documentation/Interoperability/architecture design proposal.md

Table of Contents

• Terms used
• Content related
• Actor related
• General
• Principals of the changes proposed to the ActivityPub plugin
• Goals
• Goals of the WordPress Event Federation project
• Tangential Objectives:
• Transformer Management
• Current situation
• Frontend
• Backend
• Proposal
• Frontend
• Backend
• Mapping: Actors and Dispatching (early draft)
• Current situation
• Possibilities
• What features does our Event Federation project need?
• Other features
• Proposed starting point for evaluating use cases
• Actor collisions
• Existing collisions
• New collisions
• Conclusion
• Appendix
• The approach of youtube-dl

This document outlines suggestions for the further development of the WordPress ActivityPub plugin regarding its interoperability with other WordPress plugins that could enhance ActivityPub functionality.

Terms used

The following terms are used:

Content related

• post type: post types and custom post types within WordPress
• object type: ActivityPub object type
• transformers: a piece of code that converts items of particular WordPress post types to an ActivityPub object. For example a transformer that can transform:
  • every post type to a Note
  • post or page to Note, Article, Image, Audio or Article depending on the post-format
  • tribe_events (Event post the of The Event Calendar) to Event
  • podcast (Podcast post of Podlove Podcast Publisher) to PodcastEpisode (as proposed and implemented by Castopod)

Actor related

• actors: ActivityPub Actor Objects
• actor types: ActivityPub Actor Types
• WordPress actor types: Things on WordPress that can be mapped to actors, e.g.:
  • WordPress users →Person
  • Whole Blog →Application or Group
  • Post types → e.g., Organization
• Actor mapper: Describes the mapping of WordPress actor types to any ActivityPub actors.

General

• publish/dispatch: Sending a Create, Update or Announce Activity.

Principals of the changes proposed to the ActivityPub plugin

The WordPress ActivityPub plugin

• should be aware (or even control) the whole chain from creating some content within WordPress to being published via ActivityPub. For sure, it should be the only plugin that directly sends and receives ActivityPub.
• should not get any more complex by default, the out-of-the box functionality/features and simplicity should be similar to version 1.0.0.
• offers other plugins the possibility to register custom transformers.
• (maybe) offers other plugins the possibility to register new WordPress actor types, e.g. the tribe_organizer post type of The Event Calendar.
• handles the controls collisions of actor names:
  • collisions that are already present during activation of a WordPress actor type on the settings page
  • collisions that happen afterwards (e.g., a new user registering with the same username as the blog-wide actor)
  • (maybe) show notifications/warnings when using an actor name which has been used and followed by in the past, as there may be issues due to caching of the old public key.
• maybe distinguishes between the default user interface and advanced sections which give the user more fine-grained control:
  • transformer management
  • actor management

Goals

Goals of the WordPress Event Federation project

The aim of the event federations project is to enable seamless federation of events created within an event plugin as an ActivityPub object with Type Event and accompanying meta-data, as typically seen in common ActivityPub implementations like Mobilizon. This will guarantee optimal compatibility between various services.

Tangential Objectives:

Nevertheless, we see a lot of other cases that can benefit, if the primary goals of the Event Federation project are achieved this using a modularized approach as proposed below. The following list indicates examples what might be those other benefits:

• Other types of content get federated in a more feature rich way:
  • PodcastEpisode as proposed by CastoPod
  • Question
  • Product
  • …
• ActivityPub offers features how to deal with multilingual content (contentMap - see Example 115) which could be implemented differently by multilingual and translation plugins (example search for the current use of ->set_content_map within the ActivityPub plugin).

Note: CastoPod, to give an example, has currently decided that podcast episodes will not be sent to followers as a Create activity directly containing the PodcastEpisode object, but will send a Note which is linking to the PodcastEpisode, because most Fediverse implementations currently ignore by default all object types they don't know. This is likely the same reason flohmarkt is currently also using the Note and not Product even though that one would be available in the official specification.

Transformer Management

Current situation

Frontend

The admin user interface in v1.0.0 lets one choose to which object type all post get transformed to. The options are:

• Note (default) — Should work with most platforms.
• Article — The presentation of the "Article" might change on different platforms.
• WordPress Post-Format — Maps the WordPress Post-Format to the ActivityPub Object Type.

Backend

Currently, all this functionality is handled via one hard-coded transformer in includes/transformer/class post.php (Source) and the actual logic that assigns the type is somewhere else.

/**
 * WordPress Post Transformer
 *
 * The Post Transformer is responsible for transforming a WP_Post object into different othe
 * Object-Types.
 *
 * Currently supported are:
 *
 * - Activitypub\Activity\Base_Object
 */
class Post {
    ...
    	/**
         * Transforms the WP_Post object to an ActivityPub Object
         *
         * @see \Activitypub\Activity\Base_Object
         *
         * @return \Activitypub\Activity\Base_Object The ActivityPub Object
         */
        public function to_object() {
    ...

Proposal

All publicly accessible WordPress post types can become active for ActivityPub federation when selecting a transformer available for the relevant post type. It is up to the transformer to determine the target ActivityPub object type, which may vary depending on variables such as post type or post format. If there is no specific transformer available for a given post type, it cannot be assigned through the user interface.

Frontend

	Note	Post-Format	The Events Calendar Transformer	Custom Event Transformer
post	X	O	-	-
page	O	O	-	-
tribe_events	O	-	X	O
custom_event	O	-	-	O
…	O	-	-	-

columns: Available transformers
rows: Public and non-password protected WordPress post types
Note: Only one or zero selections can be made in each row.

• For any custom post type, if there is a custom transformer registered, indicate that one should be selected as default (or select it right ahead?).
• Should not differ too much from the current view: Maybe completely hide the current activity object type part of the settings and only show the buttons for enable and disable and move the transformer table to an advanced settings page.
• Maybe make the tables more advanced and clearly show the origin of the transformer (built-in, etc.) and maybe a Pop-Up with a description, or even a link to a configuration page of the transformer.

Backend

A solution could be to define a transformer interface and let the transformers be implementations.

interface Transformer {
    public function get_supported_post_types(): array;
    public function get_transformer_info(): array;
    public function to_object(WP_Post $post);
    ....
}

Use WordPress's hook system or a public API function to let other plugins register their custom transformer implementations to the ActivityPub plugin.

Problem: With the design above the transformer does a lot and leaves a lot of responsibility to the developer implementing it.

Possible alternatives and solutions:

• The ActivityPub plugin could provide reusable Traits for common tasks.
• Provide useful functions, documentation, and examples for automatic testing of transformers.
• Highly unlikely: The ActivityPub plugin provides an even more high level framework for adding transformers. For example for events the ActivityPub plugin could provide a built-in transformer to the object type Event which only needs a mapping (might make things harder, instead of making them easier). See Appendix.
• Highly unlikely: don't use an interface with implementations, (miss)use class extensions.

Mapping: Actors and Dispatching (early draft)

More comprehensive actor management would benefit our project aims and potentially meet the needs of others in the future. Nevertheless, its importance is considerably lower than that of Transformer management. In the future, the following factors may become more important when larger websites should be using the ActivityPub plugin.

Current situation

Currently, the admin user interface in v1.0.0 gives very limited options.

- [ ] Enable `blog`-actor
- [ ] Enable `author`-actors

Note that depending on which options are enabled, the method of federating via ActivityPub varies significantly:

• If the blog-actor is enabled, but the author-actors are not, posts are attributed and created by the blog actor.
• If both options are enabled the blog-actor will announce (boost) the posts of the author-actors.

Currently, the whole logic controlling this is written within the Activity_Dispatcher class which is currently triggered only within the Scheduler class. Furthermore, although a blog actor is available, the ActivityPub plugin is still written in such a way that WordPress users are the default ActivityPub actors. This also reflects in things like the class which is responsible for the actor lookup is still called Users and yet handles the Blog and the Application user too, see the Source.

Possibilities

ActivityPub knows several actor types:

• Application
• Group
• Organization
• Person
• Service

The ActivityPub specification allows for immense flexibility in its application. As WordPress websites fulfill a variety of tasks and objectives, it is challenging to implement control features and options for actor management that are more capable without sacrificing user-friendliness.

What features does our Event Federation project need?

• For maximum Mobilizon compatibility we would love to have simply an actor of type Application, preferable called @relay@wordpress.site, that announces all events. Nonetheless, in theory, a Mobilizon instance should have the capability to follow any other actor as well. More or less this can already be achieved with the current blog-actor.
• Additionally, for example, organizers in The Event Calendar could also offer their own actor of type Organizer or Group to publicize events (getting referred to via attributedTo) if the author-actors are enabled, or create events (being the actor directly) if the author-actors are not enabled.
• Optional: As events are sometimes published a long time before they start, adding the possibility that events get announced again at a scheduled time before the start, e.g. by sending an Announce activity.

Other features

Other WordPress actor types might have valid use cases, like actors for categories, or specific post types in general: "I only want to see the blog updates, but I do not want to spam my timeline with each product they post on their website, even if they choose to federate them."

Problems:

• Overall complexity may lead to programming and usage errors.
• Naming collisions become more likely. See below.
• Who is the actor, who is the attributedTo, who is just sending an Announce of a post?

Proposed starting point for evaluating use cases

We keep in mind that every post shall only ever be sent as a Create Activity once, by one single actor. Therefore when the ActivityPub plugin makes use of a certain WordPress actor type, the WordPress actor type must exactly return one actor, which may depend on the current post being published and on the user doing the action (latter information should also be available within the WP_Post object).

User Interface:

	User	Event organizers	Product Relay	Relay(Blog)	Categories
post	C	-	-	A	A
page	C	-	-	A	O
tribe_events	O	C	-	A	0
product	O	-	C	0	0

x: Actor mapper y: WordPress post types
C: Create — can only be assigned once per row
A: Announce — can be assigned multiple times per row, but only if a Create has been assigned before

• Only allow setting something to Announce, when a Create is already set.
• Maybe forbid, that a user can automatically announce a post, because it's the "lowest level".

Actor collisions

Each ActivityPub actor must have a unique ID. In our case this is an HTTPS URI. But in reality webfinger is used: @actor-name@instance.tld. Actors like Persons and Groups and Applications have public keys attached to them, as well as their ID is probably cached by most software. So assigning some actor-name to something new also can cause unintended behavior.

If multiple plugins want to individually federate their content (like events, products, and blog posts or even the built-in WordPress categories), they must be able to have something like an actor API, the ActivityPub plugin can register. The ActivityPub plugin being the main coordinator is necessary because actors are globally unique.

• For existing (collision was created before the installation/activation of the plugin)
• For new collisions (collision was/will be created after the installation/activation of the plugin)

It seems like it is the best if collisions are avoided in the first place by encouraging the use of prefixes, like category_<category>.

Existing collisions

• Easy: Either we just tell the user to resolve all conflicts before a WordPress actor type can be activated at all
• Complex: We let him choose alternatives for conflicts.

New collisions

• Easy Forbid the saving of new users or posts that would conflict with an existing actor.
• Complex …

Conclusion

• Anyway it seems the question has to be raised whether the ActivityPub plugin should store a history of all past and currently mapped actors.
• Additionally, shouldn't the user have a detailed overview in each case if the actor management should get more complex?

Appendix

The approach of youtube-dl

youtube-dl is solving a similar problem with "extractors" that return data to the processing-chain of youtube-dl. They provide a base class that includes useful functions for common issues e.g. automatic testing, geo-bypassing, login/cookie/header management. The most basic extractor is about 40 lines of code. They can also chain extractors recursively, like a extractor for mastodon videos, that gets the video from the post and returns the source of the video, that could be YouTube, Vimeo or anything else youtube-dl supports.