Varnish Cache: What It Is & How It Works
Varnish Cache speeds up a website by caching the most-requested pages: what it is, how it works, and when to use it without touching your code.
In this article
Varnish Cache is an open-source web accelerator that makes a website noticeably faster. In technical terms it is also called an HTTP accelerator: a request accelerator that makes browsing the site feel more dynamic and responsive.
It achieves this without touching the site's code at all — a perfect fit for anyone who wants a faster, better-performing website without a rewrite.
A site whose pages load quickly is simply more pleasant to use and easier to navigate. That, in turn, is more convenient for visitors and encourages them to come back.
What Varnish Cache Is For
This is exactly where Varnish Cache helps: it supports the webmaster through this whole process. It's a tool that significantly improves both front-end delivery and page-load speed, without any code changes required. Even operators with limited experience can run Varnish Cache, which is part of why it's such a widely used, genuinely effective tool.
It's used, among others, by large newsrooms, Facebook, Reddit, Twitter and Wikipedia — in short, sites with very high traffic. As mentioned, it's an open-source project, freely accessible and open to external contributions, built specifically for the HTTP protocol. It's a proven way to speed up how a site is served without major code intervention; getting it running only takes a few, fairly simple configuration steps.
The Installation Process
Installing Varnish Cache requires root access on a dedicated server, a VPS, or a server you own. VCL — Varnish Configuration Language — is the dedicated configuration language Varnish uses; it's compiled down to machine code and executed whenever a request comes in from a client device, i.e. any visitor trying to reach the site.
Varnish Cache essentially runs in the background: it's invisible to the people browsing the site. It simply builds a cache holding as many pages as possible — typically the most visited, i.e. the most requested ones.
That way the site can serve them to visitors in the shortest possible time. Page delivery ends up noticeably faster, and the load on the central database drops, reducing the resources needed to keep the site fast and operational.
Varnish Cache Features
Varnish Cache offers many features: the best known are dynamic and static caching, but it's also worth mentioning URL-based traffic routing and load balancing.
Several providers bundle this software into their hosting plans — for example specialist WordPress hosts that pair Varnish with Apache for excellent performance. There are also VPS providers optimized specifically for high-traffic sites (roughly 100,000+ visits a month), typically running Varnish alongside Nginx with configurations tuned for heavy spikes and sustained consumption. Some shared-hosting providers pair Varnish with Zend Optimizer for their shared plans.
CMS Platforms Supported by Varnish Cache
As we've seen, Varnish Cache is useful wherever the underlying code alone can't deliver the speed a site needs. It works with the most common, well-known CMS platforms, including WordPress, Magento, Joomla and vBulletin.
Even for sites that don't run on one of these platforms, there's usually a way forward: a custom CMS integration, sometimes already available off the shelf — one of the more interesting traits of open-source software in general. That said, it's worth remembering that deploying Varnish Cache on a server does require some specialized skill.
Conclusion
Need support with Varnish Cache or have any other IT need? Get in touch with TN Solutions: we provide dedicated virtualization support and performance tuning — reach out with no obligation and we'll walk you through what you need.
Related reading
- Nginx: what it is and how it works, the tool most often paired with (or used instead of) Varnish to serve static content and act as a reverse proxy.
- Introduction to Server Monitoring, for a broader picture of keeping the server that runs Varnish healthy and fast.
- Choosing the Right RAID Level, if your real performance bottleneck is storage rather than application-level caching.
- SSH and OpenSSH server, for secure remote management of the server Varnish runs on.
Related Service: Linux Infrastructure for Varnish
Varnish is typically installed on a dedicated Linux virtual server: our system support service handles installation, VCL tuning and ongoing performance monitoring.
Frequently asked questions
How does Varnish Cache work with dynamic CMS platforms like WordPress or Magento without serving stale content?
Varnish can be configured to bypass the cache for logged-in users and to selectively invalidate pages whenever content is published or updated. It's often paired with plugins or webhooks that send PURGE or BAN commands to the Varnish backend.
What's the difference between PURGE, BAN and TTL in Varnish?
PURGE removes one specific cached object by its exact URL. BAN is more flexible: it flags content as "banned" using rules (for example by path or header) and only replaces it on the next request. TTL sets how long an object stays valid, which is useful when content has a predictable expiry. BAN is the better choice when invalidating groups of related content without a heavy load.
How do you handle caching for multilingual or multi-region content?
You need to vary the cache based on the Accept-Language header or on cookies/geolocation, defining custom rules in VCL. Alternatively, you can use a different subdomain or path per language/region and hash the cache separately for each.
Is Varnish compatible with HTTPS?
Not natively: Varnish doesn't handle SSL/TLS connections directly. To use it with HTTPS, it needs to sit behind a TLS terminator such as Nginx or HAProxy, which accepts the HTTPS request, decrypts it, and forwards it to Varnish over plain HTTP.
How do you prevent cache-miss-based DDoS attacks?
With rate limiting and rules that block or throttle suspicious requests. Dynamic counters in VCL, combined with backend health checks, let you spot abnormal behaviour and respond accordingly — for example by returning an HTTP 429.
Can you monitor in real time what's being cached or discarded?
Yes: tools like Varnishlog, Varnishncsa and Varnishstat provide detailed logs of hits/misses, headers and VCL decisions. In production it's worth pairing Varnish with Grafana and Prometheus for a full dashboard.
How do you avoid caching dynamic content like shopping carts or user areas by mistake?
You need to exclude from the cache any request that carries session cookies, authentication headers or dynamic query strings. In VCL you can write conditions that bypass the cache for these requests, serving the content directly from the backend instead.
Technology partners
Want to discuss it with our team?
We analyse your infrastructure for free and propose the most suitable solution.