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03a266d73b
This patch implements RAFT extension that allows to perform linearisable reads by accessing local state machine. The extension is described in section 6.4 of the PhD. To sum it up to perform a read barrier on a follower it needs to asks a leader the last committed index that it knows about. The leader must make sure that it is still a leader before answering by communicating with a quorum. When follower gets the index back it waits for it to be applied and by that completes read_barrier invocation. The patch adds three new RPC: read_barrier, read_barrier_reply and execute_read_barrier_on_leader. The last one is the one a follower uses to ask a leader about safe index it can read. First two are used by a leader to communicate with a quorum.