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Add option to accept any snapshot that allows replication
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Prior to this commit, the leader would not take into account snapshots
reported by a follower unless they matched or exceeded the tracked
PendingSnapshot index (which is the leader's last index at the time of
requesting the snapshot). This is too inflexible: the leader should take
into account any snapshot that reconnects the follower to its log. This
PR adds a config option ResumeReplicateBelowPendingSnapshot that enables
this behavior.

In doing so, it addresses long-standing problems that we've encountered
in CockroachDB. Unless you create the snapshot immediately and locally
when raft emits an MsgSnap, it's difficult/impossible to later
synthesize a snapshot at the requested index. It is possible to get one
above the requested index which raft always accepted, but CockroachDB
delegates snapshots to followers who might be behind on applying the
log, and it is awkward to have to wait for log application to send the
snapshot just to satisfy an overly strict condition in raft.
Additionally, CockroachDB also sends snapshots preemptively when adding
a new replica since there are qualitative differences between an initial
snapshot and one needed to reconnect to the log and one does not want to
wait for raft to round-trip to the follower to realize that a snapshot
is needed. In this case, the sent snapshot is commonly behind the
PendingSnapshot since the leader transitions the follower into
StateProbe when a snapshot is already in flight.

Touches cockroachdb/cockroach#84242.
Touches cockroachdb/cockroach#87553.
Touches cockroachdb/cockroach#87554.
Touches cockroachdb/cockroach#97971.
Touches cockroachdb/cockroach#114349.
See also https://github.com/cockroachdb/cockroach/blob/2b91c3829270eb512c5380201c26a3d838fc567a/pkg/kv/kvserver/raft_snapshot_queue.go#L131-L143.

Signed-off-by: Erik Grinaker <[email protected]>
Signed-off-by: Tobias Grieger <[email protected]>
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@tbg @erikgrinaker
tbg authored and erikgrinaker committed Nov 17, 2023
1 parent 27f6ebc commit cc1e858
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Showing 3 changed files with 66 additions and 38 deletions.
66 changes: 39 additions & 27 deletions raft.go
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Original file line number Diff line number Diff line change
Expand Up @@ -283,6 +283,22 @@ type Config struct {
// This behavior will become unconditional in the future. See:
// https://github.com/etcd-io/raft/issues/83
StepDownOnRemoval bool

// ResumeReplicateBelowPendingSnapshot allows the leader to resume replication
// to a follower (moving it to StateReplicate) if the follower is currently in
// StateSnapshot and we receive a MsgAppResp from it below the PendingSnapshot
// index but within the leader's log. In other words, the follower may apply a
// snapshot below the leader's PendingSnapshot, and the leader will resume
// replication as long as it connects the follower to the leader's log.
//
// Consider that complex systems may delegate the sending of snapshots to
// alternative datasources (i.e. not the leader). In such setups, it is
// difficult to manufacture a snapshot at a particular index requested by raft
// and the actual index may be ahead or behind. This should be okay, as long
// as the snapshot allows replication to resume.
//
// TODO(erikgrinaker): Consider making this the default behavior.
ResumeReplicateBelowPendingSnapshot bool
}

func (c *Config) validate() error {
Expand Down Expand Up @@ -413,9 +429,10 @@ type raft struct {
// randomizedElectionTimeout is a random number between
// [electiontimeout, 2 * electiontimeout - 1]. It gets reset
// when raft changes its state to follower or candidate.
randomizedElectionTimeout int
disableProposalForwarding bool
stepDownOnRemoval bool
randomizedElectionTimeout int
disableProposalForwarding bool
stepDownOnRemoval bool
resumeReplicateBelowPendingSnapshot bool

tick func()
step stepFunc
Expand All @@ -440,22 +457,23 @@ func newRaft(c *Config) *raft {
}

r := &raft{
id: c.ID,
lead: None,
isLearner: false,
raftLog: raftlog,
maxMsgSize: entryEncodingSize(c.MaxSizePerMsg),
maxUncommittedSize: entryPayloadSize(c.MaxUncommittedEntriesSize),
prs: tracker.MakeProgressTracker(c.MaxInflightMsgs, c.MaxInflightBytes),
electionTimeout: c.ElectionTick,
heartbeatTimeout: c.HeartbeatTick,
logger: c.Logger,
checkQuorum: c.CheckQuorum,
preVote: c.PreVote,
readOnly: newReadOnly(c.ReadOnlyOption),
disableProposalForwarding: c.DisableProposalForwarding,
disableConfChangeValidation: c.DisableConfChangeValidation,
stepDownOnRemoval: c.StepDownOnRemoval,
id: c.ID,
lead: None,
isLearner: false,
raftLog: raftlog,
maxMsgSize: entryEncodingSize(c.MaxSizePerMsg),
maxUncommittedSize: entryPayloadSize(c.MaxUncommittedEntriesSize),
prs: tracker.MakeProgressTracker(c.MaxInflightMsgs, c.MaxInflightBytes),
electionTimeout: c.ElectionTick,
heartbeatTimeout: c.HeartbeatTick,
logger: c.Logger,
checkQuorum: c.CheckQuorum,
preVote: c.PreVote,
readOnly: newReadOnly(c.ReadOnlyOption),
disableProposalForwarding: c.DisableProposalForwarding,
disableConfChangeValidation: c.DisableConfChangeValidation,
stepDownOnRemoval: c.StepDownOnRemoval,
resumeReplicateBelowPendingSnapshot: c.ResumeReplicateBelowPendingSnapshot,
}

cfg, prs, err := confchange.Restore(confchange.Changer{
Expand Down Expand Up @@ -1478,10 +1496,8 @@ func stepLeader(r *raft, m pb.Message) error {
switch {
case pr.State == tracker.StateProbe:
pr.BecomeReplicate()
case pr.State == tracker.StateSnapshot && pr.Match >= pr.PendingSnapshot:
// TODO(tbg): we should also enter this branch if a snapshot is
// received that is below pr.PendingSnapshot but which makes it
// possible to use the log again.
case pr.State == tracker.StateSnapshot && (pr.Match >= pr.PendingSnapshot ||
r.resumeReplicateBelowPendingSnapshot && pr.Match >= max(0, r.raftLog.firstIndex()-1)):
r.logger.Debugf("%x recovered from needing snapshot, resumed sending replication messages to %x [%s]", r.id, m.From, pr)
// Transition back to replicating state via probing state
// (which takes the snapshot into account). If we didn't
Expand Down Expand Up @@ -1560,10 +1576,6 @@ func stepLeader(r *raft, m pb.Message) error {
if pr.State != tracker.StateSnapshot {
return nil
}
// TODO(tbg): this code is very similar to the snapshot handling in
// MsgAppResp above. In fact, the code there is more correct than the
// code here and should likely be updated to match (or even better, the
// logic pulled into a newly created Progress state machine handler).
if !m.Reject {
pr.BecomeProbe()
r.logger.Debugf("%x snapshot succeeded, resumed sending replication messages to %x [%s]", r.id, m.From, pr)
Expand Down
16 changes: 10 additions & 6 deletions testdata/snapshot_succeed_via_app_resp_behind.txt
View file
Original file line number Diff line number Diff line change
@@ -1,8 +1,7 @@
# This is a variant of snapshot_succeed_via_app_resp in which the snapshot
# that is being sent is behind the PendingSnapshot index tracked by the leader.
#
# In this case, the leader keeps the follower in StateSnapshot even though it
# could move it back to StateReplicate.
# The test verifies that the leader will move the follower back to StateReplicate
# regardless, since it is able to catch up the follower from the snapshot's index.

# Turn off output during the setup of the test.
log-level none
Expand Down Expand Up @@ -123,11 +122,16 @@ stabilize 1
----
> 1 receiving messages
3->1 MsgAppResp Term:1 Log:0/10
DEBUG 1 recovered from needing snapshot, resumed sending replication messages to 3 [StateSnapshot match=10 next=11 paused pendingSnap=12]
> 1 handling Ready
Ready MustSync=false:
Messages:
1->3 MsgApp Term:1 Log:1/10 Commit:12 Entries:[1/11 EntryNormal "", 1/12 EntryNormal "\"foo\""]

# 3 is still pending a snapshot, even though its match is 10 which
# the leader could catch the follower up from.
# 3 is in StateReplicate thanks to receiving the snapshot at index 10.
# This is despite its PendingSnapshot having been 12.
status 1
----
1: StateReplicate match=12 next=13
2: StateReplicate match=12 next=13
3: StateSnapshot match=10 next=11 paused pendingSnap=12
3: StateReplicate match=10 next=13 inflight=1
22 changes: 17 additions & 5 deletions tracker/progress.go
View file
Original file line number Diff line number Diff line change
Expand Up @@ -42,11 +42,23 @@ type Progress struct {
// before and stops sending any replication message.
State StateType

// PendingSnapshot is used in StateSnapshot.
// If there is a pending snapshot, the pendingSnapshot will be set to the
// index of the snapshot. If pendingSnapshot is set, the replication process of
// this Progress will be paused. raft will not resend snapshot until the pending one
// is reported to be failed.
// PendingSnapshot is used in StateSnapshot and tracks the last index of the
// leader at the time at which it realized a snapshot was necessary. This
// matches the index in the MsgSnap message emitted from raft.
//
// While there is a pending snapshot, replication to the follower is paused.
// The follower will transition back to StateReplicate if the leader
// receives an MsgAppResp from it at or above PendingSnapshot, or if
// ResumeReplicateBelowPendingSnapshot is enabled, one that reconnects the follower to the
// leader's log (such an MsgAppResp is emitted when the follower applies a
// snapshot).
//
// The follower will transition to StateProbe if ReportSnapshot is called on
// the leader; if SnapshotFinish is passed then PendingSnapshot becomes the
// basis for the next attempt to append. In practice, the first mechanism is
// the one that is relevant in most cases. However, if this MsgAppResp is
// lost (fallible network) then the second mechanism ensures that in this
// case the follower does not erroneously remain in StateSnapshot.
PendingSnapshot uint64

// RecentActive is true if the progress is recently active. Receiving any messages
Expand Down

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