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801bf42ea2
Currently, we use storage_proxy/get_cas_shard -> sharder.shard_for_reads to decide which shard to use for LWT code execution on both replicas and the coordinator. If the coordinator is not a replica, shard_for_reads returns 0 — the 'default' shard. This behavior has at least two problems: * Shard 0 may become overloaded, because all LWT coordinators that are not replicas will be served on it. * The zero shard does not match shard_for_reads on replicas, which hinders the "same shard for client and server" RPC-level optimization. To fix this, we need to know whether the current node hosts a replica for the tablet corresponding to the given token. Currently, there is no API we could use for this. For historical reasons, sharder::shard_for_reads returns 0 when the node does not host the shard, which leads to ambiguity. This commit introduces try_get_shard_for_reads, which returns a disengaged std::optional when the tablet is not present on the local node. We leave shard_for_reads method in the base sharder class, it calls try_get_shard_for_reads and returns zero by default. We need to rename tablet_sharder private methods shard_for_reads and shard_for_writes so that they don't conflict with the sharder::shard_for_reads.