On Mastodon, user profiles can be hosted either locally on the same website as yours, or remotely on a completely different website. The same username may be used on a different domain. Therefore, a Mastodon user's full mention consists of both the username and the domain, in the form `@username@domain`. In practical terms, `@user@example.com` is not the same as `@user@example.org`. If the domain is not included, Mastodon will try to find a local user named `@username`. However, in order to deliver to someone over ActivityPub, the `@username@domain` mention is not enough -- **mentions must be translated to an HTTPS URI first**, so that the remote actor's inbox and outbox can be found.
Enter WebFinger. WebFinger as described in [RFC 7033](https://tools.ietf.org/html/rfc7033) is a spec that defines **a method for resolving links to a resource**, given only a URI on a particular server. This allows anyone to look up where a resource is located without having to know its exact location beforehand; for example, by email or phone number. This lookup is directed at the endpoint `/.well-known/webfinger`, and a `resource` query parameter is passed along with the lookup. The resource URI used with Mastodon is the `acct:` URI as described in [RFC 7565](https://tools.ietf.org/html/rfc7565), with the username of a profile that is hosted on a particular domain.
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**Because Mastodon heavily relies on mentions for addressing other profiles, WebFinger is required for fully interoperating with Mastodon.** Users can generally load profiles by searching for the direct HTTPS URI if they know it, or for the `username@domain` address, but Mastodon's internal logic depends almost completely on `acct`: URIs or `username@domain` representations. Searching for any objects or profiles from an ActivityPub implementation without WebFinger will fail because the author cannot be converted to a user in the local database.
You can parse this JSON response to find a link with your desired type. For ActivityPub `id`, we are interested in finding `application/activity+json` specifically.
This way, we have translated `@Gargron@mastodon.social` to `https://mastodon.social/users/Gargron` and we can now interact over ActivityPub by referring to this URI as `id` where appropriate.
Note in the above example that `social.example` does not use the same URI structure as Mastodon. Thus, we cannot guess the actor `id` given only the username and domain. However, if `social.example` supports WebFinger, then we can get this `id` by requesting `https://social.example/.well-known/webfinger?resource=acct:username@social.example`and parsing the response for a link with the `application/activity+json` type.
