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raft: don't apply entries when applying snapshot

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This commit removes the ability to apply log entries at the same time as
applying a snapshot. Doing so it possible, but it leads to complex code and
raises questions about what should be applied first. It also raises additional
complexity when we start allowing concurrent, asynchronous log appends and log
application. It's easiest to just disallow this.

Signed-off-by: Nathan VanBenschoten <nvanbenschoten@gmail.com>
This commit is contained in:
Nathan VanBenschoten 2022-10-18 14:19:00 -04:00
parent 95c5fed3cf
commit 539a8410f4
2 changed files with 42 additions and 17 deletions
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19
raft/log.go
View File

@ -181,9 +181,13 @@ func (l *raftLog) unstableEntries() []pb.Entry {
// If applied is smaller than the index of snapshot, it returns all committed
// entries after the index of snapshot.
func (l *raftLog) nextCommittedEnts() (ents []pb.Entry) {
off := max(l.applied+1, l.firstIndex())
if l.committed+1 > off {
ents, err := l.slice(off, l.committed+1, l.maxNextCommittedEntsSize)
if l.hasPendingSnapshot() {
// See comment in hasNextCommittedEnts.
return nil
}
if l.committed > l.applied {
lo, hi := l.applied+1, l.committed+1 // [lo, hi)
ents, err := l.slice(lo, hi, l.maxNextCommittedEntsSize)
if err != nil {
l.logger.Panicf("unexpected error when getting unapplied entries (%v)", err)
}
@ -195,8 +199,13 @@ func (l *raftLog) nextCommittedEnts() (ents []pb.Entry) {
// hasNextCommittedEnts returns if there is any available entries for execution.
// This is a fast check without heavy raftLog.slice() in nextCommittedEnts().
func (l *raftLog) hasNextCommittedEnts() bool {
off := max(l.applied+1, l.firstIndex())
return l.committed+1 > off
if l.hasPendingSnapshot() {
// If we have a snapshot to apply, don't also return any committed
// entries. Doing so raises questions about what should be applied
// first.
return false
}
return l.committed > l.applied
}
// hasPendingSnapshot returns if there is pending snapshot waiting for applying.

40
raft/log_test.go
View File

@ -349,13 +349,16 @@ func TestHasNextCommittedEnts(t *testing.T) {
{Term: 1, Index: 6},
}
tests := []struct {
applied uint64
hasNext bool
applied uint64
snap bool
whasNext bool
}{
{0, true},
{3, true},
{4, true},
{5, false},
{applied: 0, snap: false, whasNext: true},
{applied: 3, snap: false, whasNext: true},
{applied: 4, snap: false, whasNext: true},
{applied: 5, snap: false, whasNext: false},
// With snapshot.
{applied: 3, snap: true, whasNext: false},
}
for i, tt := range tests {
storage := NewMemoryStorage()
@ -364,10 +367,15 @@ func TestHasNextCommittedEnts(t *testing.T) {
raftLog.append(ents...)
raftLog.maybeCommit(5, 1)
raftLog.appliedTo(tt.applied)
if tt.snap {
newSnap := snap
newSnap.Metadata.Index++
raftLog.restore(newSnap)
}
hasNext := raftLog.hasNextCommittedEnts()
if hasNext != tt.hasNext {
t.Errorf("#%d: hasNext = %v, want %v", i, hasNext, tt.hasNext)
if hasNext != tt.whasNext {
t.Errorf("#%d: hasNext = %v, want %v", i, hasNext, tt.whasNext)
}
}
}
@ -383,12 +391,15 @@ func TestNextCommittedEnts(t *testing.T) {
}
tests := []struct {
applied uint64
snap bool
wents []pb.Entry
}{
{0, ents[:2]},
{3, ents[:2]},
{4, ents[1:2]},
{5, nil},
{applied: 0, snap: false, wents: ents[:2]},
{applied: 3, snap: false, wents: ents[:2]},
{applied: 4, snap: false, wents: ents[1:2]},
{applied: 5, snap: false, wents: nil},
// With snapshot.
{applied: 3, snap: true, wents: nil},
}
for i, tt := range tests {
storage := NewMemoryStorage()
@ -397,6 +408,11 @@ func TestNextCommittedEnts(t *testing.T) {
raftLog.append(ents...)
raftLog.maybeCommit(5, 1)
raftLog.appliedTo(tt.applied)
if tt.snap {
newSnap := snap
newSnap.Metadata.Index++
raftLog.restore(newSnap)
}
nents := raftLog.nextCommittedEnts()
if !reflect.DeepEqual(nents, tt.wents) {