Delivery of public content to sharedInbox

[evanp]

ActivityPub says in section 7.1.3:

Additionally, if an object is addressed to the Public special collection, a server MAY deliver that object to all known sharedInbox endpoints on the network.

This can have the effect that servers which implement sharedInbox endpoints to reduce traffic will get much, much more traffic from public posts of popular servers.

The mechanism for allowing delivery of an activity to all addressees on a single server should be separate from the mechanism for receiving all public activities from a remote server. Combining the two takes a network optimization and make it into a firehose; the exact opposite effect than intended.

[evanp]

I think the best next step here is to define an endpoint that is only for delivering to all addressees at once. Implementers that don't want to receive a firehose to sharedInbox can skip it, and implement to the multipleAddresseeInbox instead.

[github-actions[bot]]

This issue has been labelled as potentially needing a FEP, and contributors are welcome to submit a FEP on the topic. Note that issues may be closed without the FEP being created; that does not mean that the FEP is no longer needed.

[evanp]

FEP 0499 provides a similar mechanism to the one described.

[trwnh]

There are some related paths forward here:

• Recommend against servers implementing the "deliver to all known sharedInbox endpoints" behavior. This effectively makes an errata from MAY to SHOULD NOT.
• Recommend against publishing a sharedInbox endpoint if a server is not willing or able to handle additional traffic. This is essentially keeping the spec behavior as-is, but giving guidance on dealing with its effects.
  • Recommend a new endpoint like https://w3id.org/fep/0499/multibox which allows for delivering to multiple inboxes in the same request
  • Or, recommend a different architecture where the server is an actor with its own inbox and it "owns" or "manages" its users, where activities sent to this singular inbox get routed internally to those users based on certain criteria. (Most likely, this would require a mechanism to signal that users are being hosted by a service.)