diff --git a/api/converter/from_pb.go b/api/converter/from_pb.go
index ac4d6effa..f63585a2d 100644
--- a/api/converter/from_pb.go
+++ b/api/converter/from_pb.go
@@ -29,7 +29,6 @@ import (
 	"github.com/yorkie-team/yorkie/pkg/document/key"
 	"github.com/yorkie-team/yorkie/pkg/document/operations"
 	"github.com/yorkie-team/yorkie/pkg/document/time"
-	"github.com/yorkie-team/yorkie/server/backend/sync"
 )
 
 // FromUser converts the given Protobuf formats to model format.
@@ -192,16 +191,6 @@ func fromChangeID(id *api.ChangeID) (change.ID, error) {
 	), nil
 }
 
-// FromDocumentKey converts the given Protobuf formats to model format.
-func FromDocumentKey(pbKey string) (key.Key, error) {
-	k := key.Key(pbKey)
-	if err := k.Validate(); err != nil {
-		return "", err
-	}
-
-	return k, nil
-}
-
 // FromDocumentID converts the given Protobuf formats to model format.
 func FromDocumentID(pbID string) (types.ID, error) {
 	id := types.ID(pbID)
@@ -225,24 +214,6 @@ func FromEventType(pbDocEventType api.DocEventType) (types.DocEventType, error)
 	return "", fmt.Errorf("%v: %w", pbDocEventType, ErrUnsupportedEventType)
 }
 
-// FromDocEvent converts the given Protobuf formats to model format.
-func FromDocEvent(docEvent *api.DocEvent) (*sync.DocEvent, error) {
-	client, err := time.ActorIDFromBytes(docEvent.Publisher)
-	if err != nil {
-		return nil, err
-	}
-
-	eventType, err := FromEventType(docEvent.Type)
-	if err != nil {
-		return nil, err
-	}
-
-	return &sync.DocEvent{
-		Type:      eventType,
-		Publisher: client,
-	}, nil
-}
-
 // FromOperations converts the given Protobuf formats to model format.
 func FromOperations(pbOps []*api.Operation) ([]operations.Operation, error) {
 	var ops []operations.Operation
diff --git a/api/converter/to_pb.go b/api/converter/to_pb.go
index 48362c6a1..79a3114ac 100644
--- a/api/converter/to_pb.go
+++ b/api/converter/to_pb.go
@@ -29,7 +29,6 @@ import (
 	"github.com/yorkie-team/yorkie/pkg/document/innerpresence"
 	"github.com/yorkie-team/yorkie/pkg/document/operations"
 	"github.com/yorkie-team/yorkie/pkg/document/time"
-	"github.com/yorkie-team/yorkie/server/backend/sync"
 )
 
 // ToUser converts the given model format to Protobuf format.
@@ -214,19 +213,6 @@ func ToDocEventType(eventType types.DocEventType) (api.DocEventType, error) {
 	}
 }
 
-// ToDocEvent converts the given model to Protobuf format.
-func ToDocEvent(docEvent sync.DocEvent) (*api.DocEvent, error) {
-	eventType, err := ToDocEventType(docEvent.Type)
-	if err != nil {
-		return nil, err
-	}
-
-	return &api.DocEvent{
-		Type:      eventType,
-		Publisher: docEvent.Publisher.Bytes(),
-	}, nil
-}
-
 // ToOperations converts the given model format to Protobuf format.
 func ToOperations(ops []operations.Operation) ([]*api.Operation, error) {
 	var pbOperations []*api.Operation
diff --git a/api/types/change_summary_test.go b/api/types/change_summary_test.go
index 7a9a729d5..773a3b021 100644
--- a/api/types/change_summary_test.go
+++ b/api/types/change_summary_test.go
@@ -122,7 +122,7 @@ func TestChangeSummary(t *testing.T) {
 			PageSize:  3,
 			IsForward: true,
 		}, lastSeq)
-		assert.Equal(t, int64(lastSeq+1), from)
-		assert.Equal(t, int64(lastSeq+1), to)
+		assert.Equal(t, lastSeq+1, from)
+		assert.Equal(t, lastSeq+1, to)
 	})
 }
diff --git a/api/yorkie/v1/resources.proto b/api/yorkie/v1/resources.proto
index 46d879aad..7f6b3ca1b 100644
--- a/api/yorkie/v1/resources.proto
+++ b/api/yorkie/v1/resources.proto
@@ -95,6 +95,10 @@ message Operation {
     TimeTicket executed_at = 6;
     map<string, string> attributes = 7;
   }
+  // NOTE(hackerwins): Select Operation is not used in the current version.
+  // In the previous version, it was used to represent selection of Text.
+  // However, it has been replaced by Presence now. It is retained for backward
+  // compatibility purposes.
   message Select {
     TimeTicket parent_created_at = 1;
     TextNodePos from = 2;
diff --git a/cmd/yorkie/server.go b/cmd/yorkie/server.go
index 583188698..3fdb18a97 100644
--- a/cmd/yorkie/server.go
+++ b/cmd/yorkie/server.go
@@ -51,12 +51,6 @@ var (
 	authWebhookCacheAuthTTL    time.Duration
 	authWebhookCacheUnauthTTL  time.Duration
 
-	etcdEndpoints     []string
-	etcdDialTimeout   time.Duration
-	etcdUsername      string
-	etcdPassword      string
-	etcdLockLeaseTime time.Duration
-
 	conf = server.NewConfig()
 )
 
diff --git a/pkg/document/crdt/primitive.go b/pkg/document/crdt/primitive.go
index 0308db5c9..c109007aa 100644
--- a/pkg/document/crdt/primitive.go
+++ b/pkg/document/crdt/primitive.go
@@ -52,7 +52,7 @@ func ValueFromBytes(valueType ValueType, value []byte) interface{} {
 		return false
 	case Integer:
 		val := int32(binary.LittleEndian.Uint32(value))
-		return int32(val)
+		return val
 	case Long:
 		return int64(binary.LittleEndian.Uint64(value))
 	case Double:
diff --git a/pkg/document/crdt/tree.go b/pkg/document/crdt/tree.go
index 0e0894676..efd14500e 100644
--- a/pkg/document/crdt/tree.go
+++ b/pkg/document/crdt/tree.go
@@ -117,7 +117,7 @@ func NewTreeNode(pos *TreePos, nodeType string, attributes *RHT, value ...string
 
 // Type returns the type of the Node.
 func (n *TreeNode) Type() string {
-	return string(n.IndexTreeNode.Type)
+	return n.IndexTreeNode.Type
 }
 
 // Len returns the length of the Node.
diff --git a/pkg/index/tree.go b/pkg/index/tree.go
index d0b29ab63..468ff3a3a 100644
--- a/pkg/index/tree.go
+++ b/pkg/index/tree.go
@@ -107,21 +107,6 @@ func postorderTraversal[V Value](node *Node[V], callback func(node *Node[V], dep
 	callback(node, depth)
 }
 
-// postorderTraversalAll traverses the whole tree (include tombstones) with postorder traversal.
-func postorderTraversalAll[V Value](node *Node[V], callback func(node *Node[V], depth int) error, depth int) error {
-	if node == nil {
-		return nil
-	}
-
-	for _, child := range node.children {
-		if err := postorderTraversalAll(child, callback, depth+1); err != nil {
-			return err
-		}
-	}
-
-	return callback(node, depth)
-}
-
 // nodesBetween iterates the nodes between the given range.
 // If the given range is collapsed, the callback is not called.
 // It traverses the tree with postorder traversal.
@@ -193,11 +178,6 @@ func Traverse[V Value](tree *Tree[V], callback func(node *Node[V], depth int)) {
 	postorderTraversal(tree.root, callback, 0)
 }
 
-// TraverseAll traverses the whole tree (include tombstones) with postorder traversal.
-func TraverseAll[V Value](tree *Tree[V], callback func(node *Node[V], depth int) error) error {
-	return postorderTraversalAll(tree.root, callback, 0)
-}
-
 // Value represents the data stored in the nodes of Tree.
 type Value interface {
 	IsRemoved() bool
diff --git a/server/backend/database/database.go b/server/backend/database/database.go
index 2d4922fee..894f89e93 100644
--- a/server/backend/database/database.go
+++ b/server/backend/database/database.go
@@ -47,9 +47,6 @@ var (
 	// ErrDocumentNotFound is returned when the document could not be found.
 	ErrDocumentNotFound = errors.New("document not found")
 
-	// ErrClientDocNotFound is returned when mapping between client and document could not be found.
-	ErrClientDocNotFound = errors.New("client document not found")
-
 	// ErrConflictOnUpdate is returned when a conflict occurs during update.
 	ErrConflictOnUpdate = errors.New("conflict on update")
 
diff --git a/server/backend/database/project_info.go b/server/backend/database/project_info.go
index 2be73d0d6..8c5fe7eca 100644
--- a/server/backend/database/project_info.go
+++ b/server/backend/database/project_info.go
@@ -81,22 +81,6 @@ func NewProjectInfo(name string, owner types.ID, clientDeactivateThreshold strin
 	}
 }
 
-// ToProjectInfo converts the given types.Project to ProjectInfo.
-func ToProjectInfo(project *types.Project) *ProjectInfo {
-	return &ProjectInfo{
-		ID:                        project.ID,
-		Name:                      project.Name,
-		Owner:                     project.Owner,
-		PublicKey:                 project.PublicKey,
-		SecretKey:                 project.SecretKey,
-		AuthWebhookURL:            project.AuthWebhookURL,
-		AuthWebhookMethods:        project.AuthWebhookMethods,
-		ClientDeactivateThreshold: project.ClientDeactivateThreshold,
-		CreatedAt:                 project.CreatedAt,
-		UpdatedAt:                 project.UpdatedAt,
-	}
-}
-
 // DeepCopy returns a deep copy of the ProjectInfo.
 func (i *ProjectInfo) DeepCopy() *ProjectInfo {
 	if i == nil {
diff --git a/server/users/users.go b/server/users/users.go
index cf158a0ae..e24d383f0 100644
--- a/server/users/users.go
+++ b/server/users/users.go
@@ -38,7 +38,7 @@ func SignUp(
 		return nil, fmt.Errorf("cannot hash password: %w", err)
 	}
 
-	info, err := be.DB.CreateUserInfo(ctx, username, string(hashed))
+	info, err := be.DB.CreateUserInfo(ctx, username, hashed)
 	if err != nil {
 		return nil, err
 	}
diff --git a/test/bench/grpc_bench_test.go b/test/bench/grpc_bench_test.go
index 05650a643..254bed7f1 100644
--- a/test/bench/grpc_bench_test.go
+++ b/test/bench/grpc_bench_test.go
@@ -33,7 +33,6 @@ import (
 	"github.com/yorkie-team/yorkie/client"
 	"github.com/yorkie-team/yorkie/pkg/document"
 	"github.com/yorkie-team/yorkie/pkg/document/json"
-	"github.com/yorkie-team/yorkie/pkg/document/key"
 	"github.com/yorkie-team/yorkie/pkg/document/presence"
 	"github.com/yorkie-team/yorkie/server"
 	"github.com/yorkie-team/yorkie/server/backend/database"
@@ -105,7 +104,7 @@ func benchmarkUpdateProject(ctx context.Context, b *testing.B, cnt int, adminCli
 	for i := 0; i < cnt; i++ {
 		name := fmt.Sprintf("name%d", i)
 		authWebhookURL := fmt.Sprintf("http://authWebhookURL%d", i)
-		authWebhookMethods := []string{}
+		var authWebhookMethods []string
 		for _, m := range types.AuthMethods() {
 			authWebhookMethods = append(authWebhookMethods, string(m))
 		}
@@ -203,7 +202,7 @@ func BenchmarkRPC(b *testing.B) {
 
 		ctx := context.Background()
 
-		d1 := document.New(key.Key(helper.TestDocKey(b)))
+		d1 := document.New(helper.TestDocKey(b))
 		err := c1.Attach(ctx, d1)
 		assert.NoError(b, err)
 		testKey1 := "testKey1"
@@ -213,7 +212,7 @@ func BenchmarkRPC(b *testing.B) {
 		})
 		assert.NoError(b, err)
 
-		d2 := document.New(key.Key(helper.TestDocKey(b)))
+		d2 := document.New(helper.TestDocKey(b))
 		err = c2.Attach(ctx, d2)
 		assert.NoError(b, err)
 		testKey2 := "testKey2"
@@ -268,8 +267,8 @@ func BenchmarkRPC(b *testing.B) {
 				defer cleanupClients(b, clients)
 
 				ctx := context.Background()
-				doc1 := document.New(key.Key(helper.TestDocKey(b)))
-				doc2 := document.New(key.Key(helper.TestDocKey(b)))
+				doc1 := document.New(helper.TestDocKey(b))
+				doc2 := document.New(helper.TestDocKey(b))
 
 				err := doc1.Update(func(root *json.Object, p *presence.Presence) error {
 					text := root.SetNewText("k1")
diff --git a/test/integration/document_test.go b/test/integration/document_test.go
index 2a5981c3d..c190fe9ae 100644
--- a/test/integration/document_test.go
+++ b/test/integration/document_test.go
@@ -30,7 +30,6 @@ import (
 	"github.com/yorkie-team/yorkie/pkg/document"
 	"github.com/yorkie-team/yorkie/pkg/document/innerpresence"
 	"github.com/yorkie-team/yorkie/pkg/document/json"
-	"github.com/yorkie-team/yorkie/pkg/document/key"
 	"github.com/yorkie-team/yorkie/pkg/document/presence"
 	"github.com/yorkie-team/yorkie/test/helper"
 )
@@ -68,7 +67,7 @@ func TestDocument(t *testing.T) {
 		assert.True(t, doc2.IsAttached())
 		assert.Equal(t, `{"k1":"v2"}`, doc2.Marshal())
 
-		doc3 := document.New(key.Key("invalid$key"))
+		doc3 := document.New("invalid$key")
 		err = c1.Attach(ctx, doc3)
 		assert.Error(t, err)
 	})