CF_Transforms_Cache_Warmer

Pre-warms Cloudflare image transform caches,

Crawls any site for /cdn-cgi/image/ variants and heats AVIF + WebP at the edge before visitors arrive.

Why

Cloudflare generates each /cdn-cgi/image/... variant on first request (cold), so whichever real visitor hits it first pays the generation latency. srcset makes this worse: a browser only ever fetches the ONE variant matching its viewport, so simply loading pages never warms them all. This tool crawls the deployed HTML/CSS for every declared variant and requests each in both AVIF and WebP ahead of time, moving the cold cost off the visitor path.

Billing note: Cloudflare counts a unique transformation once per billing period; cache hits after that are free, so re-warming an already-counted variant costs nothing.

Build

Requires Go 1.21+. Produces a fully static binary with zero runtime dependencies (no Go needed on the target host):

CGO_ENABLED=0 go build -o cache-warmer .

Run

./cache-warmer                 # use defaults below
DOMAIN=https://example.com WEBROOT=/var/www/html ./cache-warmer

Env var	Default	Purpose
DOMAIN	https://example.com	Public origin to warm through
WEBROOT	/var/www/html	Deployed site directory to crawl
UA	CF-Transforms-Cache-Warmer/1.0	User-Agent (easy to allowlist in CF WAF)
MAX_TIME	20	Per-request timeout (seconds)

Exit code is non-zero if anything failed, so cron mail/logs flag it. Output includes the CF-Cache-Status of every request (HIT / MISS / EXPIRED / REVALIDATED) and a summary.

Install as a cron job

install.sh installs the binary to /usr/local/bin and registers a cron entry. Every config value resolves in this order: environment variable (used silently) → interactive prompt → built-in default.

sudo ./install.sh                                                 # interactive
sudo DOMAIN=https://example.com WEBROOT=/var/www/html ./install.sh  # non-interactive
./install.sh --dry-run                                            # print the plan, change nothing

Default schedule: 6am on the 1st of each month, logging to /var/log/warm-cache.log. The entry carries its config inline and ends with a # cache-warmer marker, so re-running install.sh replaces the old entry instead of duplicating it.

How it works

1. Walks WEBROOT for *.html / *.css files and extracts every unique /cdn-cgi/image/... path (same regex as grep -rhoE).
2. For each path, issues two requests against DOMAIN: one with an AVIF-preferring Accept header, one WebP-only — because format=auto keys the Cloudflare cache on that header.
3. Reports status code, CF-Cache-Status, and elapsed time per request; exits non-zero on any failure.