Minor Description
The design purpose of the code is that only the leader broker can determine the preferred read-replica.
readFromLocalLog()
....
// If we are the leader, determine the preferred read-replica
val preferredReadReplica = clientMetadata.flatMap(
metadata => findPreferredReadReplica(partition, metadata, replicaId, fetchInfo.fetchOffset, fetchTimeMs))
But in fact, since the broker does not judge whether it is the leader or not, the follower will also execute the preferred read-replica selection.
partition.leaderReplicaIdOpt.flatMap { leaderReplicaId =>
// Don't look up preferred for follower fetches via normal replication and
if (Request.isValidBrokerId(replicaId))
None
else {
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