Add direction to ListDocuments API

api/admin.proto

@@ -27,6 +27,7 @@ service Admin {
 message ListDocumentsRequest {
   string previous_id = 1;
   int32 page_size = 2;
+  bool is_forward = 3;
 }
 
 message ListDocumentsResponse {

yorkie/admin/server.go

@@ -120,6 +120,7 @@ func (s *Server) ListDocuments(
 		s.backend,
 		db.ID(req.PreviousId),
 		int(req.PageSize),
+		req.IsForward,
 	)
 	if err != nil {
 		return nil, err

yorkie/backend/db/db.go

@@ -124,5 +124,6 @@ type DB interface {
 		ctx context.Context,
 		previousID ID,
 		pageSize int,
+		isForward bool,
 	) ([]*DocInfo, error)
 }

yorkie/backend/db/memory/db.go

@@ -567,15 +567,34 @@ func (d *DB) FindDocInfosByPreviousIDAndPageSize(
 	ctx context.Context,
 	previousID db.ID,
 	pageSize int,
+	isForward bool,
 ) ([]*db.DocInfo, error) {
 	txn := d.db.Txn(false)
 	defer txn.Abort()
 
-	iterator, err := txn.LowerBound(
-		tblDocuments,
-		"id",
-		previousID.String(),
-	)
+	var iterator memdb.ResultIterator
+	var err error
+	if isForward {
+		iterator, err = txn.LowerBound(
+			tblDocuments,
+			"id",
+			previousID.String(),
+		)
+	} else {
+		if previousID == "" {
+			iterator, err = txn.GetReverse(
+				tblDocuments,
+				"id",
+			)
+		} else {
+			iterator, err = txn.ReverseLowerBound(
+				tblDocuments,
+				"id",
+				previousID.String(),
+			)
+		}
+	}
+
 	if err != nil {
 		return nil, err
 	}

yorkie/backend/db/memory/db_test.go

@@ -192,33 +192,49 @@ func TestDB(t *testing.T) {
 		assert.Equal(t, uint64(1), snapshot.ServerSeq)
 	})
 
-	t.Run("paging docInfo test", func(t *testing.T) {
+	t.Run("docInfo pagination test", func(t *testing.T) {
 		localDB, err := memory.New()
 		assert.NoError(t, err)
 
-		clientInfo, _ := localDB.ActivateClient(ctx, t.Name())
-
-		var givenKeys []string
-		for i := 0; i < 11; i++ {
-			docInfo, err := localDB.FindDocInfoByKey(ctx, clientInfo, fmt.Sprintf("tests$%s-%d", t.Name(), i), true)
-			assert.NoError(t, err)
-			givenKeys = append(givenKeys, docInfo.CombinedKey)
+		assertKeys := func(expectedKeys []string, infos []*db.DocInfo) {
+			var keys []string
+			for _, info := range infos {
+				keys = append(keys, info.CombinedKey)
+			}
+			assert.EqualValues(t, expectedKeys, keys)
 		}
 
-		var keys []string
-		previousID := db.ID("")
-		for {
-			docInfos, err := localDB.FindDocInfosByPreviousIDAndPageSize(ctx, previousID, 10)
+		pageSize := 5
+		totalSize := 9
+		clientInfo, _ := localDB.ActivateClient(ctx, t.Name())
+		for i := 0; i < totalSize; i++ {
+			_, err := localDB.FindDocInfoByKey(ctx, clientInfo, fmt.Sprintf("%d", i), true)
 			assert.NoError(t, err)
-			if len(docInfos) == 0 {
-				break
-			}
-			for _, docInfo := range docInfos {
-				keys = append(keys, docInfo.CombinedKey)
-			}
-			previousID = docInfos[len(docInfos)-1].ID
 		}
 
-		assert.Equal(t, givenKeys, keys)
+		// initial page, previousID is empty
+		infos, err := localDB.FindDocInfosByPreviousIDAndPageSize(ctx, "", pageSize, false)
+		assert.NoError(t, err)
+		assertKeys([]string{"8", "7", "6", "5", "4"}, infos)
+
+		// backward
+		infos, err = localDB.FindDocInfosByPreviousIDAndPageSize(ctx, infos[len(infos)-1].ID, pageSize, false)
+		assert.NoError(t, err)
+		assertKeys([]string{"3", "2", "1", "0"}, infos)
+
+		// backward again
+		emptyInfos, err := localDB.FindDocInfosByPreviousIDAndPageSize(ctx, infos[len(infos)-1].ID, pageSize, false)
+		assert.NoError(t, err)
+		assertKeys(nil, emptyInfos)
+
+		// forward
+		infos, err = localDB.FindDocInfosByPreviousIDAndPageSize(ctx, infos[0].ID, pageSize, true)
+		assert.NoError(t, err)
+		assertKeys([]string{"4", "5", "6", "7", "8"}, infos)
+
+		// forward again
+		emptyInfos, err = localDB.FindDocInfosByPreviousIDAndPageSize(ctx, infos[len(infos)-1].ID, pageSize, true)
+		assert.NoError(t, err)
+		assertKeys(nil, emptyInfos)
 	})
 }

yorkie/backend/db/mongo/client.go

@@ -558,25 +558,30 @@ func (c *Client) FindDocInfosByPreviousIDAndPageSize(
 	ctx context.Context,
 	previousID db.ID,
 	pageSize int,
+	isForward bool,
 ) ([]*db.DocInfo, error) {
 	filter := bson.M{}
 	if previousID != "" {
 		encodedPreviousID, err := encodeID(previousID)
 		if err != nil {
 			return nil, err
 		}
+
+		key := "$lt"
+		if isForward {
+			key = "$gt"
+		}
 		filter = bson.M{
-			"_id": bson.M{
-				"$gt": encodedPreviousID,
-			},
+			"_id": bson.M{key: encodedPreviousID},
 		}
 	}
 
-	cursor, err := c.collection(colDocuments).Find(
-		ctx,
-		filter,
-		options.Find().SetLimit(int64(pageSize)),
-	)
+	opts := options.Find().SetLimit(int64(pageSize))
+	if !isForward {
+		opts = opts.SetSort(map[string]int{"_id": -1})
+	}
+
+	cursor, err := c.collection(colDocuments).Find(ctx, filter, opts)
 	if err != nil {
 		logging.From(ctx).Error(err)
 		return nil, err

yorkie/documents/admin.go

@@ -31,8 +31,9 @@ func ListDocumentSummaries(
 	be *backend.Backend,
 	previousID db.ID,
 	pageSize int,
+	isForward bool,
 ) ([]*types.DocumentSummary, error) {
-	docInfo, err := be.DB.FindDocInfosByPreviousIDAndPageSize(ctx, previousID, pageSize)
+	docInfo, err := be.DB.FindDocInfosByPreviousIDAndPageSize(ctx, previousID, pageSize, isForward)
 	if err != nil {
 		return nil, err
 	}