mobilizon/docs/contribute/activity_pub.md

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# Federation
## ActivityPub
Mobilizon uses [ActivityPub](http://activitypub.rocks/) to federate content between instances. It only supports the server-to-server part of [the ActivityPub spec](https://www.w3.org/TR/activitypub/).
It implements the [HTTP signatures spec](https://tools.ietf.org/html/draft-cavage-http-signatures-12) for authentication of inbox deliveries, but doesn't implement Linked Data Signatures for forwarded payloads, and instead fetches content when needed.
To match usernames to actors, Mobilizon uses [WebFinger](https://tools.ietf.org/html/rfc7033).
## Instance subscriptions
Instances subscribe to each other through an internal actor named `relay@instance.tld` that publishes (through `Announce`) every created content to it's followers. Each content creation share is saved so that updates and deletes are correctly sent to every relay subscriber.
## Activities
Supported Activity | Supported Object
------------ | -------------
`Accept` | `Follow`, `Join`
`Announce` | `Object`
`Create` | `Note`, `Event`
`Delete` | `Object`
`Flag` | `Object`
`Follow` | `Object`
`Join` | `Event`
`Reject` | `Follow`, `Join`
`Remove` | `Note`, `Event`
`Undo` | `Announce`, `Follow`
`Update` | `Object`
## Extensions
### Event
The vocabulary for Event is based on [the Event object in ActivityStreams](https://www.w3.org/TR/activitystreams-vocabulary/#dfn-event), extended with :
* the [Event Schema](https://schema.org/Event) from Schema.org
* some properties from [iCalendar](https://tools.ietf.org/html/rfc5545), such as `ical:status` (see [this issue](https://framagit.org/framasoft/mobilizon/issues/320))
The following properties are added.
#### repliesModeration
Disabling replies is [an ongoing issue with ActivityPub](https://github.com/w3c/activitypub/issues/319) so we use a temporary property.
See [the corresponding issue](https://framagit.org/framasoft/mobilizon/issues/321).
Accepted values: `allow_all`, `closed`, `moderated` (not used at the moment)
!!! info
We also support PeerTube's `commentEnabled` property as a fallback. It is set to `true` only when `repliesModeration` is equal to `allow_all`.
Example:
```json
{
"@context": [
"...",
{
"mz": "https://joinmobilizon.org/ns#",
"pt": "https://joinpeertube.org/ns#",
"repliesModerationOption": {
"@id": "mz:repliesModerationOption",
"@type": "mz:repliesModerationOptionType"
},
"repliesModerationOptionType": {
"@id": "mz:repliesModerationOptionType",
"@type": "rdfs:Class"
},
"commentsEnabled": {
"@id": "pt:commentsEnabled",
"@type": "sc:Boolean"
}
}
],
"...": "...",
"repliesModerationOption": "allow_all",
"commentsEnabled": true,
"type": "Event",
"url": "http://mobilizon1.com/events/8cf76e9f-c426-4912-9cd6-c7030b969611"
}
```
#### joinMode
Indicator of how new members may be able to join.
See [the corresponding issue](https://framagit.org/framasoft/mobilizon/issues/321).
Accepted values: `free`, `restricted`, `invite` (not used at the moment)
Example:
```json
{
"@context": [
"...",
{
"mz": "https://joinmobilizon.org/ns#",
"joinMode": {
"@id": "mz:joinMode",
"@type": "mz:joinModeType"
},
"joinModeType": {
"@id": "mz:joinModeType",
"@type": "rdfs:Class"
}
}
],
"...": "...",
"joinMode": "restricted",
"type": "Event",
"url": "http://mobilizon1.com/events/8cf76e9f-c426-4912-9cd6-c7030b969611"
}
```
#### location
We use Schema.org's `location` property on `Event`.
[The ActivityStream vocabulary to represent places](https://www.w3.org/TR/activitystreams-vocabulary/#places) is quite limited so instead of just using `Place` from ActivityStreams we also add a few properties from Schema.org's `Place` vocabulary.
We add [an `address` property](https://schema.org/address), which we assume to be [of `PostalAddress` type](https://schema.org/PostalAddress).
```json
{
"@context": [
"...",
{
"PostalAddress": "sc:PostalAddress",
"address": {
"@id": "sc:address",
"@type": "sc:PostalAddress"
},
"addressCountry": "sc:addressCountry",
"addressLocality": "sc:addressLocality",
"addressRegion": "sc:addressRegion",
"postalCode": "sc:postalCode",
"sc": "http://schema.org#",
"streetAddress": "sc:streetAddress",
}
],
"id": "http://mobilizon2.com/events/945f350d-a3e6-4bcd-9bf2-0bd2e4d353c5",
"location": {
"address": {
"addressCountry": "France",
"addressLocality": "Lyon",
"addressRegion": "Auvergne-Rhône-Alpes",
"postalCode": "69007",
"streetAddress": "10 Rue Jangot",
"type": "PostalAddress"
},
"latitude": 4.8425657,
"longitude": 45.7517141,
"id": "http://mobilizon2.com/address/bdf7fb53-7177-46f3-8fb3-93c25a802522",
"name": "10 Rue Jangot",
"type": "Place"
},
"type": "Event"
}
```
### Join
#### participationMessage
We add a `participationMessage` property on a `Join` activity so that participants may transmit a note to event organizers, to motivate their participation when event participations are manually approved. This field is restricted to plain text.
```json
{
"type": "Join",
"object": "http://mobilizon.test/events/some-uuid",
"id": "http://mobilizon2.test/@admin/join/event/1",
"actor": "http://mobilizon2.test/@admin",
"participationMessage": "I want to join !",
"@context": [
{
"participationMessage": {
"@id": "mz:participationMessage",
"@type": "sc:Text"
}
}
]
}
```