Add worker

Author: davidbarratt

README.md

@@ -14,3 +14,27 @@ Despite this limitation, an index can be built using [Workers](https://developer
   <source media="(prefers-color-scheme: light)" srcset="./cache-tag-light.svg">
   <img alt="Architecture Diagram" src="./cache-tag-light.svg">
 </picture>
+
+This application is broken up into three [Workers](https://developers.cloudflare.com/workers/), three
+[Queues](https://developers.cloudflare.com/queues/), and one [D1](https://developers.cloudflare.com/d1/) database.
+
+### Workers
+
+#### [Watcher](./workers/watcher/)
+
+This worker watches requests to the Cloudflare Cache / Origin, captures the tags, and sends them to the
+[Controller](./workers/controller/) in order to be persisted.
+
+> [!IMPORTANT]
+> By the time a response from an origin reaches a [Worker](https://developers.cloudflare.com/workers/), Cloudflare
+> has already swallowed the `Cache-Tag` header and it is not longer available. To get around this, the worker reads the
+> custom `X-Cache-Tag` header instead.
+
+The worker also exposes a `/.cloudflare/purge` endpoint that allows tags to be purged. This endpoint matches the
+[interface of the Cloudflare endpoint](https://developers.cloudflare.com/api/operations/zone-purge#purge-cached-content-by-tag-host-or-prefix), but only allows `tags`. The tags that are purged will be scoped
+to the zone in which the request is made too. For example, a purge request to `https://example.com/.cloudflare/purge`
+would only purge resources from the `example.com` zone.
+
+- [Watcher](./workers/watcher/)
+- [Controller](./workers/controller/)
+- [Handler](./workers/handler/)