ActivityPub -> Announce -> Post
2023/01/23 (487 words)

I have been investigating how ActivityPub is meant to work. What follows is how a boost/announce/retweet works in Mastodon and any other system that implements against ActivityPub.

You can find the full collection on github https://github.com/boyter/activitypub and contribute your own details.

ActivityPub -> Announce -> Post

When a user clicks on bonk/boost/retweet the following sequence happens.

The users intent is converted to an announce. This announce is sent to every follower they have as well as the owner of the post that the user is boosting.

In the case of follower instances, they receive the announce, then fetch the content of the announce using the object field.

In the case of the owner, they take the announce and increment their boost count.

┌────────────┐ ┌────────────┐ ┌───────────────┐ ┌────────────────┐        
│    User    │ │  Instance  │ │Remote Instance│ │ Owner Instance │        
└────────────┘ └────────────┘ └───────────────┘ └────────────────┘        
      │              │               │                  │                 
      │              │               │                  │                 
      │              │               │                  │                 
      │───Announce──▶│               │                  │                 
      │              │               │                  │                 
      │              │───────┐       │                  │                 
      │              │       │       │                  │                 
      │              │Fetch Followers│                  │                 
      │              │       │       │                  │                 
      │              │◀──────┘       │                  │                 
      │              │               │                  │                 
      │              │  Announce to  │                  │                 
      │              │──Follower(s)─▶│───Fetch Outbox──▶│                 
      │              │               │                  │                 
      │              │               │◀─Update Content──│                 
      │              │               │                  │                 
      │              │               │                  │                 
      │              │───────Announce to Owner─────────▶│                 
      │              │               │                  │ ───────┐        
      │              │               │                  │        │        
      │              │               │                  │ Increment Count 
      │              │               │                  │        │        
      │              │               │                  │ ◀──────┘        
      │              │               │                  │                 
      │              │               │                  │

An example announce appears like the below. Actor is whoever did the boost, cc is the user who’s post is being boosted and object contains a link to the post itself. The below assumes the instance honk.boyter.org was boosting a post from another instance named some.instance in this case.

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "actor": "https://honk.boyter.org/u/boyter",
  "cc": [
    "https://some.instance/users/someonemadup"
  ],
  "context": "tag:mastodon.social,2023-01-17:objectId=380216002:objectType=Conversation",
  "id": "https://honk.boyter.org/u/boyter/bonk/234wetsdfw35345",
  "object": "https://some.instance/users/someonemadup/statuses/1231235634623412",
  "published": "2023-01-19T22:41:49Z",
  "to": "https://www.w3.org/ns/activitystreams#Public",
  "type": "Announce"
}

When fetching content for the above every remote instance needs to call back to the owner instance in order to fetch the content for display. Instances can require a signed request for this to ensure only followers can see the post, and return 404 generally

The fetch is done using the object value, with the appropriate header sent.

curl --location --request GET 'https://some.instance/users/someuser/statuses/123413513412312' --header 'Accept: application/json'

Note that the header should be set to application/activity+json or application/ld+json; profile="https://www.w3.org/ns/activitystreams to be even more precise, but that some instance implementations such as honk will ignore application/json for this and not return JSON. This seems to vary by instant type.

It is vital that the owner instance caches the result for these GET requests as popular content could be queried many times before a copy being made on all the remote instances.