Post a federated reply comment as a user of another ActivityPub server, not the episode’s comment server

A client-to-server (or c2s) ActivityPub interaction

In this scenario, the podcast client app facilitates logging in as an authenticated user of an ActivityPub server that is not the server specified in the RSS feed.

​ Federated Pleroma example (replying to Mastodon)

• Fetch the root comment url specific in the item's podcast:socialInteract tag (or a subcomment url) using the ActivityPub Accept header
  • Note the "id" and "conversation" of the response
• Collect the user's desired login server from your UI
• Determine if the login server is a Pleroma instance using Fetch instance Open external link
  • It's Pleroma if the response succeeds, returns an instance Open external link payload as json, and the "version" contains Pleroma
• Register your client app with the Pleroma server using Create an application Open external link
  • Use a minimal OAuth scope such as: read:accounts write:statuses
  • Only needs to be done once per server
• Compute the OAuth login url using Authorize a user Open external link
  • After logging in, the user will be redirected to the callback you specified
• Use the code returned to Obtain an Access Token Open external link
• Verify the Access Token using Verify account credentials Open external link
  • If valid, you'll receive an Account Open external link payload
  • You can now publish statuses on behalf of the logged-in user
  • Note the Account "url"
• Fetch the Account url using the ActivityPub Accept header
  • Will return an ActivityPub Person object
  • Note the Person "outbox" url
• Post the reply comment to the user's outbox url using the ActivityPub standard Create Activity API Open external link
  • Pleroma requires a wrapped activity (as shown below), it does not support automatic wrapping
  • Pleroma requires a "published" date
  • Use the "id" of the comment or subcomment above for "inReplyTo"
  • Pleroma will take care of federating the comment to the target Mastodon server
  • Mastodon requires "conversation" to be set properly in order to show the federated comment in the reply chain

POST https://pleroma-example.com/users/the-user/outbox
Authorization: Bearer <access-token>

{
    "@context": "https://www.w3.org/ns/activitystreams",
    "type": "Create",
    "to": [
        "https://www.w3.org/ns/activitystreams#Public"
        "https://mastodon-example.com/users/a-user"
    ],
    "object": {
        "type": "Note",
        "to": [
        "https://www.w3.org/ns/activitystreams#Public",
        "https://mastodon-example.com/users/a-user"
        ],
        "conversation": "<conversation-from-comment-above>",
        "published": "2021-12-15T18:46:37.830Z",
        "content": "This is the reply comment!",
        "inReplyTo": "<id-from-comment-above>"
    }
}