Replica Comes Back Stale

Key cart:8812 on $N = 3$ replicas A, B and C, with $R = 2$ and $W = 2$ . Versions are stamped with a vector clock, and each write here was issued by the same client after reading the previous one, so v3 descends from v2, which descends from v1.

step	event	A	B	C
1	write `v1`, acked by A, B, C	`v1`	`v1`	`v1`
2	B unreachable; write `v2` acked by A, C	`v2`	`v1`	`v2`
3	B returns; write `v3` acked by A, B	`v3`	`v3`	`v2`
4	read at $R = 2$ reaches B and C	`v3`	`v3`	`v2`

What does the read return, and what happens to C?

It returns v2. B skipped the v2 write while it was down, so its copy is the one that cannot be trusted and C's is authoritative.

It returns v3, and on that same read the coordinator pushes v3 to C as read repair.

It returns v3, but C is left alone. Read repair is a scheduled background job, and closing the gap here is anti-entropy's work.

It returns v2 and v3 as siblings for the client to reconcile, because two replicas answered with different versions.