consul: First pass to reduce KV watch cost

consul/state_store.go

@@ -11,6 +11,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/armon/go-radix"
 	"github.com/armon/gomdb"
 	"github.com/hashicorp/consul/consul/structs"
 )
@@ -63,6 +64,14 @@ type StateStore struct {
 	watch             map[*MDBTable]*NotifyGroup
 	queryTables       map[string]MDBTables
 
+	// kvWatch is a more optimized way of watching for KV changes.
+	// Instead of just using a NotifyGroup for the entire table,
+	// a watcher is instantiated on a given prefix. When a change happens,
+	// only the relevant watchers are woken up. This reduces the cost of
+	// watching for KV changes.
+	kvWatch     *radix.Tree
+	kvWatchLock sync.Mutex
+
 	// lockDelay is used to mark certain locks as unacquirable.
 	// When a lock is forcefully released (failing health
 	// check, destroyed session, etc), it is subject to the LockDelay
@@ -131,6 +140,7 @@ func NewStateStorePath(gc *TombstoneGC, path string, logOutput io.Writer) (*Stat
 		path:      path,
 		env:       env,
 		watch:     make(map[*MDBTable]*NotifyGroup),
+		kvWatch:   radix.New(),
 		lockDelay: make(map[string]time.Time),
 		gc:        gc,
 	}
@@ -414,6 +424,58 @@ func (s *StateStore) Watch(tables MDBTables, notify chan struct{}) {
 	}
 }
 
+// WatchKV is used to subscribe a channel to changes in KV data
+func (s *StateStore) WatchKV(prefix string, notify chan struct{}) {
+	s.kvWatchLock.Lock()
+	defer s.kvWatchLock.Unlock()
+
+	// Check for an existing notify group
+	if raw, ok := s.kvWatch.Get(prefix); ok {
+		grp := raw.(*NotifyGroup)
+		grp.Wait(notify)
+		return
+	}
+
+	// Create new notify group
+	grp := &NotifyGroup{}
+	grp.Wait(notify)
+	s.kvWatch.Insert(prefix, grp)
+}
+
+// notifyKV is used to notify any KV listeners of a change
+// on a prefix
+func (s *StateStore) notifyKV(path string, prefix bool) {
+	// Backwards compatibility for old listeners
+	s.watch[s.kvsTable].Notify()
+
+	s.kvWatchLock.Lock()
+	defer s.kvWatchLock.Unlock()
+
+	var toDelete []string
+	fn := func(s string, v interface{}) bool {
+		group := v.(*NotifyGroup)
+		group.Notify()
+		if s != "" {
+			toDelete = append(toDelete, s)
+		}
+		return false
+	}
+
+	// Invoke any watcher on the path downward to the key.
+	s.kvWatch.WalkPath(path, fn)
+
+	// If the entire prefix may be affected (e.g. delete tree),
+	// invoke the entire prefix
+	if prefix {
+		s.kvWatch.WalkPrefix(path, fn)
+	}
+
+	// Delete the old watch groups
+	for i := len(toDelete) - 1; i >= 0; i-- {
+		s.kvWatch.Delete(toDelete[i])
+	}
+}
+
 // QueryTables returns the Tables that are queried for a given query
 func (s *StateStore) QueryTables(q string) MDBTables {
 	return s.queryTables[q]
@@ -1298,7 +1360,17 @@ func (s *StateStore) kvsDeleteWithIndexTxn(index uint64, tx *MDBTxn, tableIndex
 			return err
 		}
 		tx.Defer(func() {
-			s.watch[s.kvsTable].Notify()
+			// Trigger the most fine grained notifications if possible
+			switch {
+			case len(parts) == 0:
+				s.notifyKV("", true)
+			case tableIndex == "id":
+				s.notifyKV(parts[0], false)
+			case tableIndex == "id_prefix":
+				s.notifyKV(parts[0], true)
+			default:
+				s.notifyKV("", true)
+			}
 			if s.gc != nil {
 				// If GC is configured, then we hint that this index
 				// required expiration.
@@ -1426,7 +1498,7 @@ func (s *StateStore) kvsSet(
 	if err := s.kvsTable.SetLastIndexTxn(tx, index); err != nil {
 		return false, err
 	}
-	tx.Defer(func() { s.watch[s.kvsTable].Notify() })
+	tx.Defer(func() { s.notifyKV(d.Key, false) })
 	return true, tx.Commit()
 }
 
@@ -1785,12 +1857,12 @@ func (s *StateStore) invalidateLocks(index uint64, tx *MDBTxn,
 				s.lockDelayLock.Unlock()
 			})
 		}
+		tx.Defer(func() { s.notifyKV(kv.Key, false) })
 	}
 	if len(pairs) > 0 {
 		if err := s.kvsTable.SetLastIndexTxn(tx, index); err != nil {
 			return err
 		}
-		tx.Defer(func() { s.watch[s.kvsTable].Notify() })
 	}
 	return nil
 }