removeNode reduces the required quorum size, so some pending entries may
be able to commit after it is applied.
Discovered in cockroachdb/cockroach#3642
We need to be able to force an election (on one node) after creating a
new group (cockroachdb/cockroach#1384), but it is difficult to ensure
that our call to Campaign does not race with an election that may be
started by raft itself. A redundant call to Campaign should be a no-op
instead of a panic. (But the panic in becomeCandidate remains, because
we don't want to update the term or change the committed index in this
case)
etcd is going to support incremental snapshot, and we design to let it
send at most one snapshot out at first stage. So when one snapshot is in
flight, snapshot request will return error.
When failing to get snapshot when sending MsgSnap, raft prints out
related log and abort sending this message.
sendApp accesses the storage several times. Perviously, we
assume that the storage will not be modified during the read
opeartions. The assumption is not true since the storage can
be compacted between the read operations. If a compaction
causes a read entries error, we should not painc. Instead, we
can simply retry the sendApp logic until succeed.
Each progress has a inflighs sliding window. When the progress
is in replicate state, inflights will control the sending speed
of the leader.
The leader can have at most maxInflight number of inflight
messages for each replicate progress. Receving a appResp moves
forward the sliding window. Heartbeat response free one
slot if the window is full.
limit the max size of entries sent per message.
Lower the cost at probing state as we limit the size per message;
lower the penalty when aggressively decrease to a too low next.
Follower should not reject the append message with a smaller index than its commit
index. Or it will trigger the leader's resending logic, which might have a high cost.
raft relies on the link layer to report the status of the sent snapshot.
If the snapshot is still sending, the replication to that remote peer will
be paused. If the snapshot finish sending, the replication will begin
optimistically after electionTimeout. If the snapshot fails, raft will
try to resend it.
This addresses a problem that comes up in the cockroach tests,
in which the order of messages may lead to deadlocks (due to
the fact that we don't have regular heartbeat timers in most
of our tests).
