diff --git a/kv/compactor.go b/kv/compactor.go
deleted file mode 100644
index 1687225e82..0000000000
--- a/kv/compactor.go
+++ /dev/null
@@ -1,40 +0,0 @@
-// Copyright 2015 PingCAP, Inc.
-//
-// 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,
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-package kv
-
-import "time"
-
-// CompactPolicy defines gc policy of MVCC storage.
-type CompactPolicy struct {
-	// SafePoint specifies
-	SafePoint int
-	// TriggerInterval specifies how often should the compactor
-	// scans outdated data.
-	TriggerInterval time.Duration
-	// BatchDeleteCnt specifies the batch size for
-	// deleting outdated data transaction.
-	BatchDeleteCnt int
-}
-
-// Compactor compacts MVCC storage.
-type Compactor interface {
-	// OnGet is the hook point on Txn.Get.
-	OnGet(k Key)
-	// OnSet is the hook point on Txn.Set.
-	OnSet(k Key)
-	// OnDelete is the hook point on Txn.Delete.
-	OnDelete(k Key)
-	// Compact is the function removes the given key.
-	Compact(k Key) error
-}
diff --git a/kv/error.go b/kv/error.go
new file mode 100644
index 0000000000..23523a1e16
--- /dev/null
+++ b/kv/error.go
@@ -0,0 +1,75 @@
+// Copyright 2015 PingCAP, Inc.
+//
+// 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,
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package kv
+
+import (
+	"errors"
+	"strings"
+
+	"github.com/pingcap/go-themis"
+	"github.com/pingcap/tidb/terror"
+)
+
+var (
+	// ErrClosed is used when close an already closed txn.
+	ErrClosed = errors.New("Error: Transaction already closed")
+	// ErrNotExist is used when try to get an entry with an unexist key from KV store.
+	ErrNotExist = errors.New("Error: key not exist")
+	// ErrKeyExists is used when try to put an entry to KV store.
+	ErrKeyExists = errors.New("Error: key already exist")
+	// ErrConditionNotMatch is used when condition is not met.
+	ErrConditionNotMatch = errors.New("Error: Condition not match")
+	// ErrLockConflict is used when try to lock an already locked key.
+	ErrLockConflict = errors.New("Error: Lock conflict")
+	// ErrLazyConditionPairsNotMatch is used when value in store differs from expect pairs.
+	ErrLazyConditionPairsNotMatch = errors.New("Error: Lazy condition pairs not match")
+	// ErrRetryable is used when KV store occurs RPC error or some other
+	// errors which SQL layer can safely retry.
+	ErrRetryable = errors.New("Error: KV error safe to retry")
+	// ErrCannotSetNilValue is the error when sets an empty value.
+	ErrCannotSetNilValue = errors.New("can not set nil value")
+	// ErrInvalidTxn is the error when commits or rollbacks in an invalid transaction.
+	ErrInvalidTxn = errors.New("invalid transaction")
+
+	// ErrNotCommitted is the error returned by CommitVersion when this
+	// transaction is not committed.
+	ErrNotCommitted = errors.New("this transaction has not committed")
+)
+
+// IsRetryableError checks if the err is a fatal error and the under going operation is worth to retry.
+func IsRetryableError(err error) bool {
+	if err == nil {
+		return false
+	}
+
+	if terror.ErrorEqual(err, ErrRetryable) ||
+		terror.ErrorEqual(err, ErrLockConflict) ||
+		terror.ErrorEqual(err, ErrConditionNotMatch) ||
+		terror.ErrorEqual(err, themis.ErrRetryable) ||
+		// HBase exception message will tell you if you should retry or not
+		strings.Contains(err.Error(), "try again later") {
+		return true
+	}
+
+	return false
+}
+
+// IsErrNotFound checks if err is a kind of NotFound error.
+func IsErrNotFound(err error) bool {
+	if terror.ErrorEqual(err, ErrNotExist) {
+		return true
+	}
+
+	return false
+}
diff --git a/kv/iter.go b/kv/iter.go
index b65d4e963e..d2d4dd7dcd 100644
--- a/kv/iter.go
+++ b/kv/iter.go
@@ -15,28 +15,6 @@ package kv
 
 import "github.com/juju/errors"
 
-var (
-	// ErrClosed is used when close an already closed txn.
-	ErrClosed = errors.New("Error: Transaction already closed")
-	// ErrNotExist is used when try to get an entry with an unexist key from KV store.
-	ErrNotExist = errors.New("Error: key not exist")
-	// ErrKeyExists is used when try to put an entry to KV store.
-	ErrKeyExists = errors.New("Error: key already exist")
-	// ErrConditionNotMatch is used when condition is not met.
-	ErrConditionNotMatch = errors.New("Error: Condition not match")
-	// ErrLockConflict is used when try to lock an already locked key.
-	ErrLockConflict = errors.New("Error: Lock conflict")
-	// ErrLazyConditionPairsNotMatch is used when value in store differs from expect pairs.
-	ErrLazyConditionPairsNotMatch = errors.New("Error: Lazy condition pairs not match")
-	// ErrRetryable is used when KV store occurs RPC error or some other
-	// errors which SQL layer can safely retry.
-	ErrRetryable = errors.New("Error: KV error safe to retry")
-	// ErrCannotSetNilValue is the error when sets an empty value.
-	ErrCannotSetNilValue = errors.New("can not set nil value")
-	// ErrInvalidTxn is the error when commits or rollbacks in an invalid transaction.
-	ErrInvalidTxn = errors.New("invalid transaction")
-)
-
 // NextUntil applies FnKeyCmp to each entry of the iterator until meets some condition.
 // It will stop when fn returns true, or iterator is invalid or an error occurs.
 func NextUntil(it Iterator, fn FnKeyCmp) error {
diff --git a/kv/key.go b/kv/key.go
new file mode 100644
index 0000000000..d76f505fd3
--- /dev/null
+++ b/kv/key.go
@@ -0,0 +1,47 @@
+// Copyright 2015 PingCAP, Inc.
+//
+// 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,
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package kv
+
+import "bytes"
+
+// Key represents high-level Key type.
+type Key []byte
+
+// Next returns the next key in byte-order.
+func (k Key) Next() Key {
+	// add 0x0 to the end of key
+	buf := make([]byte, len([]byte(k))+1)
+	copy(buf, []byte(k))
+	return buf
+}
+
+// Cmp returns the comparison result of two key.
+// The result will be 0 if a==b, -1 if a < b, and +1 if a > b.
+func (k Key) Cmp(another Key) int {
+	return bytes.Compare(k, another)
+}
+
+// EncodedKey represents encoded key in low-level storage engine.
+type EncodedKey []byte
+
+// Cmp returns the comparison result of two key.
+// The result will be 0 if a==b, -1 if a < b, and +1 if a > b.
+func (k EncodedKey) Cmp(another EncodedKey) int {
+	return bytes.Compare(k, another)
+}
+
+// Next returns the next key in byte-order.
+func (k EncodedKey) Next() EncodedKey {
+	return EncodedKey(bytes.Join([][]byte{k, Key{0}}, nil))
+}
diff --git a/kv/kv.go b/kv/kv.go
index fc918338cb..fab9416c82 100644
--- a/kv/kv.go
+++ b/kv/kv.go
@@ -13,98 +13,6 @@
 
 package kv
 
-import (
-	"bytes"
-	"math"
-
-	"github.com/juju/errors"
-)
-
-// Key represents high-level Key type.
-type Key []byte
-
-// Next returns the next key in byte-order.
-func (k Key) Next() Key {
-	// add 0x0 to the end of key
-	buf := make([]byte, len([]byte(k))+1)
-	copy(buf, []byte(k))
-	return buf
-}
-
-// Cmp returns the comparison result of two key.
-// The result will be 0 if a==b, -1 if a < b, and +1 if a > b.
-func (k Key) Cmp(another Key) int {
-	return bytes.Compare(k, another)
-}
-
-// EncodedKey represents encoded key in low-level storage engine.
-type EncodedKey []byte
-
-// Cmp returns the comparison result of two key.
-// The result will be 0 if a==b, -1 if a < b, and +1 if a > b.
-func (k EncodedKey) Cmp(another EncodedKey) int {
-	return bytes.Compare(k, another)
-}
-
-// Next returns the next key in byte-order.
-func (k EncodedKey) Next() EncodedKey {
-	return EncodedKey(bytes.Join([][]byte{k, Key{0}}, nil))
-}
-
-// VersionProvider provides increasing IDs.
-type VersionProvider interface {
-	CurrentVersion() (Version, error)
-}
-
-// Version is the wrapper of KV's version.
-type Version struct {
-	Ver uint64
-}
-
-var (
-	// MaxVersion is the maximum version, notice that it's not a valid version.
-	MaxVersion = Version{Ver: math.MaxUint64}
-	// MinVersion is the minimum version, it's not a valid version, too.
-	MinVersion = Version{Ver: 0}
-)
-
-// NewVersion creates a new Version struct.
-func NewVersion(v uint64) Version {
-	return Version{
-		Ver: v,
-	}
-}
-
-// Cmp returns the comparison result of two versions.
-// The result will be 0 if a==b, -1 if a < b, and +1 if a > b.
-func (v Version) Cmp(another Version) int {
-	if v.Ver > another.Ver {
-		return 1
-	} else if v.Ver < another.Ver {
-		return -1
-	}
-	return 0
-}
-
-// DecodeFn is a function that decodes data after fetching from store.
-type DecodeFn func(raw interface{}) (interface{}, error)
-
-// EncodeFn is a function that encodes data before putting into store.
-type EncodeFn func(raw interface{}) (interface{}, error)
-
-// ErrNotCommitted is the error returned by CommitVersion when this
-// transaction is not committed.
-var ErrNotCommitted = errors.New("this transaction is not committed")
-
-// Option is used for customizing kv store's behaviors during a transaction.
-type Option int
-
-// Options is an interface of a set of options. Each option is associated with a value.
-type Options interface {
-	// Get gets an option value.
-	Get(opt Option) (v interface{}, ok bool)
-}
-
 const (
 	// RangePrefetchOnCacheMiss directives that when dealing with a Get operation but failing to read data from cache,
 	// it will launch a RangePrefetch to underlying storage instead of Get. The range starts from requested key and
@@ -152,33 +60,6 @@ type MemBuffer interface {
 	Release()
 }
 
-// UnionStore is a store that wraps a snapshot for read and a BufferStore for buffered write.
-// Also, it provides some transaction related utilities.
-type UnionStore interface {
-	MemBuffer
-	// Inc increases the value for key k in KV storage by step.
-	Inc(k Key, step int64) (int64, error)
-	// GetInt64 get int64 which created by Inc method.
-	GetInt64(k Key) (int64, error)
-	// CheckLazyConditionPairs loads all lazy values from store then checks if all values are matched.
-	// Lazy condition pairs should be checked before transaction commit.
-	CheckLazyConditionPairs() error
-	// BatchPrefetch fetches values from KV storage to cache for later use.
-	BatchPrefetch(keys []Key) error
-	// RangePrefetch fetches values in the range [start, end] from KV storage
-	// to cache for later use. Maximum number of values is up to limit.
-	RangePrefetch(start, end Key, limit int) error
-	// WalkBuffer iterates all buffered kv pairs.
-	WalkBuffer(f func(k Key, v []byte) error) error
-	// SetOption sets an option with a value, when val is nil, uses the default
-	// value of this option.
-	SetOption(opt Option, val interface{})
-	// DelOption deletes an option.
-	DelOption(opt Option)
-	// ReleaseSnapshot releases underlying snapshot.
-	ReleaseSnapshot()
-}
-
 // Transaction defines the interface for operations inside a Transaction.
 // This is not thread safe.
 type Transaction interface {
diff --git a/kv/txn.go b/kv/txn.go
index 2bf483970d..197a6fc5c5 100644
--- a/kv/txn.go
+++ b/kv/txn.go
@@ -16,33 +16,12 @@ package kv
 import (
 	"math"
 	"math/rand"
-	"strings"
 	"time"
 
 	"github.com/juju/errors"
 	"github.com/ngaut/log"
-	"github.com/pingcap/go-themis"
-	"github.com/pingcap/tidb/terror"
 )
 
-// IsRetryableError checks if the err is a fatal error and the under going operation is worth to retry.
-func IsRetryableError(err error) bool {
-	if err == nil {
-		return false
-	}
-
-	if terror.ErrorEqual(err, ErrRetryable) ||
-		terror.ErrorEqual(err, ErrLockConflict) ||
-		terror.ErrorEqual(err, ErrConditionNotMatch) ||
-		terror.ErrorEqual(err, themis.ErrRetryable) ||
-		// HBase exception message will tell you if you should retry or not
-		strings.Contains(err.Error(), "try again later") {
-		return true
-	}
-
-	return false
-}
-
 // RunInNewTxn will run the f in a new transaction environment.
 func RunInNewTxn(store Storage, retryable bool, f func(txn Transaction) error) error {
 	for i := 0; i < maxRetryCnt; i++ {
diff --git a/kv/union_store.go b/kv/union_store.go
index 7e11b13610..d286e5786c 100644
--- a/kv/union_store.go
+++ b/kv/union_store.go
@@ -19,24 +19,50 @@ import (
 
 	"github.com/juju/errors"
 	"github.com/ngaut/pool"
-	"github.com/pingcap/tidb/terror"
 )
 
+// UnionStore is a store that wraps a snapshot for read and a BufferStore for buffered write.
+// Also, it provides some transaction related utilities.
+type UnionStore interface {
+	MemBuffer
+	// Inc increases the value for key k in KV storage by step.
+	Inc(k Key, step int64) (int64, error)
+	// GetInt64 get int64 which created by Inc method.
+	GetInt64(k Key) (int64, error)
+	// CheckLazyConditionPairs loads all lazy values from store then checks if all values are matched.
+	// Lazy condition pairs should be checked before transaction commit.
+	CheckLazyConditionPairs() error
+	// BatchPrefetch fetches values from KV storage to cache for later use.
+	BatchPrefetch(keys []Key) error
+	// RangePrefetch fetches values in the range [start, end] from KV storage
+	// to cache for later use. Maximum number of values is up to limit.
+	RangePrefetch(start, end Key, limit int) error
+	// WalkBuffer iterates all buffered kv pairs.
+	WalkBuffer(f func(k Key, v []byte) error) error
+	// SetOption sets an option with a value, when val is nil, uses the default
+	// value of this option.
+	SetOption(opt Option, val interface{})
+	// DelOption deletes an option.
+	DelOption(opt Option)
+	// ReleaseSnapshot releases underlying snapshot.
+	ReleaseSnapshot()
+}
+
+// Option is used for customizing kv store's behaviors during a transaction.
+type Option int
+
+// Options is an interface of a set of options. Each option is associated with a value.
+type Options interface {
+	// Get gets an option value.
+	Get(opt Option) (v interface{}, ok bool)
+}
+
 var (
 	p = pool.NewCache("memdb pool", 100, func() interface{} {
 		return NewMemDbBuffer()
 	})
 )
 
-// IsErrNotFound checks if err is a kind of NotFound error.
-func IsErrNotFound(err error) bool {
-	if terror.ErrorEqual(err, ErrNotExist) {
-		return true
-	}
-
-	return false
-}
-
 // UnionStore is an in-memory Store which contains a buffer for write and a
 // snapshot for read.
 type unionStore struct {
diff --git a/kv/version.go b/kv/version.go
new file mode 100644
index 0000000000..f009215863
--- /dev/null
+++ b/kv/version.go
@@ -0,0 +1,51 @@
+// Copyright 2015 PingCAP, Inc.
+//
+// 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,
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package kv
+
+import "math"
+
+// VersionProvider provides increasing IDs.
+type VersionProvider interface {
+	CurrentVersion() (Version, error)
+}
+
+// Version is the wrapper of KV's version.
+type Version struct {
+	Ver uint64
+}
+
+var (
+	// MaxVersion is the maximum version, notice that it's not a valid version.
+	MaxVersion = Version{Ver: math.MaxUint64}
+	// MinVersion is the minimum version, it's not a valid version, too.
+	MinVersion = Version{Ver: 0}
+)
+
+// NewVersion creates a new Version struct.
+func NewVersion(v uint64) Version {
+	return Version{
+		Ver: v,
+	}
+}
+
+// Cmp returns the comparison result of two versions.
+// The result will be 0 if a==b, -1 if a < b, and +1 if a > b.
+func (v Version) Cmp(another Version) int {
+	if v.Ver > another.Ver {
+		return 1
+	} else if v.Ver < another.Ver {
+		return -1
+	}
+	return 0
+}
diff --git a/store/localstore/compactor.go b/store/localstore/compactor.go
index 2a70276261..97698fd831 100644
--- a/store/localstore/compactor.go
+++ b/store/localstore/compactor.go
@@ -25,13 +25,23 @@ import (
 	"github.com/pingcap/tidb/util/bytes"
 )
 
-var _ kv.Compactor = (*localstoreCompactor)(nil)
-
 const (
 	deleteWorkerCnt = 3
 )
 
-var localCompactDefaultPolicy = kv.CompactPolicy{
+// compactPolicy defines gc policy of MVCC storage.
+type compactPolicy struct {
+	// SafePoint specifies
+	SafePoint int
+	// TriggerInterval specifies how often should the compactor
+	// scans outdated data.
+	TriggerInterval time.Duration
+	// BatchDeleteCnt specifies the batch size for
+	// deleting outdated data transaction.
+	BatchDeleteCnt int
+}
+
+var localCompactDefaultPolicy = compactPolicy{
 	SafePoint:       20 * 1000, // in ms
 	TriggerInterval: 10 * time.Second,
 	BatchDeleteCnt:  100,
@@ -45,7 +55,7 @@ type localstoreCompactor struct {
 	workerWaitGroup *sync.WaitGroup
 	ticker          *time.Ticker
 	db              engine.DB
-	policy          kv.CompactPolicy
+	policy          compactPolicy
 }
 
 func (gc *localstoreCompactor) OnSet(k kv.Key) {
@@ -196,7 +206,7 @@ func (gc *localstoreCompactor) Stop() {
 	gc.workerWaitGroup.Wait()
 }
 
-func newLocalCompactor(policy kv.CompactPolicy, db engine.DB) *localstoreCompactor {
+func newLocalCompactor(policy compactPolicy, db engine.DB) *localstoreCompactor {
 	return &localstoreCompactor{
 		recentKeys:      make(map[string]struct{}),
 		stopCh:          make(chan struct{}),
diff --git a/store/localstore/compactor_test.go b/store/localstore/compactor_test.go
index 0f4da22d13..b3bf436b6f 100644
--- a/store/localstore/compactor_test.go
+++ b/store/localstore/compactor_test.go
@@ -46,7 +46,7 @@ func (s *localstoreCompactorTestSuite) TestCompactor(c *C) {
 	db := store.(*dbStore).db
 	store.(*dbStore).compactor.Stop()
 
-	policy := kv.CompactPolicy{
+	policy := compactPolicy{
 		SafePoint:       500,
 		BatchDeleteCnt:  1,
 		TriggerInterval: 100 * time.Millisecond,
@@ -119,7 +119,7 @@ func (s *localstoreCompactorTestSuite) TestStartStop(c *C) {
 	db := store.(*dbStore).db
 
 	for i := 0; i < 10000; i++ {
-		policy := kv.CompactPolicy{
+		policy := compactPolicy{
 			SafePoint:       500,
 			BatchDeleteCnt:  1,
 			TriggerInterval: 100 * time.Millisecond,