varden

The caching proxy that slashes latency from seconds to microseconds starts with an often‑misspelled four‑letter word: varden.

When our team first mumbled "varden" during a 3 a m on‑call session, I assumed someone had spun up yet another internally‑named microservice. Within minutes the chat was flooded with laughing emojis and a link to Varnish Cache's official documentation. What began as a typo became the nickname for our custom edge‑caching layer-a Varnish‑based system that today absorbs over 120 000 requests per second for a global mobile back end. Behind the joking name, Varden taught us fundamentals of HTTP caching that every senior engineer should know.

This article is a behind‑the‑scenes look at the architecture - configuration language. And operational lessons we accumulated while scaling Varden. Treat it as both a Varnish‑centric deep dive and a case study in how a playful misspelling can anchor an entire caching strategy. By the end, you'll understand VCL, ESI, TTL tuning. And the subtle plumbing that keeps a distributed reverse proxy reliable-even when your Slack channel calls it by the wrong name.

What Exactly Is Varden (and Why Varnish Still Matters)?

Varden is our internal code name for a deployment of Varnish Cache, an open‑source HTTP reverse proxy that accelerates web applications by storing responses in memory. The name stuck after a junior engineer mistyped "varnish" into a Jira ticket,, and and the joke gained tractionUnder the hood, however, Varden is entirely serious: it's a layer‑7 gateway that sits between our load balancers and backend microservices, applying caching logic defined in Varnish Configuration Language (VCL).

Critically, Varden exploits two often‑underused aspects of HTTP: the caching directives in RFC 7234 and the power of server‑side composition via Edge Side Includes (ESI). While many engineers reach for general‑purpose proxies like NGINX or HAProxy for caching, Varnish was purpose‑built for this task. Varden's architecture leans on that specialization, yielding sub‑millisecond cache‑hit latencies that directly improved our mobile app's perceived responsiveness.

The Internal Architecture That Makes Varden Blazing Fast

Varden employs Varnish's classic two‑process model: a manager process that compiles VCL to C code, and a child process that executes requests. When VCL is loaded, the manager translates it into a shared object, forks the child. And the child runs a tight event‑driven loop on top of kqueue or epoll. This avoids the threading overhead common in other proxies and lets Varden handle tens of thousands of concurrent connections on modest hardware.

We tuned the startup parameters through trial and error. For our workload-mostly small JSON responses from a mobile API-we allocated 4 GB of malloc storage and set workspace_client to 64 kB to accommodate large headers from some poorly‑behaved device SDKs. One lesson: under‑allocating workspaces led to cryptic "workspace overflow" errors that were hard to debug without enabling VSL (Varnish Shared Log) in raw mode. We now mandate that every Varden instance ships with VSL streaming to a central observability stack.

VCL: The Configuration Language That Powers Varden's Caching Logic

VCL is a domain‑specific language that hooks into Varnish's request‑processing finite state machine. In Varden's repository, you'll find around 300 lines of custom VCL, structured as a series of subroutines like vcl_recv, vcl_backend_response, vcl_deliver. Each subroutine gives you a chance to inspect headers, rewrite URLs, or alter caching behaviour before the request moves to the next state.

Here's a taste of what we run in production, simplified for readability:

  sub vcl_recv { if (req method == "PURGE") { if (, and clientip ~ purgers) { return (synth(405, "Not allowed")); } return (purge); } if (req url ~ "^/api/v1/real-time") { return (pass); } unset req, and httpCookie; }  

The snippet illustrates two Varden‑specific decisions: we use a dedicated ACL for purge requests (so Content‑Management‑System updates instantly invalidate caches). And we short‑circuit caching for a real‑time data endpoint where freshness beats speed. This combination of selective purging and smart pass rules keeps the cache hit ratio above 94% while ensuring zero‑stale‑data complaints from product owners.

Key Caching Strategies: From Time‑Based TTL to Event‑Driven Invalidation

Varden leans heavily on the beresp ttl directive in vcl_backend_response to set time‑to‑live. For static assets, we push TTLs to 30 days; for semi‑dynamic API responses, we drop to 60 seconds. The magic lies in combining TTL with beresp grace-a feature that lets Varden serve stale content while asynchronously revalidating in the background. We set a grace of 10 minutes for most endpoints. Which shielded our backends during a flash‑sale traffic spike that tripled load in 90 seconds.

But time‑based eviction alone isn't enough. Varden also implements event‑driven invalidation via vcl_purge and Varnish's ban mechanism. Whenever our CMS publishes a new article, a webhook fires a curl