Subscribe

• Subscriber: The user who subscribes. If Joe subscribes to Alice, Joe is the subscriber.
• Target: The entity subscribed to. If Joe subscribes to Alice, Alice is the target.

Only entity types whose page says they accept subscriptions can be a target. In the core, that is the User. An extension can make its own types accept subscriptions, as Groups does, and states on its page what their subscribers receive.

Definition

Subscribe has no author. It MUST be sent by the subscriber’s instance.

Checking the Sender

None of the four subscription entities has an author. When a receiver checks Authors and Signers, it MUST treat subscriber as the author of a Subscribe or an Unsubscribe, and target as the author of a SubscribeAccept or a SubscribeReject.

target_type MUST be the type of the entity that target points to. A receiver that holds that entity MUST reject a subscription entity whose target_type does not match it.

{
    "type": "Subscribe",
    "created_at": "2021-01-01T00:00:00Z",
    "subscriber": "example.org:033wv9k6n0007",
    "target": "example.com:033qtdbdkj006",
    "target_type": "User"
}

Behavior

Joe, a user on joe.example, wants to subscribe to Alice, a user on alice.example.

Sending a Subscription Request

To start a subscription, joe.example MUST do the following:

1. Create a Subscribe entity with Joe as the subscriber and Alice as the target.
2. Send the Subscribe entity to alice.example’s inbox.
3. Treat the subscription as “processing” until a SubscribeAccept or SubscribeReject arrives, or until Joe sends an Unsubscribe.

A Subscribe whose target_type does not accept subscriptions MUST be rejected.

Accepting the Subscription Request

alice.example MUST answer every Subscribe with a SubscribeAccept or a SubscribeReject, even if manually_approves_subscribers is false. If Joe is already subscribed to Alice and sends another Subscribe, alice.example MUST send a SubscribeAccept again, in case the first was lost.

The answer can wait for Alice to decide. There is no time limit on a pending Subscribe.

To accept the subscription request, alice.example MUST do the following:

1. Create a SubscribeAccept entity with Joe as the subscriber and Alice as the target.
2. Send the SubscribeAccept entity to joe.example’s inbox.

It then treats Joe as a subscriber of Alice.

Rejecting the Subscription Request

To reject the subscription request, alice.example MUST do the following:

1. Create a SubscribeReject entity with Joe as the subscriber and Alice as the target.
2. Send the SubscribeReject entity to joe.example’s inbox.

Cancelling the Subscription Request

To cancel a subscription request that has not been answered, joe.example sends an Unsubscribe.

Final Steps

joe.example MUST mark the subscription as “accepted” when it receives a SubscribeAccept, and remove it when it receives a SubscribeReject.

Delivering Entities

Which entities reach a subscriber is defined in Delivery, and by the page of each type that accepts subscriptions.

Ending the Subscription

Joe ends the subscription by sending an Unsubscribe. Alice ends it by sending a SubscribeReject, which removes Joe as a subscriber. An Unsubscribe is not used to remove a subscriber.

Flows

Every message here is sent to the inbox of the other user’s instance. A subscription request is either accepted:

Or rejected:

Once accepted, the subscription ends when Joe unsubscribes:

Or when Alice removes Joe as a subscriber: