Varnish – VCL, cache tagging, ESI, invalidation, DDEV setup

Magento’s built-in Full Page Cache stores pages in files or Redis. It works, but it handles requests inside PHP – nginx and PHP-FPM still have to start for every request. Varnish sits in front of nginx and serves pages from memory without touching PHP. For a shop with traffic that is the difference between a second and milliseconds.

How Varnish works with Magento 2

Stack without Varnish: Browser – nginx – PHP-FPM – Magento – Redis/file cache

With Varnish: Browser – Varnish – (cache HIT: immediate response) or (cache MISS: nginx – PHP-FPM – Magento)

Varnish holds pages in RAM. A cache HIT is literally a few milliseconds – Varnish fires no PHP process. A cache MISS goes to the backend (nginx + PHP), which generates the page and hands it to Varnish for caching.

Configuring Magento for Varnish

# Enable Varnish as the FPC engine
bin/magento config:set system/full_page_cache/caching_application 2

# Generate the VCL configuration file for Varnish
bin/magento varnish:vcl:generate \
    --backend-host=nginx \
    --backend-port=80 \
    --export-version=6 \
    --output-file=/etc/varnish/magento.vcl

VCL basics – how Varnish makes decisions

# Fragment of magento.vcl - simplified for readability

sub vcl_recv {
    # Do not cache requests from logged-in customers
    if (req.http.cookie ~ "PHPSESSID" && req.http.cookie ~ "customer_logged_in") {
        return (pass);
    }

    # Do not cache POST requests
    if (req.method == "POST") {
        return (pass);
    }

    # Normalise URL - remove tracking parameters
    if (req.url ~ "(\?|&)(utm_source|utm_medium|utm_campaign|gclid)=") {
        set req.url = regsuball(req.url, "&(utm_source|utm_medium|utm_campaign|gclid)=[^&]+", "");
    }

    return (hash);
}

sub vcl_backend_response {
    # Cache product page for 1 hour
    if (bereq.url ~ "^/catalog/product/view") {
        set beresp.ttl = 1h;
    }
    # Home page - 5 minutes
    if (bereq.url == "/") {
        set beresp.ttl = 5m;
    }
}

Cache tagging and invalidation

Magento tags every page with an X-Magento-Tags header containing identifiers of the entities used on that page. When a product changes, Magento sends a PURGE request to Varnish with that product’s tag – Varnish removes from cache all pages containing that tag:

<?php

namespace Vendor\Module\Model;

use Magento\CacheInvalidate\Model\PurgeCache;
use Magento\Framework\App\Cache\Tag\Resolver;

class ProductCacheInvalidator
{
    public function __construct(
        private PurgeCache $purgeCache,
        private Resolver $tagResolver
    ) {}

    public function invalidateProduct(\Magento\Catalog\Model\Product $product): void
    {
        $tags = $this->tagResolver->getTags($product);
        $this->purgeCache->sendPurgeRequest(implode('|', $tags));
    }
}

ESI – dynamic blocks on cached pages

Problem: the product page is cached, but the “Recently viewed” block is per-user. ESI (Edge Side Includes) solves this through separate caching of blocks:

<!-- Varnish replaces this tag with a separate backend request -->
<esi:include src="/page_cache/block/esi/blocks/" />

Varnish in DDEV

# .ddev/docker-compose.varnish.yaml
version: '3.6'
services:
  varnish:
    image: varnish:6.4
    ports:
      - "6081:6081"
    volumes:
      - ./magento.vcl:/etc/varnish/default.vcl
    environment:
      - VARNISH_SIZE=256m
    depends_on: [web]
    labels:
      com.ddev.site-name: ${DDEV_SITENAME}

# Varnish statistics
varnishstat

# Real-time logs
varnishlog -q "ReqURL ~ '/catalog/product'"

# Manual PURGE of entire cache
curl -X PURGE http://localhost:6081/.*

# Check if a page came from Varnish
curl -I https://magento2-dev.ddev.site/ | grep X-Cache
# X-Cache: HIT or MISS

Summary

Varnish is one of the biggest performance improvements you can make for a Magento shop under load. A cache HIT eliminates PHP from handling the request entirely. The key is a good understanding of VCL and the tagging mechanism – then you can precisely control what is cached and for how long, without the risk of serving stale data.