diff --git a/pkg/sorted/buffer/buffer.go b/pkg/sorted/buffer/buffer.go
index df940ec36..34c1a6d75 100644
--- a/pkg/sorted/buffer/buffer.go
+++ b/pkg/sorted/buffer/buffer.go
@@ -33,7 +33,7 @@ import (
 // maxBufferBytes <= 0, no automatic flushing is performed.
 func New(buffer, backing sorted.KeyValue, maxBufferBytes int64) *KeyValue {
 	return &KeyValue{
-		buffer:    buffer,
+		buf:       buffer,
 		back:      backing,
 		maxBuffer: maxBufferBytes,
 	}
@@ -42,19 +42,52 @@ func New(buffer, backing sorted.KeyValue, maxBufferBytes int64) *KeyValue {
 var _ sorted.KeyValue = (*KeyValue)(nil)
 
 type KeyValue struct {
-	buffer, back sorted.KeyValue
-	maxBuffer    int64
+	buf, back sorted.KeyValue
+	maxBuffer int64
 
-	mu       sync.Mutex
+	bufMu    sync.Mutex
 	buffered int64
+
+	// This read lock should be held during Set/Get/Delete/BatchCommit,
+	// and the write lock should be held during Flush.
+	mu sync.RWMutex
 }
 
 func (kv *KeyValue) Flush() error {
-	panic("TODO: implement Flush")
+	kv.mu.Lock()
+	defer kv.mu.Unlock()
+	var (
+		bmback = kv.back.BeginBatch()
+		bmbuf  = kv.buf.BeginBatch()
+		commit = false
+		it     = kv.buf.Find("", "")
+	)
+	for it.Next() {
+		bmback.Set(it.Key(), it.Value())
+		bmbuf.Delete(it.Key())
+		commit = true
+	}
+	if err := it.Close(); err != nil {
+		return err
+	}
+	if commit {
+		if err := kv.back.CommitBatch(bmback); err != nil {
+			return err
+		}
+		if err := kv.buf.CommitBatch(bmbuf); err != nil {
+			return err
+		}
+		kv.bufMu.Lock()
+		kv.buffered = 0
+		kv.bufMu.Unlock()
+	}
+	return nil
 }
 
 func (kv *KeyValue) Get(key string) (string, error) {
-	v, err := kv.buffer.Get(key)
+	kv.mu.RLock()
+	defer kv.mu.RUnlock()
+	v, err := kv.buf.Get(key)
 	switch err {
 	case sorted.ErrNotFound:
 		break
@@ -67,17 +100,31 @@ func (kv *KeyValue) Get(key string) (string, error) {
 }
 
 func (kv *KeyValue) Set(key, value string) error {
-	return kv.buffer.Set(key, value)
+	kv.mu.RLock()
+	err := kv.buf.Set(key, value)
+	kv.mu.RUnlock()
+	if err == nil {
+		kv.bufMu.Lock()
+		kv.buffered += int64(len(key) + len(value))
+		doFlush := kv.buffered > kv.maxBuffer
+		kv.bufMu.Unlock()
+		if doFlush {
+			err = kv.Flush()
+		}
+	}
+	return err
 }
 
 func (kv *KeyValue) Delete(key string) error {
+	kv.mu.RLock()
+	defer kv.mu.RUnlock()
 	// This isn't an ideal implementation, since it synchronously
 	// deletes from the backing store. But deletes aren't really
 	// used, so ignoring for now.
 	// Could also use a syncutil.Group to do these in parallel,
 	// but the buffer should be an in-memory implementation
 	// anyway, so should be fast.
-	err1 := kv.buffer.Delete(key)
+	err1 := kv.buf.Delete(key)
 	err2 := kv.back.Delete(key)
 	if err1 != nil {
 		return err1
@@ -90,12 +137,37 @@ func (kv *KeyValue) BeginBatch() sorted.BatchMutation {
 }
 
 func (kv *KeyValue) CommitBatch(bm sorted.BatchMutation) error {
+	kv.mu.RLock()
+	defer kv.mu.RUnlock()
 	b, ok := bm.(*batch)
 	if !ok {
 		return fmt.Errorf("unexpected BatchMutation type %T", bm)
 	}
-	_ = b
-	panic("TODO")
+	var (
+		// A batch mutation for applying this mutation to the buffer.
+		bmbuf = kv.buf.BeginBatch()
+		// A lazily created batch mutation for deleting from the backing
+		// storage; this should be rare. (See Delete above.)
+		bmback sorted.BatchMutation
+	)
+	for _, m := range b.mods {
+		if m.isDelete {
+			bmbuf.Delete(m.key)
+			if bmback == nil {
+				bmback = kv.back.BeginBatch()
+			}
+			bmback.Delete(m.key)
+			continue
+		}
+		bmbuf.Set(m.key, m.value)
+	}
+	if err := kv.buf.CommitBatch(bmbuf); err != nil {
+		return err
+	}
+	if bmback != nil {
+		return kv.back.CommitBatch(bmback)
+	}
+	return nil
 }
 
 func (kv *KeyValue) Close() error {
@@ -106,11 +178,12 @@ func (kv *KeyValue) Close() error {
 }
 
 func (kv *KeyValue) Find(start, end string) sorted.Iterator {
-	ibuf := kv.buffer.Find(start, end)
+	// TODO(adg): hold read lock while iterating? seems complicated
+	ibuf := kv.buf.Find(start, end)
 	iback := kv.back.Find(start, end)
 	return &iter{
-		ibuf:  ibuf,
-		iback: iback,
+		buf:  subIter{Iterator: ibuf},
+		back: subIter{Iterator: iback},
 	}
 }
 
@@ -137,23 +210,103 @@ func (b *batch) Delete(key string) {
 }
 
 type iter struct {
-	ibuf    sorted.Iterator
-	iback   sorted.Iterator
-	bufEOF  bool
-	backEOF bool
+	buf, back subIter
 }
 
-func (it *iter) Key() string        { panic("TODO") }
-func (it *iter) Value() string      { panic("TODO") }
-func (it *iter) KeyBytes() []byte   { panic("TODO") }
-func (it *iter) ValueBytes() []byte { panic("TODO") }
-func (it *iter) Next() bool         { panic("TODO") }
+func (it *iter) current() *subIter {
+	switch {
+	case it.back.eof:
+		return &it.buf
+	case it.buf.eof:
+		return &it.back
+	case it.buf.key <= it.back.key:
+		return &it.buf
+	default:
+		return &it.back
+	}
+}
+
+func (it *iter) Next() bool {
+	// Call Next on both iterators for the first time, if we haven't
+	// already, so that the key comparisons below are valid.
+	start := false
+	if it.buf.key == "" && !it.buf.eof {
+		start = it.buf.next()
+	}
+	if it.back.key == "" && !it.buf.eof {
+		start = it.back.next() || start
+	}
+	if start {
+		// We started iterating with at least one value.
+		return true
+	}
+	// Bail if both iterators are done.
+	if it.buf.eof && it.back.eof {
+		return false
+	}
+	// If one iterator is done, advance the other.
+	if it.buf.eof {
+		return it.back.next()
+	}
+	if it.back.eof {
+		return it.buf.next()
+	}
+	// If both iterators still going,
+	// advance the one that is further behind,
+	// or both simultaneously if they point to the same key.
+	switch {
+	case it.buf.key < it.back.key:
+		it.buf.next()
+	case it.buf.key > it.back.key:
+		it.back.next()
+	case it.buf.key == it.back.key:
+		n1, n2 := it.buf.next(), it.back.next()
+		if !n1 && !n2 {
+			// Both finished simultaneously.
+			return false
+		}
+	}
+	return true
+}
+
+func (it *iter) Key() string {
+	return it.current().key
+}
+
+func (it *iter) Value() string {
+	return it.current().Value()
+}
+
+func (it *iter) KeyBytes() []byte {
+	return it.current().KeyBytes()
+}
+
+func (it *iter) ValueBytes() []byte {
+	return it.current().ValueBytes()
+}
 
 func (it *iter) Close() error {
-	err1 := it.ibuf.Close()
-	err2 := it.iback.Close()
+	err1 := it.buf.Close()
+	err2 := it.back.Close()
 	if err1 != nil {
 		return err1
 	}
 	return err2
 }
+
+// subIter is an iterator (either the backing storage or the buffer) that
+// keeps track of the current key and whether it has reached EOF.
+type subIter struct {
+	sorted.Iterator
+	key string
+	eof bool
+}
+
+func (it *subIter) next() bool {
+	if it.Next() {
+		it.key = it.Key()
+		return true
+	}
+	it.eof = true
+	return false
+}
diff --git a/pkg/sorted/buffer/buffer_test.go b/pkg/sorted/buffer/buffer_test.go
new file mode 100644
index 000000000..dbb646485
--- /dev/null
+++ b/pkg/sorted/buffer/buffer_test.go
@@ -0,0 +1,104 @@
+/*
+Copyright 2014 The Camlistore Authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+     http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package buffer
+
+import (
+	"testing"
+
+	"camlistore.org/pkg/sorted"
+)
+
+// TODO(adg): test batch mutations
+// TODO(adg): test auto-flush behavior
+
+func TestBuffer(t *testing.T) {
+	var (
+		toBack = []mod{
+			{false, "b", "b1"},
+			{false, "d", "d1"},
+			{false, "f", "f1"},
+		}
+		toBuf = []mod{
+			{false, "a", "a2"},
+			{false, "b", "b2"},
+			{false, "c", "c2"},
+			{false, "e", "e2"},
+			{true, "f", ""},
+			{false, "g", "g2"},
+		}
+		backBeforeFlush = []mod{
+			{false, "b", "b1"},
+			{false, "d", "d1"},
+			// f deleted
+		}
+		want = []mod{
+			{false, "a", "a2"},
+			{false, "b", "b2"},
+			{false, "c", "c2"},
+			{false, "d", "d1"},
+			{false, "e", "e2"},
+			// f deleted
+			{false, "g", "g2"},
+		}
+	)
+
+	// Populate backing storage.
+	backing := sorted.NewMemoryKeyValue()
+	for _, m := range toBack {
+		backing.Set(m.key, m.value)
+	}
+	// Wrap with buffered storage, populate.
+	buf := New(sorted.NewMemoryKeyValue(), backing, 1<<20)
+	for _, m := range toBuf {
+		if m.isDelete {
+			buf.Delete(m.key)
+		} else {
+			buf.Set(m.key, m.value)
+		}
+	}
+
+	// Check contents of buffered storage.
+	check(t, buf, "buffered", want)
+	check(t, backing, "backing before flush", backBeforeFlush)
+
+	// Flush.
+	if err := buf.Flush(); err != nil {
+		t.Fatal("flush error: ", err)
+	}
+
+	// Check contents of backing storage.
+	check(t, backing, "backing after flush", want)
+}
+
+func check(t *testing.T, kv sorted.KeyValue, prefix string, want []mod) {
+	it := kv.Find("", "")
+	for i, m := range want {
+		if !it.Next() {
+			t.Fatalf("%v: unexpected it.Next == false on iteration %d", prefix, i)
+		}
+		if k, v := it.Key(), it.Value(); k != m.key || v != m.value {
+			t.Errorf("%v: got key == %q value == %q, want key == %q value == %q on iteration %d",
+				prefix, k, v, m.key, m.value, i)
+		}
+	}
+	if it.Next() {
+		t.Errorf("%v: unexpected it.Next == true after complete iteration", prefix)
+	}
+	if err := it.Close(); err != nil {
+		t.Errorf("%v: error closing iterator: %v", prefix, err)
+	}
+}