Varnish

Varnish is a web application accelerator. It is also called a caching HTTP reverse proxy. It sits between users and a web server and stores copies of content in memory. When users request the same content again, Varnish serves it directly without contacting the server.

Varnish is used to make websites load faster. It reduces server load and speeds up response time. It is especially useful for sites that serve many visitors or receive repeated requests for the same content. It supports complex caching rules and works well with large content management systems.

History and Development

Varnish was created in 2005 by Danish developer Poul-Henning Kamp, funded by the Norwegian newspaper Verdens Gang. They needed a high-performance caching system for their busy news website.

Key motivations and design choices:

• Existing tools like Squid were focused on forward proxying, not web acceleration
• Squid's legacy code made deep customization difficult
• Kamp chose to build Varnish from scratch, optimized for modern web needs
• Written in the C programming language for speed and low-level control

Major milestones and features:

• First release in 2006
• Introduced VCL (Varnish Configuration Language), enabling custom caching rules
• Focused on serving high volumes of requests quickly and efficiently
• Gained adoption across:
  • News and media outlets
  • E-commerce platforms
  • Hosting and infrastructure providers

Varnish is now maintained by Varnish Software, which provides commercial services around the project. The open-source core remains available under the BSD license.

Comparison with Previous Tools

Before Varnish, many sites used Squid or built their own cache layers. Squid was created in the 1990s and focused on forward caching. It was good for saving bandwidth by storing frequently accessed resources from other websites. But it lacked flexibility for acting as a reverse proxy in front of a single web server.

Varnish took a different approach. It was built for reverse caching. That means it stores and serves content from a specific backend, such as Apache or Nginx. It supports content-heavy websites and handles high loads better.

Varnish introduced VCL, which gave users direct control over how content is cached, served, and purged. This was missing in earlier tools. Varnish also relied heavily on memory for storage, which helped deliver faster results.

Compared to newer tools like Nginx caching or Content Delivery Networks (CDNs), Varnish gives more control at the server level. CDNs offload content to global networks, while Varnish keeps it local and configurable.

Key Features and Concepts

Varnish has several features that help it speed up websites and reduce load.

• Reverse Proxy Caching - Varnish works as a reverse proxy. It sits between users and the backend server. When a request comes in, Varnish checks if a copy has been stored. If it does, it returns that copy. If not, it forwards the request to the backend, stores the response, and returns it to the user.
  
  This reduces the number of times the server needs to generate the same response.
• VCL (Varnish Configuration Language) - VCL is the language used to control Varnish behavior. It lets users write rules for how requests are handled. This includes how long content is cached, when it should be purged, or how different URLs should be treated.
  
  VCL scripts are compiled into C code for speed. This makes custom logic fast and efficient.
• In-Memory Storage - Varnish stores data in memory. This means it can serve cached content much faster than systems that rely on disk. It uses memory-efficient structures and can handle thousands of requests per second.
• Backend Health Checks - Varnish can monitor the health of backend servers. If a backend becomes slow or unresponsive, Varnish can skip it and use others. This adds a layer of fault tolerance.
• Grace Mode - Varnish supports a "grace" period. When the cached content has expired, it can still be served while the new version is fetched in the background. This avoids delays and keeps the site responsive during high traffic or backend issues.
• Purge and Invalidation - Content in Varnish can be purged manually or automatically. Developers can send special requests to remove stale content. VCL can also include rules for when to invalidate cached data.
• Logging and Monitoring - Varnish includes tools like varnishlog, varnishstat, and varnishhist. These tools help track performance, view logs, and analyze traffic in real time.
• Edge Side Includes (ESI) - Varnish supports ESI, a feature that allows pages to be split into fragments. Some parts can be cached longer, while others remain dynamic. This helps build flexible pages that respond quickly.

Advantages of Varnish

Varnish offers many benefits for developers, administrators, and hosting providers.

• Speed - Because Varnish stores content in memory, it delivers results faster than disk-based systems. This reduces server response time and improves user experience.
• Reduced Backend Load - Varnish handles repeated requests without hitting the backend. This reduces CPU usage, database queries, and application processing.
• Flexibility - VCL allows detailed control over request and response behavior. Users can create fine-tuned caching rules that match their site structure and content needs.
• Scalability - Varnish handles thousands of requests per second with low resource usage. It scales well as traffic grows.
• Compatibility - Varnish works with most backend servers and CMS platforms. It integrates well into existing setups and needs few changes to start.
• Better Uptime - With grace mode, Varnish keeps serving content even if the backend slows down. This helps websites stay online during traffic spikes or brief server problems.

Limitations

While Varnish is powerful, it has some limits.

• It does not support HTTPS natively. You need to use tools like Nginx or Hitch to handle SSL.
• It is not a full web server. It works best as part of a larger stack.
• Dynamic content must be managed carefully to avoid caching errors.
• Memory use grows with the amount of cached content. Servers need enough RAM to match demand.

Varnish Use Cases with NTC Hosting

Varnish primary use case is to speed up websites and is available with almost all web hosting plans offered by NTC Hosting, as well as all semi-dedicated, virtual private servers, and dedicated servers that feature the Hepsia Control Panel. It works well with content management systems like WordPress, Drupal, and Magento. Hosting providers use it to handle high traffic and reduce server strain.

• News and Media Sites - News sites serve many visitors at once. Most requests are for the same articles. Varnish stores the pages in memory and delivers them instantly, even under load.
• E-commerce Platforms - Shops often serve product pages that do not change often. Varnish can cache these pages and serve them without going to the backend. For example, category pages, product lists, and image galleries can be cached, while checkout pages remain dynamic.
• APIs and Microservices - APIs that return the same data repeatedly benefit from Varnish. It reduces the number of database queries and speeds up response time. This is helpful for rate-limited APIs or services under heavy use.
• Multi-Site Hosting - Varnish can serve many domains from one setup. Each site can have its own rules for caching and purging. Hosting companies use this to manage large numbers of sites on shared infrastructure.