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An impact report from the front-lines of nostr client development.
Negentropy
NIP-77, Negentropy Syncing
A set reconciliation protocol for finding which records each side is missing without exchanging every ID.
Negentropy is an existing set reconciliation protocol. It lets two sides compare large sets efficiently, learn which IDs each side has that the other side needs, and then transfer only the missing records. Nostr uses it for syncing.
In Nostr, NIP-77 brings Negentropy into the protocol so a client and relay can compare the events they have within a chosen filter and sync whatever is missing.
This makes sync cheaper when both sides already share much of the same history. It is useful for client-relay sync, relay-relay sync, and other cases where bandwidth matters.