Best (or Various) Way(s) to Check if Your Users Belong to Collections in Addressing?

ActivityPub Server to Server
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Simply, how might one handle checking (to process) addressing (to, bto, cc, bcc)?

Handling specific Object IDs or Public addressing is, obviously, easy but what about collections?

If the types of collections were limited (say to only followers and following), I could see an easy way to verify quickly but, as far as I understand the spec., any collection technically qualifies for what can be put in the addressing attributes.

And you can’t (easily) just grab the collection to check if any of your users reside in said collection since that collection could be paginated.

And large collections mean, potentially, churning through a ton of pages just to check if you see any of your users.

And there’s no provided way to request the remote instance to see if some user is a part of one of the instance’s collections so the searching, I would thing, ultimately resides with you.

So…how can you know whether an Object is addressed to any of your users with the expectation that said addressing could contain any collection, whatsoever?

I feel like I’m missing something obvious but I can’t seem to reason my way out of this conundrum.

Thanks for any help!

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In Jejune the way I handle this is by shadowing the expanded collection in audience. The audience property then contains a list of every intended recipient.

Another efficient way would be to introduce a new activity, say Forward, which has the semantics of Announce, but the side effects of Create. Then you would just treat the collection itself as a mailbox, and it would forward along using Forward activities. That would require implementations to use something like LDSIgs though.

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But doesn’t that require me to rely on those who send the Activities to shadow the collection for me?

If a project which doesn’t shadow the collections in audience sends me an Activity, I’m right back in the same problem, right (assuming I’m understanding correctly)?

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Correct. This is not a completely resolved issue as yet.

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