# Other Protocols How Versia's concepts map to ActivityPub and Matrix, for readers who know one of them. This page is for readers who already know [ActivityPub](https://www.w3.org/TR/activitypub/) or the [Matrix server-server API](https://spec.matrix.org/latest/server-server-api/). It maps Versia's concepts to theirs and explains where they differ. It is informative and sets no requirements: the linked pages define the protocol. Some ActivityPub practice comes from Mastodon and other implementations rather than from the ActivityPub specification. This page says so where it matters. ## Concepts | Versia | ActivityPub | Matrix | | -------------------------------------------------- | ---------------------------------------------- | ---------------------------------- | | [Instance](/entities/instance-metadata) | Server | Homeserver | | [Entity](/entities) | Object or Activity | Event | | [Reference](/entities#reference) | `id`, a URL | Event ID | | [User address](/entities/user#addresses) | WebFinger address, by convention | User ID | | [Inbox](/federation#inboxes), one per instance | Inbox per actor, optional shared inbox | Transactions | | [Signature](/signatures) by the instance key | HTTP Signatures by an actor key, by convention | `X-Matrix` header, by a server key | | [Extension](/extensions) names | JSON-LD contexts | Namespaced event types | | [Subscribe](/entities/subscribe) | Follow | None | | [Changing an entity](/entities#changing-an-entity) | Update | Event replacements | | [Delete](/entities/delete) | Delete and Tombstone | Redaction | | [Collection](/structures/collection) | Collection | None | | [Versions](/federation/versions) | None | Spec and endpoint versions | | [Discovery](/federation/discovery) | WebFinger, by convention | Well-known file | | [Versia Links](/links) | None | `matrix:` URIs and matrix.to links | ## No Activity Wrapper ActivityPub delivers activities, such as a `Create` that wraps a `Note`. Versia sends the entity itself: an instance posts a Note by [sending the Note](/federation#inboxes) to the inbox. Actions that have no object to send are their own entities, such as [Subscribe](/entities/subscribe) and [Delete](/entities/delete). To [change an entity](/entities#changing-an-entity), an instance sends the whole entity again with the same `id`. This matches ActivityPub's server-to-server `Update`, which replaces the whole object. ## Plain JSON and Namespaces Versia is plain JSON, like Matrix, and not JSON-LD. Extensions are named by the [reverse domain](/extensions#naming) of their author, such as `pub.versia:likes`, and `pub.versia` is reserved for this specification. Matrix does the same with event types, such as `com.example.myapp.event`, with `m.` reserved for its specification. A receiver [ignores unknown fields](/validation#unknown-fields-and-types). It answers an entity of an unknown extension type with a `2xx` and drops it. ## Instance Keys ActivityPub does not define authentication. In practice, servers sign requests with HTTP Signatures, using a key that belongs to an actor. Versia signs requests and responses between instances with [one key per instance](/signatures), published in its [InstanceMetadata](/entities/instance-metadata). This is closest to Matrix, where a homeserver signs requests with its own key. ## Signed Messages, Not Signed Entities Matrix signs each event, so a server can check an event that another server passes on. Versia signs the HTTP message, not the entity. An entity's `author` [must belong to the instance that signed the message](/validation#authors-and-signers), so an instance cannot pass on another instance's entities. This is why, in [Groups](/extensions/groups), the group's instance tells members' instances to fetch a Note from its author's instance instead of forwarding it. ## Subscriptions [Subscribe](/entities/subscribe), [SubscribeAccept](/entities/subscribe-accept), [SubscribeReject](/entities/subscribe-reject) and [Unsubscribe](/entities/unsubscribe) play the roles of ActivityPub's `Follow`, `Accept`, `Reject` and `Undo` of a `Follow`. A Subscribe can target any entity type that accepts subscriptions, such as a [Group](/extensions/groups), not only a User. Matrix has no subscription to a user. Users join rooms, and every server in a room holds a copy of it. A Versia Group lives on one instance instead, which tells members' instances what is posted to it. ## Posts Versia's core has no post type. Posts are Notes from the [Social extension](/extensions/social), which most other extensions build on. In ActivityPub, `Note` is part of the ActivityStreams vocabulary, and in Matrix, messages are `m.room.message` events. | Versia | ActivityPub | | ---------------------------------------------------------- | ------------------------------------------------- | | [Note](/extensions/social#note) | `Note` | | [Notes collection](/extensions/social#notes-collection) | Outbox | | [Featured collection](/extensions/social#user-collections) | `featured`, by Mastodon convention | | Note `mentions` | `Mention` tags | | Note `subject` and `is_sensitive` | `summary` and `sensitive`, by Mastodon convention | | [Share](/extensions/share) | `Announce` | | [Like and Dislike](/extensions/likes) | `Like` and `Dislike` | | [Reactions](/extensions/reactions) | None in ActivityStreams | | [Polls](/extensions/polls) | `Question` | | [Migration](/extensions/migration) | `Move` |