raft: provide protection against unbounded Raft log growth

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

contrib/raftexample/raft.go

@@ -274,12 +274,13 @@ func (rc *raftNode) startRaft() {
 		rpeers[i] = raft.Peer{ID: uint64(i + 1)}
 	}
 	c := &raft.Config{
-		ID:              uint64(rc.id),
-		ElectionTick:    10,
-		HeartbeatTick:   1,
-		Storage:         rc.raftStorage,
-		MaxSizePerMsg:   1024 * 1024,
-		MaxInflightMsgs: 256,
+		ID:                        uint64(rc.id),
+		ElectionTick:              10,
+		HeartbeatTick:             1,
+		Storage:                   rc.raftStorage,
+		MaxSizePerMsg:             1024 * 1024,
+		MaxInflightMsgs:           256,
+		MaxUncommittedEntriesSize: 1 << 30,
 	}
 
 	if oldwal {

raft/README.md

@@ -41,6 +41,7 @@ This raft implementation also includes a few optional enhancements:
 - Writing to leader's disk in parallel
 - Internal proposal redirection from followers to leader
 - Automatic stepping down when the leader loses quorum
+- Protection against unbounded log growth when quorum is lost
 
 ## Notable Users
 

raft/node.go

@@ -401,6 +401,7 @@ func (n *node) run(r *raft) {
 
 			r.msgs = nil
 			r.readStates = nil
+			r.reduceUncommittedSize(rd.CommittedEntries)
 			advancec = n.advancec
 		case <-advancec:
 			if applyingToI != 0 {

raft/node_test.go

@@ -997,3 +997,57 @@ func TestNodeCommitPaginationAfterRestart(t *testing.T) {
 		)
 	}
 }
+
+// TestNodeBoundedLogGrowthWithPartition tests a scenario where a leader is
+// partitioned from a quorum of nodes. It verifies that the leader's log is
+// protected from unbounded growth even as new entries continue to be proposed.
+// This protection is provided by the MaxUncommittedEntriesSize configuration.
+func TestNodeBoundedLogGrowthWithPartition(t *testing.T) {
+	const maxEntries = 16
+	data := []byte("testdata")
+	testEntry := raftpb.Entry{Data: data}
+	maxEntrySize := uint64(maxEntries * testEntry.Size())
+
+	s := NewMemoryStorage()
+	cfg := newTestConfig(1, []uint64{1}, 10, 1, s)
+	cfg.MaxUncommittedEntriesSize = maxEntrySize
+	r := newRaft(cfg)
+	n := newNode()
+	go n.run(r)
+	defer n.Stop()
+	n.Campaign(context.TODO())
+
+	rd := readyWithTimeout(&n)
+	if len(rd.CommittedEntries) != 1 {
+		t.Fatalf("expected 1 (empty) entry, got %d", len(rd.CommittedEntries))
+	}
+	s.Append(rd.Entries)
+	n.Advance()
+
+	// Simulate a network partition while we make our proposals by never
+	// committing anything. These proposals should not cause the leader's
+	// log to grow indefinitely.
+	for i := 0; i < 1024; i++ {
+		n.Propose(context.TODO(), data)
+	}
+
+	// Check the size of leader's uncommitted log tail. It should not exceed the
+	// MaxUncommittedEntriesSize limit.
+	checkUncommitted := func(exp uint64) {
+		t.Helper()
+		if a := r.uncommittedSize; exp != a {
+			t.Fatalf("expected %d uncommitted entry bytes, found %d", exp, a)
+		}
+	}
+	checkUncommitted(maxEntrySize)
+
+	// Recover from the partition. The uncommitted tail of the Raft log should
+	// disappear as entries are committed.
+	rd = readyWithTimeout(&n)
+	if len(rd.CommittedEntries) != maxEntries {
+		t.Fatalf("expected %d entries, got %d", maxEntries, len(rd.CommittedEntries))
+	}
+	s.Append(rd.Entries)
+	n.Advance()
+	checkUncommitted(0)
+}

raft/raft.go

@@ -148,12 +148,17 @@ type Config struct {
 	// applied entries. This is a very application dependent configuration.
 	Applied uint64
 
-	// MaxSizePerMsg limits the max size of each append message. Smaller value
-	// lowers the raft recovery cost(initial probing and message lost during normal
-	// operation). On the other side, it might affect the throughput during normal
-	// replication. Note: math.MaxUint64 for unlimited, 0 for at most one entry per
-	// message.
+	// MaxSizePerMsg limits the max byte size of each append message. Smaller
+	// value lowers the raft recovery cost(initial probing and message lost
+	// during normal operation). On the other side, it might affect the
+	// throughput during normal replication. Note: math.MaxUint64 for unlimited,
+	// 0 for at most one entry per message.
 	MaxSizePerMsg uint64
+	// MaxUncommittedEntriesSize limits the aggregate byte size of the
+	// uncommitted entries that may be appended to a leader's log. Once this
+	// limit is exceeded, proposals will begin to return ErrProposalDropped
+	// errors. Note: 0 for no limit.
+	MaxUncommittedEntriesSize uint64
 	// MaxInflightMsgs limits the max number of in-flight append messages during
 	// optimistic replication phase. The application transportation layer usually
 	// has its own sending buffer over TCP/UDP. Setting MaxInflightMsgs to avoid
@@ -215,6 +220,12 @@ func (c *Config) validate() error {
 		return errors.New("storage cannot be nil")
 	}
 
+	if c.MaxUncommittedEntriesSize < 0 {
+		return errors.New("max uncommitted entry size cannot be less than 0")
+	} else if c.MaxUncommittedEntriesSize == 0 {
+		c.MaxUncommittedEntriesSize = noLimit
+	}
+
 	if c.MaxInflightMsgs <= 0 {
 		return errors.New("max inflight messages must be greater than 0")
 	}
@@ -241,11 +252,12 @@ type raft struct {
 	// the log
 	raftLog *raftLog
 
-	maxInflight int
-	maxMsgSize  uint64
-	prs         map[uint64]*Progress
-	learnerPrs  map[uint64]*Progress
-	matchBuf    uint64Slice
+	maxMsgSize         uint64
+	maxUncommittedSize uint64
+	maxInflight        int
+	prs                map[uint64]*Progress
+	learnerPrs         map[uint64]*Progress
+	matchBuf           uint64Slice
 
 	state StateType
 
@@ -268,6 +280,10 @@ type raft struct {
 	// be proposed if the leader's applied index is greater than this
 	// value.
 	pendingConfIndex uint64
+	// an estimate of the size of the uncommitted tail of the Raft log. Used to
+	// prevent unbounded log growth. Only maintained by the leader. Reset on
+	// term changes.
+	uncommittedSize uint64
 
 	readOnly *readOnly
 
@@ -326,6 +342,7 @@ func newRaft(c *Config) *raft {
 		raftLog:                   raftlog,
 		maxMsgSize:                c.MaxSizePerMsg,
 		maxInflight:               c.MaxInflightMsgs,
+		maxUncommittedSize:        c.MaxUncommittedEntriesSize,
 		prs:                       make(map[uint64]*Progress),
 		learnerPrs:                make(map[uint64]*Progress),
 		electionTimeout:           c.ElectionTick,
@@ -616,6 +633,7 @@ func (r *raft) reset(term uint64) {
 	})
 
 	r.pendingConfIndex = 0
+	r.uncommittedSize = 0
 	r.readOnly = newReadOnly(r.readOnly.option)
 }
 
@@ -954,6 +972,10 @@ func stepLeader(r *raft, m pb.Message) error {
 			r.logger.Debugf("%x [term %d] transfer leadership to %x is in progress; dropping proposal", r.id, r.Term, r.leadTransferee)
 			return ErrProposalDropped
 		}
+		if !r.increaseUncommittedSize(m.Entries) {
+			r.logger.Debugf("%x appending new entries to log would exceed uncommitted entry size limit; dropping proposal", r.id)
+			return ErrProposalDropped
+		}
 
 		for i, e := range m.Entries {
 			if e.Type == pb.EntryConfChange {
@@ -1462,6 +1484,49 @@ func (r *raft) abortLeaderTransfer() {
 	r.leadTransferee = None
 }
 
+// increaseUncommittedSize computes the size of the proposed entries and
+// determines whether they would push leader over its maxUncommittedSize limit.
+// If the new entries would exceed the limit, the method returns false. If not,
+// the increase in uncommitted entry size is recorded and the method returns
+// true.
+func (r *raft) increaseUncommittedSize(ents []pb.Entry) bool {
+	var s uint64
+	for _, e := range ents {
+		s += uint64(e.Size())
+	}
+
+	if r.uncommittedSize > 0 && r.uncommittedSize+s > r.maxUncommittedSize {
+		// If the uncommitted tail of the Raft log is empty, allow any size
+		// proposal. Otherwise, limit the size of the uncommitted tail of the
+		// log and drop any proposal that would push the size over the limit.
+		return false
+	}
+	r.uncommittedSize += s
+	return true
+}
+
+// reduceUncommittedSize accounts for the newly committed entries by decreasing
+// the uncommitted entry size limit.
+func (r *raft) reduceUncommittedSize(ents []pb.Entry) {
+	if r.uncommittedSize == 0 {
+		// Fast-path for followers, who do not track or enforce the limit.
+		return
+	}
+
+	var s uint64
+	for _, e := range ents {
+		s += uint64(e.Size())
+	}
+	if s > r.uncommittedSize {
+		// uncommittedSize may underestimate the size of the uncommitted Raft
+		// log tail but will never overestimate it. Saturate at 0 instead of
+		// allowing overflow.
+		r.uncommittedSize = 0
+	} else {
+		r.uncommittedSize -= s
+	}
+}
+
 func numOfPendingConf(ents []pb.Entry) int {
 	n := 0
 	for i := range ents {

raft/raft_test.go

@@ -362,6 +362,71 @@ func TestProgressFlowControl(t *testing.T) {
 	}
 }
 
+func TestUncommittedEntryLimit(t *testing.T) {
+	const maxEntries = 16
+	testEntry := pb.Entry{Data: []byte("testdata")}
+	maxEntrySize := maxEntries * testEntry.Size()
+
+	cfg := newTestConfig(1, []uint64{1, 2, 3}, 5, 1, NewMemoryStorage())
+	cfg.MaxUncommittedEntriesSize = uint64(maxEntrySize)
+	r := newRaft(cfg)
+	r.becomeCandidate()
+	r.becomeLeader()
+
+	// Set the two followers to the replicate state. Commit to tail of log.
+	const numFollowers = 2
+	r.prs[2].becomeReplicate()
+	r.prs[3].becomeReplicate()
+	r.uncommittedSize = 0
+
+	// Send proposals to r1. The first 5 entries should be appended to the log.
+	propMsg := pb.Message{From: 1, To: 1, Type: pb.MsgProp, Entries: []pb.Entry{testEntry}}
+	propEnts := make([]pb.Entry, maxEntries)
+	for i := 0; i < maxEntries; i++ {
+		if err := r.Step(propMsg); err != nil {
+			t.Fatalf("proposal resulted in error: %v", err)
+		}
+		propEnts[i] = testEntry
+	}
+
+	// Send one more proposal to r1. It should be rejected.
+	if err := r.Step(propMsg); err != ErrProposalDropped {
+		t.Fatalf("proposal not dropped: %v", err)
+	}
+
+	// Read messages and reduce the uncommitted size as if we had committed
+	// these entries.
+	ms := r.readMessages()
+	if e := maxEntries * numFollowers; len(ms) != e {
+		t.Fatalf("expected %d messages, got %d", e, len(ms))
+	}
+	r.reduceUncommittedSize(propEnts)
+
+	// Send a single large proposal to r1. Should be accepted even though it
+	// pushes us above the limit because we were beneath it before the proposal.
+	propEnts = make([]pb.Entry, 2*maxEntries)
+	for i := range propEnts {
+		propEnts[i] = testEntry
+	}
+	propMsgLarge := pb.Message{From: 1, To: 1, Type: pb.MsgProp, Entries: propEnts}
+	if err := r.Step(propMsgLarge); err != nil {
+		t.Fatalf("proposal resulted in error: %v", err)
+	}
+
+	// Send one more proposal to r1. It should be rejected, again.
+	if err := r.Step(propMsg); err != ErrProposalDropped {
+		t.Fatalf("proposal not dropped: %v", err)
+	}
+
+	// Read messages and reduce the uncommitted size as if we had committed
+	// these entries.
+	ms = r.readMessages()
+	if e := 1 * numFollowers; len(ms) != e {
+		t.Fatalf("expected %d messages, got %d", e, len(ms))
+	}
+	r.reduceUncommittedSize(propEnts)
+}
+
 func TestLeaderElection(t *testing.T) {
 	testLeaderElection(t, false)
 }

raft/rafttest/node.go

@@ -41,12 +41,13 @@ type node struct {
 func startNode(id uint64, peers []raft.Peer, iface iface) *node {
 	st := raft.NewMemoryStorage()
 	c := &raft.Config{
-		ID:              id,
-		ElectionTick:    10,
-		HeartbeatTick:   1,
-		Storage:         st,
-		MaxSizePerMsg:   1024 * 1024,
-		MaxInflightMsgs: 256,
+		ID:                        id,
+		ElectionTick:              10,
+		HeartbeatTick:             1,
+		Storage:                   st,
+		MaxSizePerMsg:             1024 * 1024,
+		MaxInflightMsgs:           256,
+		MaxUncommittedEntriesSize: 1 << 30,
 	}
 	rn := raft.StartNode(c, peers)
 	n := &node{
@@ -125,12 +126,13 @@ func (n *node) restart() {
 	// wait for the shutdown
 	<-n.stopc
 	c := &raft.Config{
-		ID:              n.id,
-		ElectionTick:    10,
-		HeartbeatTick:   1,
-		Storage:         n.storage,
-		MaxSizePerMsg:   1024 * 1024,
-		MaxInflightMsgs: 256,
+		ID:                        n.id,
+		ElectionTick:              10,
+		HeartbeatTick:             1,
+		Storage:                   n.storage,
+		MaxSizePerMsg:             1024 * 1024,
+		MaxInflightMsgs:           256,
+		MaxUncommittedEntriesSize: 1 << 30,
 	}
 	n.Node = raft.RestartNode(c)
 	n.start()

raft/rawnode.go

@@ -198,6 +198,7 @@ func (rn *RawNode) Step(m pb.Message) error {
 func (rn *RawNode) Ready() Ready {
 	rd := rn.newReady()
 	rn.raft.msgs = nil
+	rn.raft.reduceUncommittedSize(rd.CommittedEntries)
 	return rd
 }