Add another worker

Author: davidbarratt

README.md

@@ -1,6 +1,6 @@
 # Cache Tag
 
-Cloudflare has the ability to index cached resources by _tag_ which allows those resources be
+[Cloudflare](https://www.cloudflare.com/) has the ability to index cached resources by _tag_ which allows those resources be
 [purged by tag](https://developers.cloudflare.com/cache/how-to/purge-cache/purge-by-tags/). However, this feature is
 [only available for Enterprise customers](https://developers.cloudflare.com/cache/how-to/purge-cache/purge-by-tags/#:~:text=Note%20that%20Tag%2C%20Hostname%20and%20Prefix%20purges%20are%20only%20available%20for%20Enterprise%20customers.).
 
@@ -22,22 +22,44 @@ This application is broken up into three [Workers](https://developers.cloudflare
 
 #### [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.
+This worker watches requests to the Cloudflare [Cache](https://developers.cloudflare.com/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
+> has already swallowed the `Cache-Tag` header and it is no 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.
+to the [zone](https://developers.cloudflare.com/fundamentals/setup/accounts-and-zones/#zones) 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](https://developers.cloudflare.com/fundamentals/setup/accounts-and-zones/#zones).
 
 #### [Controller](./workers/controller/)
 
-_@todo_
+A [Worker](https://developers.cloudflare.com/workers/) is an
+[account-level resource](https://developers.cloudflare.com/fundamentals/setup/accounts-and-zones/#accounts), but
+[Cache](https://developers.cloudflare.com/cache/) is a
+[zone-level resource](https://developers.cloudflare.com/fundamentals/setup/accounts-and-zones/#zones). Because of this,
+there is no way to know what zone a resource is being cached in from a
+[Worker](https://developers.cloudflare.com/workers/).
+
+To mitigate this problem, we can leverage the
+[CF-Worker](https://developers.cloudflare.com/fundamentals/reference/http-request-headers/#cf-worker) which gets added
+to outbound requests from a [Worker](https://developers.cloudflare.com/workers/). Unfortunately this header does not
+exist when using [Service Bindings](https://developers.cloudflare.com/workers/runtime-apis/bindings/service-bindings/).
+The only way to retrieve the header is by making a request to the [Worker](https://developers.cloudflare.com/workers/)
+on the provided [workers.dev subdomain](https://developers.cloudflare.com/workers/configuration/routing/workers-dev/).
+
+The [Controller](./workers/controller/) exists primarily as an intermediary between [Watcher](./workers/watcher/) and [Handler](./workers/handler/) to collect zone information. It is **not** included as a part of [Handler](./workers/handler/) in order to ensure that the [Worker](https://developers.cloudflare.com/workers/) is collocated in the
+[same data center](https://developers.cloudflare.com/workers/configuration/smart-placement/) as
+[Watcher](./workers/watcher/).
+
+The worker also exposes a `/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`. Since no
+[zone](https://developers.cloudflare.com/fundamentals/setup/accounts-and-zones/#zones) information is provided, matching
+resources from **all** zones will be purged.
 
 #### [Handler](./workers/handler/)