Background: I lately deployed a tiny little app to Cloudflare Workers to render and serve social media images. It’s nothing very fancy — just a little Rust/Wasm project that uses ril to render an image and push it up to Backblaze B2, unless it has already been rendered. I was a bit confused to discover that I was serving a lot of duplicate requests. My own basic caching strategy of “is this already in B2?” worked fine, but it was asking that question a lot more than I expected given it was running on Cloudflare.

A tiny bit of debugging — read: inspecting the response headers — told me that Cloudflare wasn’t caching it at all: there was no CF-Cache-Status header. That was pretty surprising to me, given it was sitting on a Cloudflare Worker, but it turns out Cloudflare does not automatically cache any request to a Worker unless you explicitly ask it to. The idea is (and had I read the docs end to end I would have known this!) that you are responsible to use the Cache API yourself!

It was a whopping six lines of code to add, thankfully:

let cache = Cache::default();
let cache_key = request.uri().to_string();
if let Some(resp) = cache.get(&cache_key, false).await? {
    return Ok(resp);
}

// Check B2 and rebuild the social media image if need be…

cache.put(&cache_key, response).await?;

Thenceforth everything Just Worked™ the way I expected it to in the first place!

I’ll probably say more later about Cloudflare Workers, but in the meantime, I hope this is helpful to someone else out there!