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Refactor process append entry #7224

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acb54f1
Refactor processAppendEntry()
sciascid Aug 27, 2025
134258f
Refactor processAppendEntry
sciascid Oct 1, 2025
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202 changes: 99 additions & 103 deletions server/raft.go
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Original file line number Diff line number Diff line change
Expand Up @@ -514,7 +514,7 @@ func (s *Server) initRaftNode(accName string, cfg *RaftConfig, labels pprofLabel
truncateAndErr(index - 1)
break
}
n.processAppendEntry(ae, nil)
n.recoverAppendEntry(ae)
// Check how much we have queued up so far to determine if we should pause.
for _, e := range ae.entries {
qsz += len(e.Data)
Expand Down Expand Up @@ -2156,7 +2156,7 @@ func (n *raft) processAppendEntries() {
aes := n.entry.pop()
if canProcess {
for _, ae := range aes {
n.processAppendEntry(ae, ae.sub)
n.processAppendEntry(ae)
}
}
n.entry.recycle(&aes)
Expand Down Expand Up @@ -3444,28 +3444,68 @@ func (n *raft) updateLeader(newLeader string) {
}
}

// processAppendEntry will process an appendEntry. This is called either
// during recovery or from processAppendEntries when there are new entries
// to be committed.
func (n *raft) processAppendEntry(ae *appendEntry, sub *subscription) {
// recoverAppendEntry will process an appendEntry for recovery
func (n *raft) recoverAppendEntry(ae *appendEntry) {
n.Lock()
defer n.Unlock()

n.pterm = ae.term
n.pindex = ae.pindex + 1

n.processEntriesAndCommit(ae, false)
}

// processAppendEntry will process an appendEntry. This is called from
// processAppendEntries when there are new entries to be committed.
func (n *raft) processAppendEntry(ae *appendEntry) {
var ar *appendEntryResponse

// Make a copy of the reply subject, as ae may return
// to its pool as part of processAppendEntryLocked
subject := ae.reply

n.Lock()
isNew := ae.sub != nil && ae.sub == n.aesub
isCatchup := !isNew && n.catchup != nil && ae.sub == n.catchup.sub

// Process the appendEntry if it is a new proposal, or if we are
// catching up. Ignore messages coming from old catchup subs when
// ae.lterm <= 0. Older versions of the server do not send the leader
// term when catching up. Old catchups from newer subs can be rejected
// later by checking that the appendEntry is on the correct term.
if isNew || isCatchup || ae.lterm > 0 {
ar = n.processAppendEntryLocked(ae, isNew)
} else {
n.debug("processAppendEntry ignore entry from old subscription")
}
n.Unlock()

if ar != nil {
var scratch [appendEntryResponseLen]byte
n.sendRPC(subject, ar.reply, ar.encode(scratch[:]))
arPool.Put(ar)
}
}

// Process the given appendEntry. Parameter isNew indicates whether the
// appendEntry is comes from a new proposal or from a catchup request.
// Optionally returns a appendEntryResponse. The caller is responsible
// for sending out the response and return it to its pool.
// Lock should be held.
func (n *raft) processAppendEntryLocked(ae *appendEntry, isNew bool) *appendEntryResponse {
// Don't reset here if we have been asked to assume leader position.
if !n.lxfer {
n.resetElectionTimeout()
}

// Just return if closed or we had previous write error.
if n.State() == Closed || n.werr != nil {
n.Unlock()
return
return nil
}

// Scratch buffer for responses.
var scratch [appendEntryResponseLen]byte
arbuf := scratch[:]

// Grab term from append entry. But if leader explicitly defined its term, use that instead.
// This is required during catchup if the leader catches us up on older items from previous terms.
// Grab term from append entry. But if leader explicitly defined its term,
// use that instead. This is required during catchup if the leader catches
// us up on older items from previous terms.
// While still allowing us to confirm they're matching our highest known term.
lterm := ae.term
if ae.lterm != 0 {
Expand Down Expand Up @@ -3496,12 +3536,8 @@ func (n *raft) processAppendEntry(ae *appendEntry, sub *subscription) {
n.stepdownLocked(ae.leader)
} else {
// Let them know we are the leader.
ar := newAppendEntryResponse(n.term, n.pindex, n.id, false)
n.debug("AppendEntry ignoring old term from another leader")
n.sendRPC(ae.reply, _EMPTY_, ar.encode(arbuf))
arPool.Put(ar)
n.Unlock()
return
return newAppendEntryResponse(n.term, n.pindex, n.id, false)
}
}

Expand All @@ -3524,12 +3560,6 @@ func (n *raft) processAppendEntry(ae *appendEntry, sub *subscription) {
}
}

// Catching up state.
catchingUp := n.catchup != nil
// Is this a new entry? New entries will be delivered on the append entry
// sub, rather than a catch-up sub.
isNew := sub != nil && sub == n.aesub

// Track leader directly
if isNew && ae.leader != noLeader {
if ps := n.peers[ae.leader]; ps != nil {
Expand All @@ -3539,15 +3569,6 @@ func (n *raft) processAppendEntry(ae *appendEntry, sub *subscription) {
}
}

// If we are/were catching up ignore old catchup subs, but only if catching up from an older server
// that doesn't send the leader term when catching up. We can reject old catchups from newer subs
// later, just by checking the append entry is on the correct term.
if !isNew && sub != nil && ae.lterm == 0 && (!catchingUp || sub != n.catchup.sub) {
n.Unlock()
n.debug("AppendEntry ignoring old entry from previous catchup")
return
}

// If this term is greater than ours.
if lterm > n.term {
n.term = lterm
Expand All @@ -3559,46 +3580,33 @@ func (n *raft) processAppendEntry(ae *appendEntry, sub *subscription) {
n.debug("Term higher than ours and we are not a follower: %v, stepping down to %q", n.State(), ae.leader)
n.stepdownLocked(ae.leader)
}
} else if lterm < n.term && sub != nil && (isNew || ae.lterm != 0) {
} else if lterm < n.term && (isNew || ae.lterm != 0) {
// Anything that's below our expected highest term needs to be rejected.
// Unless we're replaying (sub=nil), in which case we'll always continue.
// For backward-compatibility we shouldn't reject if we're being caught up by an old server.
if !isNew {
n.debug("AppendEntry ignoring old entry from previous catchup")
n.Unlock()
return
return nil
}
n.debug("Rejected AppendEntry from a leader (%s) with term %d which is less than ours", ae.leader, lterm)
ar := newAppendEntryResponse(n.term, n.pindex, n.id, false)
n.Unlock()
n.sendRPC(ae.reply, _EMPTY_, ar.encode(arbuf))
arPool.Put(ar)
return
return newAppendEntryResponse(n.term, n.pindex, n.id, false)
}

// Check state if we are catching up.
if catchingUp {
if cs := n.catchup; cs != nil && n.pterm >= cs.cterm && n.pindex >= cs.cindex {
if n.catchup != nil {
if n.pterm >= n.catchup.cterm && n.pindex >= n.catchup.cindex {
// If we are here we are good, so if we have a catchup pending we can cancel.
n.cancelCatchup()
// Reset our notion of catching up.
catchingUp = false
} else if isNew {
var ar *appendEntryResponse
var inbox string
// Check to see if we are stalled. If so recreate our catchup state and resend response.
if n.catchupStalled() {
n.debug("Catchup may be stalled, will request again")
inbox = n.createCatchup(ae)
ar = newAppendEntryResponse(n.pterm, n.pindex, n.id, false)
}
n.Unlock()
if ar != nil {
n.sendRPC(ae.reply, inbox, ar.encode(arbuf))
arPool.Put(ar)
inbox := n.createCatchup(ae)
ar := newAppendEntryResponse(n.pterm, n.pindex, n.id, false)
ar.reply = inbox
return ar
}
// Ignore new while catching up or replaying.
return
return nil
}
}

Expand Down Expand Up @@ -3668,21 +3676,19 @@ func (n *raft) processAppendEntry(ae *appendEntry, sub *subscription) {
// For example, if we got partial catchup, and then the "real-time" messages came in very delayed.
// If we reported "success" on those "real-time" messages, we'd wrongfully be providing
// quorum while not having an up-to-date log.
n.Unlock()
return
return nil
}

// Check if we are catching up. If we are here we know the leader did not have all of the entries
// so make sure this is a snapshot entry. If it is not start the catchup process again since it
// means we may have missed additional messages.
if catchingUp {
if n.catchup != nil {
// This means we already entered into a catchup state but what the leader sent us did not match what we expected.
// Snapshots and peerstate will always be together when a leader is catching us up in this fashion.
if len(ae.entries) != 2 || ae.entries[0].Type != EntrySnapshot || ae.entries[1].Type != EntryPeerState {
n.warn("Expected first catchup entry to be a snapshot and peerstate, will retry")
n.cancelCatchup()
n.Unlock()
return
return nil
}

if ps, err := decodePeerState(ae.entries[1].Data); err == nil {
Expand All @@ -3692,8 +3698,7 @@ func (n *raft) processAppendEntry(ae *appendEntry, sub *subscription) {
} else {
n.warn("Could not parse snapshot peerstate correctly")
n.cancelCatchup()
n.Unlock()
return
return nil
}

// Inherit state from appendEntry with the leader's snapshot.
Expand All @@ -3710,48 +3715,54 @@ func (n *raft) processAppendEntry(ae *appendEntry, sub *subscription) {
// Install the leader's snapshot as our own.
if err := n.installSnapshot(snap); err != nil {
n.setWriteErrLocked(err)
n.Unlock()
return
return nil
}
n.resetInitializing()

// Now send snapshot to upper levels. Only send the snapshot, not the peerstate entry.
n.apply.push(newCommittedEntry(n.commit, ae.entries[:1]))
n.Unlock()
return
return nil
}

// Setup our state for catching up.
n.debug("AppendEntry did not match [%d:%d] with [%d:%d]", ae.pterm, ae.pindex, n.pterm, n.pindex)
inbox := n.createCatchup(ae)
ar := newAppendEntryResponse(n.pterm, n.pindex, n.id, false)
n.Unlock()
n.sendRPC(ae.reply, inbox, ar.encode(arbuf))
arPool.Put(ar)
return
ar.reply = inbox
return ar
}

CONTINUE:
// Save to our WAL if we have entries.
if ae.shouldStore() {
// Only store if an original which will have sub != nil
if sub != nil {
if err := n.storeToWAL(ae); err != nil {
if err != ErrStoreClosed {
n.warn("Error storing entry to WAL: %v", err)
}
n.Unlock()
return
if err := n.storeToWAL(ae); err != nil {
if err != ErrStoreClosed {
n.warn("Error storing entry to WAL: %v", err)
}
n.cachePendingEntry(ae)
n.resetInitializing()
} else {
// This is a replay on startup so just take the appendEntry version.
n.pterm = ae.term
n.pindex = ae.pindex + 1
return nil
}
n.cachePendingEntry(ae)
n.resetInitializing()
}

// ae should no longer be used after this call as
// processEntriesAndCommit may return the appendEntry back to its pool
n.processEntriesAndCommit(ae, isNew)

// Only ever respond to new entries.
// Never respond to catchup messages, because providing quorum based on this is unsafe.
// The only way for the leader to receive "success" MUST be through this path.
if isNew {
// Success. Send our response.
return newAppendEntryResponse(n.pterm, n.pindex, n.id, true)
}

return nil
}

// Process all entries in appendEntry and try to commit.
// Lock should be held.
func (n *raft) processEntriesAndCommit(ae *appendEntry, isNew bool) {
// Check to see if we have any related entries to process here.
for _, e := range ae.entries {
switch e.Type {
Expand Down Expand Up @@ -3789,9 +3800,9 @@ CONTINUE:
}
}

// Make a copy of these values, as the AppendEntry might be cached and returned to the pool in applyCommit.
// Copy ae.commit as the AppendEntry might be cached and returned to the
// pool in applyCommit.
aeCommit := ae.commit
aeReply := ae.reply

// Apply anything we need here.
if aeCommit > n.commit {
Expand All @@ -3806,21 +3817,6 @@ CONTINUE:
}
}
}

// Only ever respond to new entries.
// Never respond to catchup messages, because providing quorum based on this is unsafe.
// The only way for the leader to receive "success" MUST be through this path.
var ar *appendEntryResponse
if sub != nil && isNew {
ar = newAppendEntryResponse(n.pterm, n.pindex, n.id, true)
}
n.Unlock()

// Success. Send our response.
if ar != nil {
n.sendRPC(aeReply, _EMPTY_, ar.encode(arbuf))
arPool.Put(ar)
}
}

// resetInitializing resets the notion of initializing.
Expand Down
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