The suggested pattern for Raft proposals is that they be retried
periodically until they succeed. This turns out to be an issue
when a leader cannot commit entries because the leader will continue
to append re-proposed entries to its log without committing anything.
This can result in the uncommitted tail of a leader's log growing
without bound until it is able to commit entries.
This change add a safeguard to protect against this case where a
leader's log can grow without bound during loss of quorum scenarios.
It does so by introducing a new, optional ``MaxUncommittedEntriesSize
configuration. This config limits the max aggregate size of uncommitted
entries that may be appended to a leader's log. Once this limit
is exceeded, proposals will begin to return ErrProposalDropped
errors.
See cockroachdb/cockroach#27772
In #9982, a mechanism to limit the size of `CommittedEntries` was
introduced. The way this mechanism worked was that it would load
applicable entries (passing the max size hint) and would emit a
`HardState` whose commit index was truncated to match the limitation
applied to the entries. Unfortunately, this was subtly incorrect
when the user-provided `Entries` implementation didn't exactly
match what Raft uses internally. Depending on whether a `Node` or
a `RawNode` was used, this would either lead to regressing the
HardState's commit index or outright forgetting to apply entries,
respectively.
Asking implementers to precisely match the Raft size limitation
semantics was considered but looks like a bad idea as it puts
correctness squarely in the hands of downstream users. Instead, this
PR removes the truncation of `HardState` when limiting is active
and tracks the applied index separately. This removes the old
paradigm (that the previous code tried to work around) that the
client will always apply all the way to the commit index, which
isn't true when commit entries are paginated.
See [1] for more on the discovery of this bug (CockroachDB's
implementation of `Entries` returns one more entry than Raft's when the
size limit hits).
[1]: https://github.com/cockroachdb/cockroach/issues/28918#issuecomment-418174448
The MaxSizePerMsg setting is now used to limit the size of
Ready.CommittedEntries. This prevents out-of-memory errors if the raft
log has become very large and commits all at once.
Scanning the uncommitted portion of the raft log to determine whether
there are any pending config changes can be expensive. In
cockroachdb/cockroach#18601, we've seen that a new leader can spend so
much time scanning its log post-election that it fails to send
its first heartbeats in time to prevent a second election from
starting immediately.
Instead of tracking whether a pending config change exists with a
boolean, this commit tracks the latest log index at which a pending
config change *could* exist. This is a less expensive solution to
the problem, and the impact of false positives should be minimal since
a newly-elected leader should be able to quickly commit the tail of
its log.
TestNodeTick relies on a unreliable func `waitForSchedule` when running
with GOMAXPROCS > 1. This commit changes the test to make sure we stop
the node afte it drains the tick chan. The test should be reliable now.
Getting gosimple suggestion while running test script, so this PR is for fixing gosimple S1019 check.
raft/node_test.go:456:40: should use make([]raftpb.Entry, 1) instead (S1019)
raft/node_test.go:457:49: should use make([]raftpb.Entry, 1) instead (S1019)
raft/node_test.go:458:43: should use make([]raftpb.Message, 1) instead (S1019)
Refer https://github.com/dominikh/go-tools/blob/master/cmd/gosimple/README.md#checks for more information.
n.Tick() is async. It can be racy when running with n.Stop().
n.Status() is sync and has a feedback mechnism internally. So there wont be
any race between n.Status() and n.Stop() call.
ForceGosched() performs bad when GOMAXPROCS>1. When GOMAXPROCS=1, it
could promise that other goroutines run long enough
because it always yield the processor to other goroutines. But it cannot
yield processor to goroutine running on other processors. So when
GOMAXPROCS>1, the yield may finish when goroutine on the other
processor just runs for little time.
Here is a test to confirm the case:
```
package main
import (
"fmt"
"runtime"
"testing"
)
func ForceGosched() {
// possibility enough to sched up to 10 go routines.
for i := 0; i < 10000; i++ {
runtime.Gosched()
}
}
var d int
func loop(c chan struct{}) {
for {
select {
case <-c:
for i := 0; i < 1000; i++ {
fmt.Sprintf("come to time %d", i)
}
d++
}
}
}
func TestLoop(t *testing.T) {
c := make(chan struct{}, 1)
go loop(c)
c <- struct{}{}
ForceGosched()
if d != 1 {
t.Fatal("d is not incremented")
}
}
```
`go test -v -race` runs well, but `GOMAXPROCS=2 go test -v -race` fails.
Change the functionality to waiting for schedule to happen.
raft node should set initial prev hard state to empty.
Or it will not send the first hard coded state to application
until the state changes again.
This commit fixs the issue. It introduce a small overhead, that
the same tate might send to application twice when restarting.
But this is fine.
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.
Nathan VanBenschoten