FA-75029 / CRDT convergence / Member archive

Causal delivery buffer: a redelivery of the newest applied message is buffered · case 04

A retransmitted copy of the latest message sits in the buffer forever and is not counted as a duplicate.

Member previewVariant 4 · 3 implementations · 10 checks per implementation

Case contract

A receiver starts with an empty delivered vector. An arrival [sender, vector, payload] is a duplicate when its sender entry is at or below the delivered entry or the same (sender, entry) is already buffered. Otherwise it is buffered; then, repeatedly, the oldest buffered message whose sender entry is exactly delivered+1 and whose other entries are all <= delivered is delivered, setting only the sender entry. Return delivered payloads, sorted pending payloads, duplicate count and the delivered vector.

Why this case matters

Operation-based CRDTs converge only if operations are delivered exactly once and in causal order.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
duplicate of a delivered message is discarded	`{"clock": [["a", 2]], "delivered": ["a1", "a2"], "duplicates": 3, "pending": ["a1"]}`	`{"clock": [["a", 2]], "delivered": ["a1", "a2"], "duplicates": 4, "pending": []}`	Failed

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