> the ActivityPub spec is pretty cagey on retries ("[federated servers] SHOULD additionally retry delivery to recipients if it fails due to network error"). Who knows if the existing servers implement some strategy, but the spec has no guidance AFAIK. Also, is a recipient server being down/erroring a network error? Also, nothing says POSTs have to be sent immediately.
I don't know much about ActivityPub; do you know whether updates from individual topics / subscriptions contain sequence numbers? If a recipient server receives updates 12, 13, and 14 on topic ABC, but then has a network blip and drops update number 15-17, and then successfully receives 18, it could know that it's missing some (and presumably poll for the dropped ones).
Likewise, if a receiving server notices that it hasn't received any updates on one topic for a week when before it had been receiving multiple per day, it could proactively poll the sending server.
I don't know much about ActivityPub; do you know whether updates from individual topics / subscriptions contain sequence numbers? If a recipient server receives updates 12, 13, and 14 on topic ABC, but then has a network blip and drops update number 15-17, and then successfully receives 18, it could know that it's missing some (and presumably poll for the dropped ones).
Likewise, if a receiving server notices that it hasn't received any updates on one topic for a week when before it had been receiving multiple per day, it could proactively poll the sending server.