FEP-171b: Conversation Containers

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Summary

This document specifies a model for managing conversations in ActivityPub network. It is based on the implementation of Conversation Containers in Streams.

In this model conversations are represented as collections controlled by a single actor. Such conversations take place within a specific audience and may be moderated.

Requirements

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC-2119.

Containers

A conversation container is implemented as a collection. Every item in that collection is an Add activity where object is another activity (such as Create, Update, Delete, Like, Dislike, EmojiReact or Announce). The conversation owner distributes Add activities to other participants, thus keeping their views of the conversation synchronized.

Container collection

  • Collection type MUST be OrderedCollection.
  • Collection items MUST be in chronological order.
  • Collection MUST have an attributedTo property containing the id of the conversation owner.
  • Collection SHOULD have collectionOf property with value Activity.

[!NOTE] The required collection type might be changed to a more descriptive one (such as ConversationContainer) in a future version of this document. That will allow identification of Add activities belonging to a conversation container by the value of Add.target.type.

[!NOTE] In addition to the conversation container collection, implementers MAY provide collection that represents the conversation tree and contains posts.

Adding activities to a conversation

Conversation owner can add any activity to the conversation. However, if a context property is present on the activity, its value SHOULD be identical to the ID of a conversation container.

When activity is added to the conversation, its owner sends an Add activity to the conversation audience (usually defined by a top-level post).

Add activities published by the conversation owner MUST have a target property containing a partial object:

  • type: OrderedCollection
  • id: the id of the conversation container.
  • attributedTo: the id of the collection owner.

Add activities published by the conversation owner MUST be added to the conversation container collection.

[!NOTE] The "conversation outbox" model where the target of Add activity is a collection of Add activities is not compatible with ActivityStreams definition of Add activity, according to which Add activity indicates that the actor has added the object to the target.

Top-level post

The author of a top-level post it not necessarily the conversation owner. When owner is a group, conversations can be started by any of its members.

The top-level post MUST have a contextHistory property that refers to the conversation container. The presence of this property indicates that publisher implements conversation containers.

[!NOTE] In addtion to contextHistory property, implementers MAY add a context property that refers to a collection of posts.

Interactions

All activities in a conversation SHOULD only be delivered to the conversation owner.

Conversation participants SHOULD reject conversation activities that have not been added to the conversation by its owner.

The audience of a reply MUST be copied from a conversation root.

Reply with a different audience can be created by starting a new conversation and including a FEP-e232 link to the post instead of specifying it in inReplyTo.

Moderation

When conversation owner does not want to add activity to a conversation, that activity is ignored and a corresponding Add activity is not published.

To remove a previously approved post from a conversation, its owner publishes a Delete activity where object is the post that must be removed. This activity is then wrapped in Add activity and distributed to the conversation audience.

[!NOTE] Actor shouldn't be able to delete objects it didn't create. In a future version of this document Delete might be replaced with Remove(target: Thread).

Backfilling

Conversation participants can retrieve missing activities by reading the conversation container collection.

Authentication

When an ActivityPub server receives an Add activity in its inbox, it MUST perform the authentication procedure according to FEP-fe34:

  • If Add.object is not embedded, fetch it. If location of the fetched activity has the same origin as Add.object, add it to the conversation.
  • If Add.object is embedded, check whether Add.actor and Add.object.actor have the same origin.
  • If origins are equal, add Add.object to the conversation.
  • If origins are different and FEP-8b32 integrity proof is present, verify the proof. If the proof is valid, add Add.object to the conversation.
  • If integrity proof is not present, fetch Add.object by its id. If location of the fetched activity has the same origin as Add.object.id, add fetched activity to the conversation.

The processing of unauthenticated embedded activities is strongly discouraged. If such activities are not rejected by the consumer, a malicious conversation owner may be able to perform a cache poisoning attack and overwrite any actor or a post in consumer's local cache by sending a forged Update(Actor) or Update(Object) wrapped in an Add activity.

[!WARNING] Sometimes activities have non-dereferenceable identifiers. That may prevent their authentication.

例

Example of an Add activity for a reply to a followers-only post:

{
  "@context": [
    "https://www.w3.org/ns/activitystreams"
  ],
  "type": "Add",
  "id": "https://alice.example/activities/add/1",
  "actor": "https://alice.example/actors/1",
  "object": {
    "@context": [
      "https://www.w3.org/ns/activitystreams"
    ],
    "type": "Create",
    "id": "https://bob.example/activities/create/1",
    "actor": "https://bob.example/actors/1",
    "context": "https://alice.example/contexts/1",
    "object": {
      "@context": [
        "https://www.w3.org/ns/activitystreams"
      ],
      "type": "Note",
      "id": "https://bob.example/posts/1",
      "inReplyTo": "https://alice.example/posts/1",
      "content": "This is a reply",
      "to": [
        "https://alice.example/actors/1",
        "https://alice.example/actors/1/followers"
      ]
    },
    "to": [
      "https://alice.example/actors/1",
      "https://alice.example/actors/1/followers"
    ]
  },
  "target": {
    "type": "OrderedCollection",
    "id": "https://alice.example/contexts/1",
    "attributedTo": "https://alice.example/actors/1"
  },
  "to": [
    "https://bob.example/actors/1",
    "https://alice.example/actors/1/followers"
  ]
}

Example of a container of a followers-only conversation:

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://w3id.org/fep/171b"
  ],
  "type": "OrderedCollection",
  "id": "https://alice.example/contexts/1",
  "attributedTo": "https://alice.example/actors/1",
  "collectionOf": "Activity",
  "orderedItems": [
    "https://alice.example/activities/add/1"
  ]
}

Comparison with other proposals

  • FEP-400e: The object of Add is an object, not an activity, and conversation collection contains added objects. Reject(Create) activity is generated for rejected posts. Conversation participants are expected to add a target property to posts.
  • FEP-1b12: Announce activity is used instead of Add. Conversation and related activities are synchronized between participants, but conversation backfilling mechanism is not specified.
  • GoToSocial Interaction Policy: conversation is managed separately for each post (in a conversation container the owner has authority over the entire thread). Accept or Reject activity is generated for every interaction (in a conversation container Add activity is generated when activity is approved, and rejected activities are ignored). Conversation is not synchronized between participants, but can be backfilled by recursively fetching replies collections.

実装

参考文献

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