = "gzip";
        } else if (req.http.Accept-Encoding ~ "deflate") {
            set req.http.Accept-Encoding = "deflate";
        } else {
            unset req.http.Accept-Encoding;
        }
    }
}

sub vcl_backend_response {
    # Set a long TTL for static files
    if (bereq.url ~ "(jpg|jpeg|png|gif|ico|swf|css|js|html|htm)$") {
        set beresp.ttl = 1w;
    }
}

sub vcl_deliver {
    # Add a header to indicate a cache hit or miss
    if (obj.hits > 0) {
        set resp.http.X-Cache = "HIT";
    } else {
        set resp.http.X-Cache = "MISS";
    }
}

In this example:

  • In vcl_recv, we remove cookies for static files, as they are typically the same for all users and don’t need to be personalized. We also normalize the Accept-Encoding header to reduce cache fragmentation.
  • In vcl_backend_response, we set a long Time-To-Live for static files to increase the cache hit rate.
  • In vcl_deliver, we add a custom X-Cache header to the response to indicate whether it was a cache hit or miss. This can be useful for debugging.

Tips for Configuring Varnish Using VCL

  1. Understand the VCL Syntax: VCL is a C-like language, and understanding its syntax is key to configuring Varnish effectively.
  2. Use Subroutines: VCL is structured around subroutines, which are blocks of code that define how Varnish should handle different stages of the request/response process.
  3. Define Your Caching Policy: You can use VCL to define a caching policy that fits your needs. For example, you can specify which URLs should be cached, how long they should be cached for, and what should happen when the cache expires.
  4. Test Your Configuration: Always test your VCL configuration with the varnishd -C -f /path/to/your.vcl command before deploying it. This will compile your VCL into C code and check for any syntax errors.
  5. Use Include Statements: If your VCL becomes too large, you can split it into multiple files and use include statements to include them in the main VCL file. This can make your VCL easier to manage.

Conclusion

Varnish stands out as a powerful tool for any web server administrator or webmaster seeking to enhance their web application’s performance and scalability. Its advanced caching capabilities, coupled with the flexibility offered by VCL, make it a versatile solution adaptable to various use cases.

Whether you’re running a high-traffic news site, an e-commerce platform, a streaming service, or any other web application that needs to deliver content quickly to a large number of users, Varnish can be a game-changer. By leveraging Varnish, you can provide a smoother, faster user experience, which can lead to increased user engagement, higher conversion rates, and improved SEO performance.

Hope you found this article helpful.

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FAQ

  1. What is Varnish and what does it do?

    Varnish is a high-performance HTTP accelerator designed to speed up web applications. It operates by storing a copy of a page served by the web server the first time a user visits that page. When the page is requested again, Varnish serves the copy instead of requesting the page from the web server, thereby increasing the speed of the response and reducing the load on the web server.

  2. How does Varnish improve website performance?

    Varnish enhances website performance by serving cached content directly from memory, thereby reducing the load on backend servers. This results in faster page load times and a more responsive website. Varnish also offers load balancing capabilities, which can distribute traffic evenly across multiple backend servers, preventing any single server from becoming a bottleneck and ensuring smooth performance even under high traffic loads.

  3. What is Varnish Configuration Language (VCL)?

    Varnish Configuration Language (VCL) is a domain-specific language used in Varnish to write policies that dictate how incoming requests should be handled. These policies can control various aspects of Varnish’s behavior, such as which requests should be cached, how long they should be cached for, and how to handle requests when the cache is full. VCL provides a high degree of flexibility and control, making Varnish a highly customizable solution for web caching.

  4. How does Varnish compare to other caching solutions?

    Varnish stands out among other caching solutions due to its high performance, advanced caching capabilities, and the flexibility provided by the Varnish Configuration Language (VCL). While other solutions like Squid, Nginx, and Apache Traffic Server also offer caching capabilities, they may lack some of the advanced features of Varnish. For example, Nginx does not support advanced caching features like Edge Side Includes (ESI), and while Squid supports a wide range of protocols, it is generally considered less modern and less performant than Varnish for high-traffic sites.

  5. What are the benefits of using Varnish?

    Varnish offers numerous benefits, including improved speed and performance, enhanced scalability, and a better user experience. By serving cached content directly from memory, Varnish can deliver content at lightning-fast speeds, leading to a more responsive and efficient web application. Its load balancing capabilities also enhance the scalability of web applications, allowing them to handle high traffic loads effectively. Furthermore, the improved page load times contribute to a smoother and more enjoyable user experience, which can lead to increased user engagement and conversion rates.

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