Merge pull request #5640 from bhandras/kvdb-prefetch

kvdb+channeld: extend `kvdb` with `Prefetch` for prefetching buckets in one go and speed up payment control by prefetching payments on hot paths

channeldb/payment_control.go

@@ -131,6 +131,7 @@ func (p *PaymentControl) InitPayment(paymentHash lntypes.Hash,
 		// from a previous execution of the batched db transaction.
 		updateErr = nil
 
+		prefetchPayment(tx, paymentHash)
 		bucket, err := createPaymentBucket(tx, paymentHash)
 		if err != nil {
 			return err
@@ -292,6 +293,7 @@ func (p *PaymentControl) RegisterAttempt(paymentHash lntypes.Hash,
 
 	var payment *MPPayment
 	err = kvdb.Batch(p.db.Backend, func(tx kvdb.RwTx) error {
+		prefetchPayment(tx, paymentHash)
 		bucket, err := fetchPaymentBucketUpdate(tx, paymentHash)
 		if err != nil {
 			return err
@@ -430,6 +432,7 @@ func (p *PaymentControl) updateHtlcKey(paymentHash lntypes.Hash,
 	err := kvdb.Batch(p.db.Backend, func(tx kvdb.RwTx) error {
 		payment = nil
 
+		prefetchPayment(tx, paymentHash)
 		bucket, err := fetchPaymentBucketUpdate(tx, paymentHash)
 		if err != nil {
 			return err
@@ -500,6 +503,7 @@ func (p *PaymentControl) Fail(paymentHash lntypes.Hash,
 		updateErr = nil
 		payment = nil
 
+		prefetchPayment(tx, paymentHash)
 		bucket, err := fetchPaymentBucketUpdate(tx, paymentHash)
 		if err == ErrPaymentNotInitiated {
 			updateErr = ErrPaymentNotInitiated
@@ -550,6 +554,7 @@ func (p *PaymentControl) FetchPayment(paymentHash lntypes.Hash) (
 
 	var payment *MPPayment
 	err := kvdb.View(p.db, func(tx kvdb.RTx) error {
+		prefetchPayment(tx, paymentHash)
 		bucket, err := fetchPaymentBucket(tx, paymentHash)
 		if err != nil {
 			return err
@@ -568,6 +573,26 @@ func (p *PaymentControl) FetchPayment(paymentHash lntypes.Hash) (
 	return payment, nil
 }
 
+// prefetchPayment attempts to prefetch as much of the payment as possible to
+// reduce DB roundtrips.
+func prefetchPayment(tx kvdb.RTx, paymentHash lntypes.Hash) {
+	rb := kvdb.RootBucket(tx)
+	kvdb.Prefetch(
+		rb,
+		[]string{
+			// Prefetch all keys in the payment's bucket.
+			string(paymentsRootBucket),
+			string(paymentHash[:]),
+		},
+		[]string{
+			// Prefetch all keys in the payment's htlc bucket.
+			string(paymentsRootBucket),
+			string(paymentHash[:]),
+			string(paymentHtlcsBucket),
+		},
+	)
+}
+
 // createPaymentBucket creates or fetches the sub-bucket assigned to this
 // payment hash.
 func createPaymentBucket(tx kvdb.RwTx, paymentHash lntypes.Hash) (

docs/release-notes/release-notes-0.14.0.md

@@ -267,6 +267,9 @@ you.
   payments. Deleting all failed payments beforehand makes migration safer and
   faster too.
 
+* [Prefetch payments on hot paths](https://github.com/lightningnetwork/lnd/pull/5640)
+  to reduce roundtrips to the remote DB backend.
+
 ## Performance improvements
 
 * [Update MC store in blocks](https://github.com/lightningnetwork/lnd/pull/5515)

kvdb/bolt_test.go

@@ -63,6 +63,10 @@ func TestBolt(t *testing.T) {
 			name: "tx rollback",
 			test: testTxRollback,
 		},
+		{
+			name: "prefetch",
+			test: testPrefetch,
+		},
 	}
 
 	for _, test := range tests {

kvdb/etcd/db.go

@@ -219,7 +219,8 @@ func (db *db) View(f func(tx walletdb.ReadTx) error, reset func()) error {
 		return f(newReadWriteTx(stm, etcdDefaultRootBucketId, nil))
 	}
 
-	return RunSTM(db.cli, apply, db.txQueue, db.getSTMOptions()...)
+	_, err := RunSTM(db.cli, apply, db.txQueue, db.getSTMOptions()...)
+	return err
 }
 
 // Update opens a database read/write transaction and executes the function f
@@ -240,7 +241,8 @@ func (db *db) Update(f func(tx walletdb.ReadWriteTx) error, reset func()) error
 		return f(newReadWriteTx(stm, etcdDefaultRootBucketId, nil))
 	}
 
-	return RunSTM(db.cli, apply, db.txQueue, db.getSTMOptions()...)
+	_, err := RunSTM(db.cli, apply, db.txQueue, db.getSTMOptions()...)
+	return err
 }
 
 // PrintStats returns all collected stats pretty printed into a string.

kvdb/etcd/readwrite_bucket.go

@@ -371,3 +371,37 @@ func (b *readWriteBucket) Sequence() uint64 {
 
 	return num
 }
+
+func flattenMap(m map[string]struct{}) []string {
+	result := make([]string, len(m))
+	i := 0
+
+	for key := range m {
+		result[i] = key
+		i++
+	}
+
+	return result
+}
+
+// Prefetch will prefetch all keys in the passed paths as well as all bucket
+// keys along the paths.
+func (b *readWriteBucket) Prefetch(paths ...[]string) {
+	keys := make(map[string]struct{})
+	ranges := make(map[string]struct{})
+
+	for _, path := range paths {
+		parent := b.id
+		for _, bucket := range path {
+			bucketKey := makeBucketKey(parent, []byte(bucket))
+			keys[string(bucketKey[:])] = struct{}{}
+
+			id := makeBucketID(bucketKey)
+			parent = id[:]
+		}
+
+		ranges[string(parent)] = struct{}{}
+	}
+
+	b.tx.stm.Prefetch(flattenMap(keys), flattenMap(ranges))
+}

kvdb/etcd/readwrite_tx.go

@@ -41,6 +41,13 @@ func rootBucket(tx *readWriteTx) *readWriteBucket {
 	return newReadWriteBucket(tx, tx.rootBucketID[:], tx.rootBucketID[:])
 }
 
+// RootBucket will return a handle to the root bucket. This is not a real handle
+// but just a wrapper around the root bucket ID to allow derivation of child
+// keys.
+func (tx *readWriteTx) RootBucket() walletdb.ReadBucket {
+	return rootBucket(tx)
+}
+
 // ReadBucket opens the root bucket for read only access.  If the bucket
 // described by the key does not exist, nil is returned.
 func (tx *readWriteTx) ReadBucket(key []byte) walletdb.ReadBucket {

kvdb/etcd/stm.go

@@ -8,6 +8,8 @@ import (
 	"math"
 	"strings"
 
+	"github.com/google/btree"
+	pb "go.etcd.io/etcd/api/v3/etcdserverpb"
 	v3 "go.etcd.io/etcd/client/v3"
 )
 
@@ -71,9 +73,13 @@ type STM interface {
 	// Commit may return CommitError if transaction is outdated and needs retry.
 	Commit() error
 
-	// Rollback emties the read and write sets such that a subsequent commit
+	// Rollback entries the read and write sets such that a subsequent commit
 	// won't alter the database.
 	Rollback()
+
+	// Prefetch prefetches the passed keys and prefixes. For prefixes it'll
+	// fetch the whole range.
+	Prefetch(keys []string, prefix []string)
 }
 
 // CommitError is used to check if there was an error
@@ -104,15 +110,26 @@ func (e DatabaseError) Error() string {
 	return fmt.Sprintf("etcd error: %v - %v", e.msg, e.err)
 }
 
-// stmGet is the result of a read operation,
+// stmGet is the result of a read operation, a value and the mod revision of the
-// a value and the mod revision of the key/value.
+// key/value.
 type stmGet struct {
-	val string
+	KV
 	rev int64
 }
 
+// Less implements less operator for btree.BTree.
+func (c *stmGet) Less(than btree.Item) bool {
+	return c.key < than.(*stmGet).key
+}
+
 // readSet stores all reads done in an STM.
-type readSet map[string]stmGet
+type readSet struct {
+	// tree stores the items in the read set.
+	tree *btree.BTree
+
+	// fullRanges stores full range prefixes.
+	fullRanges map[string]struct{}
+}
 
 // stmPut stores a value and an operation (put/delete).
 type stmPut struct {
@@ -141,11 +158,8 @@ type stm struct {
 	// options stores optional settings passed by the user.
 	options *STMOptions
 
-	// prefetch hold prefetched key values and revisions.
-	prefetch readSet
-
 	// rset holds read key values and revisions.
-	rset readSet
+	rset *readSet
 
 	// wset holds overwritten keys and their values.
 	wset writeSet
@@ -158,6 +172,9 @@ type stm struct {
 
 	// onCommit gets called upon commit.
 	onCommit func()
+
+	// callCount tracks the number of times we called into etcd.
+	callCount int
 }
 
 // STMOptions can be used to pass optional settings
@@ -188,9 +205,12 @@ func WithCommitStatsCallback(cb func(bool, CommitStats)) STMOptionFunc {
 // RunSTM runs the apply function by creating an STM using serializable snapshot
 // isolation, passing it to the apply and handling commit errors and retries.
 func RunSTM(cli *v3.Client, apply func(STM) error, txQueue *commitQueue,
-	so ...STMOptionFunc) error {
+	so ...STMOptionFunc) (int, error) {
 
-	return runSTM(makeSTM(cli, false, txQueue, so...), apply)
+	stm := makeSTM(cli, false, txQueue, so...)
+	err := runSTM(stm, apply)
+
+	return stm.callCount, err
 }
 
 // NewSTM creates a new STM instance, using serializable snapshot isolation.
@@ -217,11 +237,11 @@ func makeSTM(cli *v3.Client, manual bool, txQueue *commitQueue,
 		manual:  manual,
 		txQueue: txQueue,
 		options: opts,
-		prefetch: make(map[string]stmGet),
+		rset:    newReadSet(),
 	}
 
 	// Reset read and write set.
-	s.Rollback()
+	s.rollback(true)
 
 	return s
 }
@@ -262,8 +282,11 @@ func runSTM(s *stm, apply func(STM) error) error {
 				return
 			}
 
-			// Rollback before trying to re-apply.
+			// Rollback the write set before trying to re-apply.
-			s.Rollback()
+			// Upon commit we retrieved the latest version of all
+			// previously fetched keys and ranges so we don't need
+			// to rollback the read set.
+			s.rollback(false)
 			retries++
 
 			// Re-apply the transaction closure.
@@ -287,14 +310,16 @@ func runSTM(s *stm, apply func(STM) error) error {
 	// result in queueing up transactions and contending DB access.
 	// Copying these strings is cheap due to Go's immutable string which is
 	// always a reference.
-	rkeys := make([]string, len(s.rset))
+	rkeys := make([]string, s.rset.tree.Len())
 	wkeys := make([]string, len(s.wset))
 
 	i := 0
-	for key := range s.rset {
+	s.rset.tree.Ascend(func(item btree.Item) bool {
-		rkeys[i] = key
+		rkeys[i] = item.(*stmGet).key
 		i++
-	}
+
+		return true
+	})
 
 	i = 0
 	for key := range s.wset {
@@ -320,42 +345,225 @@ func runSTM(s *stm, apply func(STM) error) error {
 	return executeErr
 }
 
-// add inserts a txn response to the read set. This is useful when the txn
+func newReadSet() *readSet {
-// fails due to conflict where the txn response can be used to prefetch
+	return &readSet{
-// key/values.
+		tree:       btree.New(5),
-func (rs readSet) add(txnResp *v3.TxnResponse) {
+		fullRanges: make(map[string]struct{}),
-	for _, resp := range txnResp.Responses {
+	}
-		getResp := (*v3.GetResponse)(resp.GetResponseRange())
+}
+
+// add inserts key/values to to read set.
+func (rs *readSet) add(responses []*pb.ResponseOp) {
+	for _, resp := range responses {
+		getResp := resp.GetResponseRange()
 		for _, kv := range getResp.Kvs {
-			rs[string(kv.Key)] = stmGet{
+			rs.addItem(
-				val: string(kv.Value),
+				string(kv.Key), string(kv.Value), kv.ModRevision,
-				rev: kv.ModRevision,
+			)
-			}
 		}
 	}
 }
 
-// gets is a helper to create an op slice for transaction
+// addFullRange adds all full ranges to the read set.
-// construction.
+func (rs *readSet) addFullRange(prefixes []string, responses []*pb.ResponseOp) {
-func (rs readSet) gets() []v3.Op {
+	for i, resp := range responses {
-	ops := make([]v3.Op, 0, len(rs))
+		getResp := resp.GetResponseRange()
+		for _, kv := range getResp.Kvs {
+			rs.addItem(
+				string(kv.Key), string(kv.Value), kv.ModRevision,
+			)
+		}
 
-	for k := range rs {
+		rs.fullRanges[prefixes[i]] = struct{}{}
-		ops = append(ops, v3.OpGet(k))
+	}
+}
+
+// presetItem presets a key to zero revision if not already present in the read
+// set.
+func (rs *readSet) presetItem(key string) {
+	item := &stmGet{
+		KV: KV{
+			key: key,
+		},
+		rev: 0,
+	}
+
+	if !rs.tree.Has(item) {
+		rs.tree.ReplaceOrInsert(item)
+	}
+}
+
+// addItem adds a single new key/value to the read set (if not already present).
+func (rs *readSet) addItem(key, val string, modRevision int64) {
+	item := &stmGet{
+		KV: KV{
+			key: key,
+			val: val,
+		},
+		rev: modRevision,
+	}
+
+	rs.tree.ReplaceOrInsert(item)
+}
+
+// hasFullRange checks if the read set has a full range prefetched.
+func (rs *readSet) hasFullRange(prefix string) bool {
+	_, ok := rs.fullRanges[prefix]
+	return ok
+}
+
+// next returns the pre-fetched next value of the prefix. If matchKey is true,
+// it'll simply return the key/value that matches the passed key.
+func (rs *readSet) next(prefix, key string, matchKey bool) (*stmGet, bool) {
+	pivot := &stmGet{
+		KV: KV{
+			key: key,
+		},
+	}
+
+	var result *stmGet
+	rs.tree.AscendGreaterOrEqual(
+		pivot,
+		func(item btree.Item) bool {
+			next := item.(*stmGet)
+			if (!matchKey && next.key == key) || next.rev == 0 {
+				return true
+			}
+
+			if strings.HasPrefix(next.key, prefix) {
+				result = next
+			}
+
+			return false
+		},
+	)
+
+	return result, result != nil
+}
+
+// prev returns the pre-fetched prev key/value of the prefix from key.
+func (rs *readSet) prev(prefix, key string) (*stmGet, bool) {
+	pivot := &stmGet{
+		KV: KV{
+			key: key,
+		},
+	}
+
+	var result *stmGet
+
+	rs.tree.DescendLessOrEqual(
+		pivot, func(item btree.Item) bool {
+			prev := item.(*stmGet)
+			if prev.key == key || prev.rev == 0 {
+				return true
+			}
+
+			if strings.HasPrefix(prev.key, prefix) {
+				result = prev
+			}
+
+			return false
+		},
+	)
+
+	return result, result != nil
+}
+
+// last returns the last key/value of the passed range (if prefetched).
+func (rs *readSet) last(prefix string) (*stmGet, bool) {
+	// We create an artificial key here that is just one step away from the
+	// prefix. This way when we try to get the first item with our prefix
+	// before this newly crafted key we'll make sure it's the last element
+	// of our range.
+	key := []byte(prefix)
+	key[len(key)-1] += 1
+
+	return rs.prev(prefix, string(key))
+}
+
+// clear completely clears the readset.
+func (rs *readSet) clear() {
+	rs.tree.Clear(false)
+	rs.fullRanges = make(map[string]struct{})
+}
+
+// getItem returns the matching key/value from the readset.
+func (rs *readSet) getItem(key string) (*stmGet, bool) {
+	pivot := &stmGet{
+		KV: KV{
+			key: key,
+		},
+		rev: 0,
+	}
+	item := rs.tree.Get(pivot)
+	if item != nil {
+		return item.(*stmGet), true
+	}
+
+	// It's possible that although this key isn't in the read set, we
+	// fetched a full range the key is prefixed with. In this case we'll
+	// insert the key with zero revision.
+	for prefix := range rs.fullRanges {
+		if strings.HasPrefix(key, prefix) {
+			rs.tree.ReplaceOrInsert(pivot)
+			return pivot, true
+		}
+	}
+
+	return nil, false
+}
+
+// prefetchSet is a helper to create an op slice of all OpGet's that represent
+// fetched keys appended with a slice of all OpGet's representing all prefetched
+// full ranges.
+func (rs *readSet) prefetchSet() []v3.Op {
+	ops := make([]v3.Op, 0, rs.tree.Len())
+
+	rs.tree.Ascend(func(item btree.Item) bool {
+		key := item.(*stmGet).key
+		for prefix := range rs.fullRanges {
+			// Do not add the key if it has been prefetched in a
+			// full range.
+			if strings.HasPrefix(key, prefix) {
+				return true
+			}
+		}
+
+		ops = append(ops, v3.OpGet(key))
+		return true
+	})
+
+	for prefix := range rs.fullRanges {
+		ops = append(ops, v3.OpGet(prefix, v3.WithPrefix()))
 	}
 
 	return ops
 }
 
+// getFullRanges returns all prefixes that we prefetched.
+func (rs *readSet) getFullRanges() []string {
+	prefixes := make([]string, 0, len(rs.fullRanges))
+
+	for prefix := range rs.fullRanges {
+		prefixes = append(prefixes, prefix)
+	}
+
+	return prefixes
+}
+
 // cmps returns a compare list which will serve as a precondition testing that
 // the values in the read set didn't change.
-func (rs readSet) cmps() []v3.Cmp {
+func (rs *readSet) cmps() []v3.Cmp {
-	cmps := make([]v3.Cmp, 0, len(rs))
+	cmps := make([]v3.Cmp, 0, rs.tree.Len())
-	for key, getValue := range rs {
+
-		cmps = append(cmps, v3.Compare(
+	rs.tree.Ascend(func(item btree.Item) bool {
-			v3.ModRevision(key), "=", getValue.rev,
+		get := item.(*stmGet)
-		))
+		cmps = append(
-	}
+			cmps, v3.Compare(v3.ModRevision(get.key), "=", get.rev),
+		)
+
+		return true
+	})
 
 	return cmps
 }
@@ -384,6 +592,7 @@ func (ws writeSet) puts() []v3.Op {
 // then fetch will try to fix the STM's snapshot revision (if not already set).
 // We'll also cache the returned key/value in the read set.
 func (s *stm) fetch(key string, opts ...v3.OpOption) ([]KV, error) {
+	s.callCount++
 	resp, err := s.client.Get(
 		s.options.ctx, key, append(opts, s.getOpts...)...,
 	)
@@ -394,7 +603,7 @@ func (s *stm) fetch(key string, opts ...v3.OpOption) ([]KV, error) {
 		}
 	}
 
-	// Set revison and serializable options upon first fetch
+	// Set revision and serializable options upon first fetch
 	// for any subsequent fetches.
 	if s.getOpts == nil {
 		s.revision = resp.Header.Revision
@@ -408,26 +617,18 @@ func (s *stm) fetch(key string, opts ...v3.OpOption) ([]KV, error) {
 		// Add assertion to the read set which will extend our commit
 		// constraint such that the commit will fail if the key is
 		// present in the database.
-		s.rset[key] = stmGet{
+		s.rset.addItem(key, "", 0)
-			rev: 0,
-		}
 	}
 
 	var result []KV
 
 	// Fill the read set with key/values returned.
 	for _, kv := range resp.Kvs {
-		// Remove from prefetch.
 		key := string(kv.Key)
 		val := string(kv.Value)
 
-		delete(s.prefetch, key)
-
 		// Add to read set.
-		s.rset[key] = stmGet{
+		s.rset.addItem(key, val, kv.ModRevision)
-			val: val,
-			rev: kv.ModRevision,
-		}
 
 		result = append(result, KV{key, val})
 	}
@@ -452,20 +653,8 @@ func (s *stm) Get(key string) ([]byte, error) {
 		return []byte(put.val), nil
 	}
 
-	// Populate read set if key is present in
-	// the prefetch set.
-	if getValue, ok := s.prefetch[key]; ok {
-		delete(s.prefetch, key)
-
-		// Use the prefetched value only if it is for
-		// an existing key.
-		if getValue.rev != 0 {
-			s.rset[key] = getValue
-		}
-	}
-
 	// Return value if alread in read set.
-	if getValue, ok := s.rset[key]; ok {
+	if getValue, ok := s.rset.getItem(key); ok {
 		// Return the value if the rset contains an existing key.
 		if getValue.rev != 0 {
 			return []byte(getValue.val), nil
@@ -497,6 +686,17 @@ func (s *stm) First(prefix string) (*KV, error) {
 // Last returns the last key/value with prefix. If there's no key starting with
 // prefix, Last will return nil.
 func (s *stm) Last(prefix string) (*KV, error) {
+	var (
+		kv    KV
+		found bool
+	)
+
+	if s.rset.hasFullRange(prefix) {
+		if item, ok := s.rset.last(prefix); ok {
+			kv = item.KV
+			found = true
+		}
+	} else {
 		// As we don't know the full range, fetch the last
 		// key/value with this prefix first.
 		resp, err := s.fetch(prefix, v3.WithLastKey()...)
@@ -504,15 +704,11 @@ func (s *stm) Last(prefix string) (*KV, error) {
 			return nil, err
 		}
 
-	var (
-		kv    KV
-		found bool
-	)
-
 		if len(resp) > 0 {
 			kv = resp[0]
 			found = true
 		}
+	}
 
 	// Now make sure there's nothing in the write set
 	// that is a better match, meaning it has the same
@@ -539,14 +735,22 @@ func (s *stm) Last(prefix string) (*KV, error) {
 }
 
 // Prev returns the prior key/value before key (with prefix). If there's no such
-// key Next will return nil.
+// key Prev will return nil.
 func (s *stm) Prev(prefix, startKey string) (*KV, error) {
-	var result KV
+	var kv, result KV
 
 	fetchKey := startKey
 	matchFound := false
 
 	for {
+		if s.rset.hasFullRange(prefix) {
+			if item, ok := s.rset.prev(prefix, fetchKey); ok {
+				kv = item.KV
+			} else {
+				break
+			}
+		} else {
+
 			// Ask etcd to retrieve one key that is a
 			// match in descending order from the passed key.
 			opts := []v3.OpOption{
@@ -564,7 +768,8 @@ func (s *stm) Prev(prefix, startKey string) (*KV, error) {
 				break
 			}
 
-		kv := &kvs[0]
+			kv = kvs[0]
+		}
 
 		// WithRange and WithPrefix can't be used
 		// together, so check prefix here. If the
@@ -580,13 +785,13 @@ func (s *stm) Prev(prefix, startKey string) (*KV, error) {
 			continue
 		}
 
-		result = *kv
+		result = kv
 		matchFound = true
 
 		break
 	}
 
-	// Closre holding all checks to find a possibly
+	// Closure holding all checks to find a possibly
 	// better match.
 	matches := func(key string) bool {
 		if !strings.HasPrefix(key, prefix) {
@@ -635,13 +840,24 @@ func (s *stm) Seek(prefix, key string) (*KV, error) {
 // passed startKey. If includeStartKey is set to true, it'll return the value
 // of startKey (essentially implementing seek).
 func (s *stm) next(prefix, startKey string, includeStartKey bool) (*KV, error) {
-	var result KV
+	var kv, result KV
 
 	fetchKey := startKey
 	firstFetch := true
 	matchFound := false
 
 	for {
+		if s.rset.hasFullRange(prefix) {
+			matchKey := includeStartKey && firstFetch
+			firstFetch = false
+			if item, ok := s.rset.next(
+				prefix, fetchKey, matchKey,
+			); ok {
+				kv = item.KV
+			} else {
+				break
+			}
+		} else {
 			// Ask etcd to retrieve one key that is a
 			// match in ascending order from the passed key.
 			opts := []v3.OpOption{
@@ -669,7 +885,8 @@ func (s *stm) next(prefix, startKey string, includeStartKey bool) (*KV, error) {
 				break
 			}
 
-		kv := &kvs[0]
+			kv = kvs[0]
+
 			// WithRange and WithPrefix can't be used
 			// together, so check prefix here. If the
 			// returned key no longer has the prefix,
@@ -677,6 +894,7 @@ func (s *stm) next(prefix, startKey string, includeStartKey bool) (*KV, error) {
 			if !strings.HasPrefix(kv.key, prefix) {
 				break
 			}
+		}
 
 		// Move on to fetch starting with the next
 		// key if this one is marked deleted.
@@ -685,7 +903,7 @@ func (s *stm) next(prefix, startKey string, includeStartKey bool) (*KV, error) {
 			continue
 		}
 
-		result = *kv
+		result = kv
 		matchFound = true
 
 		break
@@ -753,6 +971,72 @@ func (s *stm) OnCommit(cb func()) {
 	s.onCommit = cb
 }
 
+// Prefetch will prefetch the passed keys and prefixes in one transaction.
+// Keys and prefixes that we already have will be skipped.
+func (s *stm) Prefetch(keys []string, prefixes []string) {
+	fetchKeys := make([]string, 0, len(keys))
+	for _, key := range keys {
+		if _, ok := s.rset.getItem(key); !ok {
+			fetchKeys = append(fetchKeys, key)
+		}
+	}
+
+	fetchPrefixes := make([]string, 0, len(prefixes))
+	for _, prefix := range prefixes {
+		if s.rset.hasFullRange(prefix) {
+			continue
+		}
+		fetchPrefixes = append(fetchPrefixes, prefix)
+	}
+
+	if len(fetchKeys) == 0 && len(fetchPrefixes) == 0 {
+		return
+	}
+
+	prefixOpts := append(
+		[]v3.OpOption{v3.WithPrefix()}, s.getOpts...,
+	)
+
+	txn := s.client.Txn(s.options.ctx)
+	ops := make([]v3.Op, 0, len(fetchKeys)+len(fetchPrefixes))
+
+	for _, key := range fetchKeys {
+		ops = append(ops, v3.OpGet(key, s.getOpts...))
+	}
+	for _, key := range fetchPrefixes {
+		ops = append(ops, v3.OpGet(key, prefixOpts...))
+	}
+
+	txn.Then(ops...)
+	txnresp, err := txn.Commit()
+	s.callCount++
+
+	if err != nil {
+		return
+	}
+
+	// Set revision and serializable options upon first fetch for any
+	// subsequent fetches.
+	if s.getOpts == nil {
+		s.revision = txnresp.Header.Revision
+		s.getOpts = []v3.OpOption{
+			v3.WithRev(s.revision),
+			v3.WithSerializable(),
+		}
+	}
+
+	// Preset keys to "not-present" (revision set to zero).
+	for _, key := range fetchKeys {
+		s.rset.presetItem(key)
+	}
+
+	// Set prefetched keys.
+	s.rset.add(txnresp.Responses[:len(fetchKeys)])
+
+	// Set prefetched ranges.
+	s.rset.addFullRange(fetchPrefixes, txnresp.Responses[len(fetchKeys):])
+}
+
 // commit builds the final transaction and tries to execute it. If commit fails
 // because the keys have changed return a CommitError, otherwise return a
 // DatabaseError.
@@ -774,10 +1058,11 @@ func (s *stm) commit() (CommitStats, error) {
 	txn = txn.If(cmps...)
 	txn = txn.Then(s.wset.puts()...)
 
-	// Prefetch keys in case of conflict to save
+	// Prefetch keys and ranges in case of conflict to save as many
-	// a round trip to etcd.
+	// round-trips as possible.
-	txn = txn.Else(s.rset.gets()...)
+	txn = txn.Else(s.rset.prefetchSet()...)
 
+	s.callCount++
 	txnresp, err := txn.Commit()
 	if err != nil {
 		return stats, DatabaseError{
@@ -786,8 +1071,7 @@ func (s *stm) commit() (CommitStats, error) {
 		}
 	}
 
-	// Call the commit callback if the transaction
+	// Call the commit callback if the transaction was successful.
-	// was successful.
 	if txnresp.Succeeded {
 		if s.onCommit != nil {
 			s.onCommit()
@@ -796,12 +1080,23 @@ func (s *stm) commit() (CommitStats, error) {
 		return stats, nil
 	}
 
-	// Load prefetch before if commit failed.
+	// Determine where our fetched full ranges begin in the response.
-	s.rset.add(txnresp)
+	prefixes := s.rset.getFullRanges()
-	s.prefetch = s.rset
+	firstPrefixResp := len(txnresp.Responses) - len(prefixes)
 
-	// Return CommitError indicating that the transaction
+	// Clear reload and preload it with the prefetched keys and ranges.
-	// can be retried.
+	s.rset.clear()
+	s.rset.add(txnresp.Responses[:firstPrefixResp])
+	s.rset.addFullRange(prefixes, txnresp.Responses[firstPrefixResp:])
+
+	// Set our revision boundary.
+	s.revision = txnresp.Header.Revision
+	s.getOpts = []v3.OpOption{
+		v3.WithRev(s.revision),
+		v3.WithSerializable(),
+	}
+
+	// Return CommitError indicating that the transaction can be retried.
 	return stats, CommitError{}
 }
 
@@ -819,8 +1114,17 @@ func (s *stm) Commit() error {
 // Rollback resets the STM. This is useful for uncommitted transaction rollback
 // and also used in the STM main loop to reset state if commit fails.
 func (s *stm) Rollback() {
-	s.rset = make(map[string]stmGet)
+	s.rollback(true)
-	s.wset = make(map[string]stmPut)
+}
-	s.getOpts = nil
+
-	s.revision = math.MaxInt64 - 1
+// rollback will reset the read and write sets. If clearReadSet is false we'll
+// only reset the the write set.
+func (s *stm) rollback(clearReadSet bool) {
+	if clearReadSet {
+		s.rset.clear()
+		s.revision = math.MaxInt64 - 1
+		s.getOpts = nil
+	}
+
+	s.wset = make(map[string]stmPut)
 }

kvdb/etcd/stm_test.go

@@ -39,8 +39,9 @@ func TestPutToEmpty(t *testing.T) {
 		return nil
 	}
 
-	err = RunSTM(db.cli, apply, txQueue)
+	callCount, err := RunSTM(db.cli, apply, txQueue)
 	require.NoError(t, err)
+	require.Equal(t, 1, callCount)
 
 	require.Equal(t, "abc", f.Get("123"))
 }
@@ -66,6 +67,9 @@ func TestGetPutDel(t *testing.T) {
 		{"e", "5"},
 	}
 
+	// Extra 2 => Get(x), Commit()
+	expectedCallCount := len(testKeyValues) + 2
+
 	for _, kv := range testKeyValues {
 		f.Put(kv.key, kv.val)
 	}
@@ -79,11 +83,12 @@ func TestGetPutDel(t *testing.T) {
 		require.NoError(t, err)
 		require.Nil(t, v)
 
+		// Fetches: 1.
 		v, err = stm.Get("x")
 		require.NoError(t, err)
 		require.Nil(t, v)
 
-		// Get all existing keys.
+		// Get all existing keys. Fetches: len(testKeyValues)
 		for _, kv := range testKeyValues {
 			v, err = stm.Get(kv.key)
 			require.NoError(t, err)
@@ -120,8 +125,9 @@ func TestGetPutDel(t *testing.T) {
 		return nil
 	}
 
-	err = RunSTM(db.cli, apply, txQueue)
+	callCount, err := RunSTM(db.cli, apply, txQueue)
 	require.NoError(t, err)
+	require.Equal(t, expectedCallCount, callCount)
 
 	require.Equal(t, "1", f.Get("a"))
 	require.Equal(t, "2", f.Get("b"))
@@ -134,6 +140,17 @@ func TestGetPutDel(t *testing.T) {
 func TestFirstLastNextPrev(t *testing.T) {
 	t.Parallel()
 
+	testFirstLastNextPrev(t, nil, nil, 41)
+	testFirstLastNextPrev(t, nil, []string{"k"}, 4)
+	testFirstLastNextPrev(t, nil, []string{"k", "w"}, 2)
+	testFirstLastNextPrev(t, []string{"kb"}, nil, 42)
+	testFirstLastNextPrev(t, []string{"kb", "ke"}, nil, 42)
+	testFirstLastNextPrev(t, []string{"kb", "ke", "w"}, []string{"k", "w"}, 2)
+}
+
+func testFirstLastNextPrev(t *testing.T, prefetchKeys []string,
+	prefetchRange []string, expectedCallCount int) {
+
 	f := NewEtcdTestFixture(t)
 	ctx, cancel := context.WithCancel(context.Background())
 
@@ -159,6 +176,8 @@ func TestFirstLastNextPrev(t *testing.T) {
 	require.NoError(t, err)
 
 	apply := func(stm STM) error {
+		stm.Prefetch(prefetchKeys, prefetchRange)
+
 		// First/Last on valid multi item interval.
 		kv, err := stm.First("k")
 		require.NoError(t, err)
@@ -177,11 +196,25 @@ func TestFirstLastNextPrev(t *testing.T) {
 		require.NoError(t, err)
 		require.Equal(t, &KV{"w", "w"}, kv)
 
+		// Non existing.
+		val, err := stm.Get("ke1")
+		require.Nil(t, val)
+		require.Nil(t, err)
+
+		val, err = stm.Get("ke2")
+		require.Nil(t, val)
+		require.Nil(t, err)
+
 		// Next/Prev on start/end.
 		kv, err = stm.Next("k", "ke")
 		require.NoError(t, err)
 		require.Nil(t, kv)
 
+		// Non existing.
+		val, err = stm.Get("ka")
+		require.Nil(t, val)
+		require.Nil(t, err)
+
 		kv, err = stm.Prev("k", "kb")
 		require.NoError(t, err)
 		require.Nil(t, kv)
@@ -277,8 +310,9 @@ func TestFirstLastNextPrev(t *testing.T) {
 		return nil
 	}
 
-	err = RunSTM(db.cli, apply, txQueue)
+	callCount, err := RunSTM(db.cli, apply, txQueue)
 	require.NoError(t, err)
+	require.Equal(t, expectedCallCount, callCount)
 
 	require.Equal(t, "0", f.Get("ka"))
 	require.Equal(t, "2", f.Get("kc"))
@@ -330,9 +364,11 @@ func TestCommitError(t *testing.T) {
 		return nil
 	}
 
-	err = RunSTM(db.cli, apply, txQueue)
+	callCount, err := RunSTM(db.cli, apply, txQueue)
 	require.NoError(t, err)
 	require.Equal(t, 2, cnt)
+	// Get() + 2 * Commit().
+	require.Equal(t, 3, callCount)
 
 	require.Equal(t, "abc", f.Get("123"))
 }

kvdb/etcd_test.go

@@ -135,6 +135,11 @@ func TestEtcd(t *testing.T) {
 			test:       testTxRollback,
 			expectedDb: map[string]string{},
 		},
+		{
+			name:       "prefetch",
+			test:       testPrefetch,
+			expectedDb: map[string]string{},
+		},
 	}
 
 	for _, test := range tests {

kvdb/go.mod

@@ -3,9 +3,12 @@ module github.com/lightningnetwork/lnd/kvdb
 require (
 	github.com/btcsuite/btclog v0.0.0-20170628155309-84c8d2346e9f
 	github.com/btcsuite/btcwallet/walletdb v1.3.6-0.20210803004036-eebed51155ec
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/google/btree v1.0.1
 	github.com/lightningnetwork/lnd/healthcheck v1.0.0
 	github.com/stretchr/testify v1.7.0
 	go.etcd.io/bbolt v1.3.6
+	go.etcd.io/etcd/api/v3 v3.5.0
 	go.etcd.io/etcd/client/pkg/v3 v3.5.0
 	go.etcd.io/etcd/client/v3 v3.5.0
 	go.etcd.io/etcd/server/v3 v3.5.0

kvdb/interface.go

@@ -93,6 +93,42 @@ type RwCursor = walletdb.ReadWriteCursor
 // writes. When only reads are necessary, consider using a RTx instead.
 type RwTx = walletdb.ReadWriteTx
 
+// ExtendedRTx is an extension to walletdb.ReadTx to allow prefetching of keys.
+type ExtendedRTx interface {
+	RTx
+
+	// RootBucket returns the "root bucket" which is pseudo bucket used
+	// when prefetching (keys from) top level buckets.
+	RootBucket() RBucket
+}
+
+// ExtendedRBucket is an extension to walletdb.ReadBucket to allow prefetching
+// of all values inside buckets.
+type ExtendedRBucket interface {
+	RBucket
+
+	// Prefetch will attempt to prefetch all values under a path.
+	Prefetch(paths ...[]string)
+}
+
+// Prefetch will attempt to prefetch all values under a path from the passed
+// bucket.
+func Prefetch(b RBucket, paths ...[]string) {
+	if bucket, ok := b.(ExtendedRBucket); ok {
+		bucket.Prefetch(paths...)
+	}
+}
+
+// RootBucket is a wrapper to ExtendedRTx.RootBucket which does nothing if
+// the implementation doesn't have ExtendedRTx.
+func RootBucket(t RTx) RBucket {
+	if tx, ok := t.(ExtendedRTx); ok {
+		return tx.RootBucket()
+	}
+
+	return nil
+}
+
 var (
 	// ErrBucketNotFound is returned when trying to access a bucket that
 	// has not been created yet.

kvdb/prefetch_test.go

@@ -0,0 +1,188 @@
+package kvdb
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/btcsuite/btcwallet/walletdb"
+	"github.com/davecgh/go-spew/spew"
+	"github.com/stretchr/testify/require"
+)
+
+func fetchBucket(t *testing.T, bucket walletdb.ReadBucket) map[string]string {
+	items := make(map[string]string)
+	err := bucket.ForEach(func(k, v []byte) error {
+		if v != nil {
+			items[string(k)] = string(v)
+		}
+
+		return nil
+	})
+	require.NoError(t, err)
+
+	return items
+}
+
+func alterBucket(t *testing.T, bucket walletdb.ReadWriteBucket,
+	put map[string]string, remove []string) {
+
+	for k, v := range put {
+		require.NoError(t, bucket.Put([]byte(k), []byte(v)))
+	}
+
+	for _, k := range remove {
+		require.NoError(t, bucket.Delete([]byte(k)))
+	}
+}
+
+func prefetchTest(t *testing.T, db walletdb.DB,
+	prefetchAt []bool, put map[string]string, remove []string) {
+
+	prefetch := func(i int, tx walletdb.ReadTx) {
+		require.Less(t, i, len(prefetchAt))
+		if prefetchAt[i] {
+			Prefetch(
+				RootBucket(tx),
+				[]string{"top"}, []string{"top", "bucket"},
+			)
+		}
+	}
+
+	items := map[string]string{
+		"a": "1",
+		"b": "2",
+		"c": "3",
+		"d": "4",
+		"e": "5",
+	}
+
+	err := Update(db, func(tx walletdb.ReadWriteTx) error {
+		top, err := tx.CreateTopLevelBucket([]byte("top"))
+		require.NoError(t, err)
+		require.NotNil(t, top)
+
+		for k, v := range items {
+			require.NoError(t, top.Put([]byte(k), []byte(v)))
+		}
+
+		bucket, err := top.CreateBucket([]byte("bucket"))
+		require.NoError(t, err)
+		require.NotNil(t, bucket)
+
+		for k, v := range items {
+			require.NoError(t, bucket.Put([]byte(k), []byte(v)))
+		}
+
+		return nil
+	}, func() {})
+	require.NoError(t, err)
+
+	for k, v := range put {
+		items[k] = v
+	}
+
+	for _, k := range remove {
+		delete(items, k)
+	}
+
+	err = Update(db, func(tx walletdb.ReadWriteTx) error {
+		prefetch(0, tx)
+		top := tx.ReadWriteBucket([]byte("top"))
+		require.NotNil(t, top)
+		alterBucket(t, top, put, remove)
+
+		prefetch(1, tx)
+		require.Equal(t, items, fetchBucket(t, top))
+
+		prefetch(2, tx)
+		bucket := top.NestedReadWriteBucket([]byte("bucket"))
+		require.NotNil(t, bucket)
+		alterBucket(t, bucket, put, remove)
+
+		prefetch(3, tx)
+		require.Equal(t, items, fetchBucket(t, bucket))
+
+		return nil
+	}, func() {})
+	require.NoError(t, err)
+
+	err = Update(db, func(tx walletdb.ReadWriteTx) error {
+		return tx.DeleteTopLevelBucket([]byte("top"))
+	}, func() {})
+	require.NoError(t, err)
+}
+
+// testPrefetch tests that prefetching buckets works as expected even when the
+// prefetch happens multiple times and the bucket contents change. Our expectation
+// is that with or without prefetches, the kvdb layer works accourding to the
+// interface specification.
+func testPrefetch(t *testing.T, db walletdb.DB) {
+	tests := []struct {
+		put    map[string]string
+		remove []string
+	}{
+		{
+			put:    nil,
+			remove: nil,
+		},
+		{
+			put: map[string]string{
+				"a":   "a",
+				"aa":  "aa",
+				"aaa": "aaa",
+				"x":   "x",
+				"y":   "y",
+			},
+			remove: nil,
+		},
+		{
+			put: map[string]string{
+				"a":   "a",
+				"aa":  "aa",
+				"aaa": "aaa",
+				"x":   "x",
+				"y":   "y",
+			},
+			remove: []string{"a", "c", "d"},
+		},
+		{
+			put:    nil,
+			remove: []string{"b", "d"},
+		},
+	}
+
+	prefetchAt := [][]bool{
+		{false, false, false, false},
+		{true, false, false, false},
+		{false, true, false, false},
+		{false, false, true, false},
+		{false, false, false, true},
+		{true, true, false, false},
+		{true, true, true, false},
+		{true, true, true, true},
+		{true, false, true, true},
+		{true, false, false, true},
+		{true, false, true, false},
+	}
+
+	for i, test := range tests {
+		test := test
+
+		for j := 0; j < len(prefetchAt); j++ {
+			if !t.Run(
+				fmt.Sprintf("prefetch %d %d", i, j),
+				func(t *testing.T) {
+					prefetchTest(
+						t, db, prefetchAt[j], test.put,
+						test.remove,
+					)
+				}) {
+
+				fmt.Printf("Prefetch test (%d, %d) failed:\n"+
+					"testcase=%v\n prefetch=%v\n",
+					i, j, spew.Sdump(test),
+					spew.Sdump(prefetchAt[j]))
+			}
+		}
+	}
+}