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-rw-r--r--synapse/federation/federation_client.py15
-rw-r--r--synapse/federation/federation_server.py12
-rw-r--r--synapse/federation/transport/client.py8
-rw-r--r--synapse/federation/transport/server/federation.py7
-rw-r--r--synapse/handlers/account_data.py2
-rw-r--r--synapse/handlers/receipts.py2
-rw-r--r--synapse/handlers/room.py9
-rw-r--r--synapse/http/servlet.py70
-rw-r--r--synapse/rest/admin/event_reports.py12
-rw-r--r--synapse/rest/admin/federation.py7
-rw-r--r--synapse/rest/admin/media.py21
-rw-r--r--synapse/rest/admin/rooms.py16
-rw-r--r--synapse/rest/admin/statistics.py11
-rw-r--r--synapse/rest/admin/users.py5
-rw-r--r--synapse/rest/client/relations.py3
-rw-r--r--synapse/rest/client/room.py5
-rw-r--r--synapse/storage/databases/main/__init__.py5
-rw-r--r--synapse/storage/databases/main/events_worker.py11
-rw-r--r--synapse/storage/databases/main/media_repository.py5
-rw-r--r--synapse/storage/databases/main/room.py9
-rw-r--r--synapse/storage/databases/main/stats.py6
-rw-r--r--synapse/storage/databases/main/transactions.py13
-rw-r--r--synapse/streams/config.py12
23 files changed, 171 insertions, 95 deletions
diff --git a/synapse/federation/federation_client.py b/synapse/federation/federation_client.py
index feb32e40e5..8493ffc2e5 100644
--- a/synapse/federation/federation_client.py
+++ b/synapse/federation/federation_client.py
@@ -37,7 +37,7 @@ from typing import (
 import attr
 from prometheus_client import Counter
 
-from synapse.api.constants import EventContentFields, EventTypes, Membership
+from synapse.api.constants import Direction, EventContentFields, EventTypes, Membership
 from synapse.api.errors import (
     CodeMessageException,
     Codes,
@@ -1680,7 +1680,12 @@ class FederationClient(FederationBase):
         return result
 
     async def timestamp_to_event(
-        self, *, destinations: List[str], room_id: str, timestamp: int, direction: str
+        self,
+        *,
+        destinations: List[str],
+        room_id: str,
+        timestamp: int,
+        direction: Direction,
     ) -> Optional["TimestampToEventResponse"]:
         """
         Calls each remote federating server from `destinations` asking for their closest
@@ -1693,7 +1698,7 @@ class FederationClient(FederationBase):
             room_id: Room to fetch the event from
             timestamp: The point in time (inclusive) we should navigate from in
                 the given direction to find the closest event.
-            direction: ["f"|"b"] to indicate whether we should navigate forward
+            direction: indicates whether we should navigate forward
                 or backward from the given timestamp to find the closest event.
 
         Returns:
@@ -1738,7 +1743,7 @@ class FederationClient(FederationBase):
             return None
 
     async def _timestamp_to_event_from_destination(
-        self, destination: str, room_id: str, timestamp: int, direction: str
+        self, destination: str, room_id: str, timestamp: int, direction: Direction
     ) -> "TimestampToEventResponse":
         """
         Calls a remote federating server at `destination` asking for their
@@ -1751,7 +1756,7 @@ class FederationClient(FederationBase):
             room_id: Room to fetch the event from
             timestamp: The point in time (inclusive) we should navigate from in
                 the given direction to find the closest event.
-            direction: ["f"|"b"] to indicate whether we should navigate forward
+            direction: indicates whether we should navigate forward
                 or backward from the given timestamp to find the closest event.
 
         Returns:
diff --git a/synapse/federation/federation_server.py b/synapse/federation/federation_server.py
index c9a6dfd1a4..8d36172484 100644
--- a/synapse/federation/federation_server.py
+++ b/synapse/federation/federation_server.py
@@ -34,7 +34,13 @@ from prometheus_client import Counter, Gauge, Histogram
 from twisted.internet.abstract import isIPAddress
 from twisted.python import failure
 
-from synapse.api.constants import EduTypes, EventContentFields, EventTypes, Membership
+from synapse.api.constants import (
+    Direction,
+    EduTypes,
+    EventContentFields,
+    EventTypes,
+    Membership,
+)
 from synapse.api.errors import (
     AuthError,
     Codes,
@@ -218,7 +224,7 @@ class FederationServer(FederationBase):
         return 200, res
 
     async def on_timestamp_to_event_request(
-        self, origin: str, room_id: str, timestamp: int, direction: str
+        self, origin: str, room_id: str, timestamp: int, direction: Direction
     ) -> Tuple[int, Dict[str, Any]]:
         """When we receive a federated `/timestamp_to_event` request,
         handle all of the logic for validating and fetching the event.
@@ -228,7 +234,7 @@ class FederationServer(FederationBase):
             room_id: Room to fetch the event from
             timestamp: The point in time (inclusive) we should navigate from in
                 the given direction to find the closest event.
-            direction: ["f"|"b"] to indicate whether we should navigate forward
+            direction: indicates whether we should navigate forward
                 or backward from the given timestamp to find the closest event.
 
         Returns:
diff --git a/synapse/federation/transport/client.py b/synapse/federation/transport/client.py
index 682666ab36..c05d598b70 100644
--- a/synapse/federation/transport/client.py
+++ b/synapse/federation/transport/client.py
@@ -32,7 +32,7 @@ from typing import (
 import attr
 import ijson
 
-from synapse.api.constants import Membership
+from synapse.api.constants import Direction, Membership
 from synapse.api.errors import Codes, HttpResponseException, SynapseError
 from synapse.api.room_versions import RoomVersion
 from synapse.api.urls import (
@@ -169,7 +169,7 @@ class TransportLayerClient:
         )
 
     async def timestamp_to_event(
-        self, destination: str, room_id: str, timestamp: int, direction: str
+        self, destination: str, room_id: str, timestamp: int, direction: Direction
     ) -> Union[JsonDict, List]:
         """
         Calls a remote federating server at `destination` asking for their
@@ -180,7 +180,7 @@ class TransportLayerClient:
             room_id: Room to fetch the event from
             timestamp: The point in time (inclusive) we should navigate from in
                 the given direction to find the closest event.
-            direction: ["f"|"b"] to indicate whether we should navigate forward
+            direction: indicates whether we should navigate forward
                 or backward from the given timestamp to find the closest event.
 
         Returns:
@@ -194,7 +194,7 @@ class TransportLayerClient:
             room_id,
         )
 
-        args = {"ts": [str(timestamp)], "dir": [direction]}
+        args = {"ts": [str(timestamp)], "dir": [direction.value]}
 
         remote_response = await self.client.get_json(
             destination, path=path, args=args, try_trailing_slash_on_400=True
diff --git a/synapse/federation/transport/server/federation.py b/synapse/federation/transport/server/federation.py
index 17c427387e..f7ca87adc4 100644
--- a/synapse/federation/transport/server/federation.py
+++ b/synapse/federation/transport/server/federation.py
@@ -26,7 +26,7 @@ from typing import (
 
 from typing_extensions import Literal
 
-from synapse.api.constants import EduTypes
+from synapse.api.constants import Direction, EduTypes
 from synapse.api.errors import Codes, SynapseError
 from synapse.api.room_versions import RoomVersions
 from synapse.api.urls import FEDERATION_UNSTABLE_PREFIX, FEDERATION_V2_PREFIX
@@ -234,9 +234,10 @@ class FederationTimestampLookupServlet(BaseFederationServerServlet):
         room_id: str,
     ) -> Tuple[int, JsonDict]:
         timestamp = parse_integer_from_args(query, "ts", required=True)
-        direction = parse_string_from_args(
-            query, "dir", default="f", allowed_values=["f", "b"], required=True
+        direction_str = parse_string_from_args(
+            query, "dir", allowed_values=["f", "b"], required=True
         )
+        direction = Direction(direction_str)
 
         return await self.handler.on_timestamp_to_event_request(
             origin, room_id, timestamp, direction
diff --git a/synapse/handlers/account_data.py b/synapse/handlers/account_data.py
index d500b21809..67e789eef7 100644
--- a/synapse/handlers/account_data.py
+++ b/synapse/handlers/account_data.py
@@ -314,7 +314,7 @@ class AccountDataEventSource(EventSource[int, JsonDict]):
     def __init__(self, hs: "HomeServer"):
         self.store = hs.get_datastores().main
 
-    def get_current_key(self, direction: str = "f") -> int:
+    def get_current_key(self) -> int:
         return self.store.get_max_account_data_stream_id()
 
     async def get_new_events(
diff --git a/synapse/handlers/receipts.py b/synapse/handlers/receipts.py
index 6a4fed1156..04c61ae3dd 100644
--- a/synapse/handlers/receipts.py
+++ b/synapse/handlers/receipts.py
@@ -315,5 +315,5 @@ class ReceiptEventSource(EventSource[int, JsonDict]):
 
         return events, to_key
 
-    def get_current_key(self, direction: str = "f") -> int:
+    def get_current_key(self) -> int:
         return self.store.get_max_receipt_stream_id()
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index 60a6d9cf3c..7ba7c4ff07 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -27,6 +27,7 @@ from typing_extensions import TypedDict
 
 import synapse.events.snapshot
 from synapse.api.constants import (
+    Direction,
     EventContentFields,
     EventTypes,
     GuestAccess,
@@ -1487,7 +1488,7 @@ class TimestampLookupHandler:
         requester: Requester,
         room_id: str,
         timestamp: int,
-        direction: str,
+        direction: Direction,
     ) -> Tuple[str, int]:
         """Find the closest event to the given timestamp in the given direction.
         If we can't find an event locally or the event we have locally is next to a gap,
@@ -1498,7 +1499,7 @@ class TimestampLookupHandler:
             room_id: Room to fetch the event from
             timestamp: The point in time (inclusive) we should navigate from in
                 the given direction to find the closest event.
-            direction: ["f"|"b"] to indicate whether we should navigate forward
+            direction: indicates whether we should navigate forward
                 or backward from the given timestamp to find the closest event.
 
         Returns:
@@ -1533,13 +1534,13 @@ class TimestampLookupHandler:
                 local_event_id, allow_none=False, allow_rejected=False
             )
 
-            if direction == "f":
+            if direction == Direction.FORWARDS:
                 # We only need to check for a backward gap if we're looking forwards
                 # to ensure there is nothing in between.
                 is_event_next_to_backward_gap = (
                     await self.store.is_event_next_to_backward_gap(local_event)
                 )
-            elif direction == "b":
+            elif direction == Direction.BACKWARDS:
                 # We only need to check for a forward gap if we're looking backwards
                 # to ensure there is nothing in between
                 is_event_next_to_forward_gap = (
diff --git a/synapse/http/servlet.py b/synapse/http/servlet.py
index dead02cd5c..0070bd2940 100644
--- a/synapse/http/servlet.py
+++ b/synapse/http/servlet.py
@@ -13,6 +13,7 @@
 # limitations under the License.
 
 """ This module contains base REST classes for constructing REST servlets. """
+import enum
 import logging
 from http import HTTPStatus
 from typing import (
@@ -362,6 +363,7 @@ def parse_string(
     request: Request,
     name: str,
     *,
+    default: Optional[str] = None,
     required: bool = False,
     allowed_values: Optional[Iterable[str]] = None,
     encoding: str = "ascii",
@@ -413,6 +415,74 @@ def parse_string(
     )
 
 
+EnumT = TypeVar("EnumT", bound=enum.Enum)
+
+
+@overload
+def parse_enum(
+    request: Request,
+    name: str,
+    E: Type[EnumT],
+    default: EnumT,
+) -> EnumT:
+    ...
+
+
+@overload
+def parse_enum(
+    request: Request,
+    name: str,
+    E: Type[EnumT],
+    *,
+    required: Literal[True],
+) -> EnumT:
+    ...
+
+
+def parse_enum(
+    request: Request,
+    name: str,
+    E: Type[EnumT],
+    default: Optional[EnumT] = None,
+    required: bool = False,
+) -> Optional[EnumT]:
+    """
+    Parse an enum parameter from the request query string.
+
+    Note that the enum *must only have string values*.
+
+    Args:
+        request: the twisted HTTP request.
+        name: the name of the query parameter.
+        E: the enum which represents valid values
+        default: enum value to use if the parameter is absent, defaults to None.
+        required: whether to raise a 400 SynapseError if the
+            parameter is absent, defaults to False.
+
+    Returns:
+        An enum value.
+
+    Raises:
+        SynapseError if the parameter is absent and required, or if the
+            parameter is present, must be one of a list of allowed values and
+            is not one of those allowed values.
+    """
+    # Assert the enum values are strings.
+    assert all(
+        isinstance(e.value, str) for e in E
+    ), "parse_enum only works with string values"
+    str_value = parse_string(
+        request,
+        name,
+        default=default.value if default is not None else None,
+        required=required,
+        allowed_values=[e.value for e in E],
+    )
+    if str_value is None:
+        return None
+    return E(str_value)
+
+
 def _parse_string_value(
     value: bytes,
     allowed_values: Optional[Iterable[str]],
diff --git a/synapse/rest/admin/event_reports.py b/synapse/rest/admin/event_reports.py
index 6d634eef70..a3beb74e2c 100644
--- a/synapse/rest/admin/event_reports.py
+++ b/synapse/rest/admin/event_reports.py
@@ -16,8 +16,9 @@ import logging
 from http import HTTPStatus
 from typing import TYPE_CHECKING, Tuple
 
+from synapse.api.constants import Direction
 from synapse.api.errors import Codes, NotFoundError, SynapseError
-from synapse.http.servlet import RestServlet, parse_integer, parse_string
+from synapse.http.servlet import RestServlet, parse_enum, parse_integer, parse_string
 from synapse.http.site import SynapseRequest
 from synapse.rest.admin._base import admin_patterns, assert_requester_is_admin
 from synapse.types import JsonDict
@@ -60,7 +61,7 @@ class EventReportsRestServlet(RestServlet):
 
         start = parse_integer(request, "from", default=0)
         limit = parse_integer(request, "limit", default=100)
-        direction = parse_string(request, "dir", default="b")
+        direction = parse_enum(request, "dir", Direction, Direction.BACKWARDS)
         user_id = parse_string(request, "user_id")
         room_id = parse_string(request, "room_id")
 
@@ -78,13 +79,6 @@ class EventReportsRestServlet(RestServlet):
                 errcode=Codes.INVALID_PARAM,
             )
 
-        if direction not in ("f", "b"):
-            raise SynapseError(
-                HTTPStatus.BAD_REQUEST,
-                "Unknown direction: %s" % (direction,),
-                errcode=Codes.INVALID_PARAM,
-            )
-
         event_reports, total = await self.store.get_event_reports_paginate(
             start, limit, direction, user_id, room_id
         )
diff --git a/synapse/rest/admin/federation.py b/synapse/rest/admin/federation.py
index 023ed92144..e0ee55bd0e 100644
--- a/synapse/rest/admin/federation.py
+++ b/synapse/rest/admin/federation.py
@@ -15,9 +15,10 @@ import logging
 from http import HTTPStatus
 from typing import TYPE_CHECKING, Tuple
 
+from synapse.api.constants import Direction
 from synapse.api.errors import Codes, NotFoundError, SynapseError
 from synapse.federation.transport.server import Authenticator
-from synapse.http.servlet import RestServlet, parse_integer, parse_string
+from synapse.http.servlet import RestServlet, parse_enum, parse_integer, parse_string
 from synapse.http.site import SynapseRequest
 from synapse.rest.admin._base import admin_patterns, assert_requester_is_admin
 from synapse.storage.databases.main.transactions import DestinationSortOrder
@@ -79,7 +80,7 @@ class ListDestinationsRestServlet(RestServlet):
             allowed_values=[dest.value for dest in DestinationSortOrder],
         )
 
-        direction = parse_string(request, "dir", default="f", allowed_values=("f", "b"))
+        direction = parse_enum(request, "dir", Direction, default=Direction.FORWARDS)
 
         destinations, total = await self._store.get_destinations_paginate(
             start, limit, destination, order_by, direction
@@ -192,7 +193,7 @@ class DestinationMembershipRestServlet(RestServlet):
                 errcode=Codes.INVALID_PARAM,
             )
 
-        direction = parse_string(request, "dir", default="f", allowed_values=("f", "b"))
+        direction = parse_enum(request, "dir", Direction, default=Direction.FORWARDS)
 
         rooms, total = await self._store.get_destination_rooms_paginate(
             destination, start, limit, direction
diff --git a/synapse/rest/admin/media.py b/synapse/rest/admin/media.py
index 73470f09ae..0d072c42a7 100644
--- a/synapse/rest/admin/media.py
+++ b/synapse/rest/admin/media.py
@@ -17,9 +17,16 @@ import logging
 from http import HTTPStatus
 from typing import TYPE_CHECKING, Tuple
 
+from synapse.api.constants import Direction
 from synapse.api.errors import Codes, NotFoundError, SynapseError
 from synapse.http.server import HttpServer
-from synapse.http.servlet import RestServlet, parse_boolean, parse_integer, parse_string
+from synapse.http.servlet import (
+    RestServlet,
+    parse_boolean,
+    parse_enum,
+    parse_integer,
+    parse_string,
+)
 from synapse.http.site import SynapseRequest
 from synapse.rest.admin._base import (
     admin_patterns,
@@ -389,7 +396,7 @@ class UserMediaRestServlet(RestServlet):
         # to newest media is on top for backward compatibility.
         if b"order_by" not in request.args and b"dir" not in request.args:
             order_by = MediaSortOrder.CREATED_TS.value
-            direction = "b"
+            direction = Direction.BACKWARDS
         else:
             order_by = parse_string(
                 request,
@@ -397,8 +404,8 @@ class UserMediaRestServlet(RestServlet):
                 default=MediaSortOrder.CREATED_TS.value,
                 allowed_values=[sort_order.value for sort_order in MediaSortOrder],
             )
-            direction = parse_string(
-                request, "dir", default="f", allowed_values=("f", "b")
+            direction = parse_enum(
+                request, "dir", Direction, default=Direction.FORWARDS
             )
 
         media, total = await self.store.get_local_media_by_user_paginate(
@@ -447,7 +454,7 @@ class UserMediaRestServlet(RestServlet):
         # to newest media is on top for backward compatibility.
         if b"order_by" not in request.args and b"dir" not in request.args:
             order_by = MediaSortOrder.CREATED_TS.value
-            direction = "b"
+            direction = Direction.BACKWARDS
         else:
             order_by = parse_string(
                 request,
@@ -455,8 +462,8 @@ class UserMediaRestServlet(RestServlet):
                 default=MediaSortOrder.CREATED_TS.value,
                 allowed_values=[sort_order.value for sort_order in MediaSortOrder],
             )
-            direction = parse_string(
-                request, "dir", default="f", allowed_values=("f", "b")
+            direction = parse_enum(
+                request, "dir", Direction, default=Direction.FORWARDS
             )
 
         media, _ = await self.store.get_local_media_by_user_paginate(
diff --git a/synapse/rest/admin/rooms.py b/synapse/rest/admin/rooms.py
index e957aa28ca..1d6e4982d7 100644
--- a/synapse/rest/admin/rooms.py
+++ b/synapse/rest/admin/rooms.py
@@ -16,13 +16,14 @@ from http import HTTPStatus
 from typing import TYPE_CHECKING, List, Optional, Tuple, cast
 from urllib import parse as urlparse
 
-from synapse.api.constants import EventTypes, JoinRules, Membership
+from synapse.api.constants import Direction, EventTypes, JoinRules, Membership
 from synapse.api.errors import AuthError, Codes, NotFoundError, SynapseError
 from synapse.api.filtering import Filter
 from synapse.http.servlet import (
     ResolveRoomIdMixin,
     RestServlet,
     assert_params_in_dict,
+    parse_enum,
     parse_integer,
     parse_json_object_from_request,
     parse_string,
@@ -224,15 +225,8 @@ class ListRoomRestServlet(RestServlet):
                 errcode=Codes.INVALID_PARAM,
             )
 
-        direction = parse_string(request, "dir", default="f")
-        if direction not in ("f", "b"):
-            raise SynapseError(
-                HTTPStatus.BAD_REQUEST,
-                "Unknown direction: %s" % (direction,),
-                errcode=Codes.INVALID_PARAM,
-            )
-
-        reverse_order = True if direction == "b" else False
+        direction = parse_enum(request, "dir", Direction, default=Direction.FORWARDS)
+        reverse_order = True if direction == Direction.BACKWARDS else False
 
         # Return list of rooms according to parameters
         rooms, total_rooms = await self.store.get_rooms_paginate(
@@ -949,7 +943,7 @@ class RoomTimestampToEventRestServlet(RestServlet):
         await assert_user_is_admin(self._auth, requester)
 
         timestamp = parse_integer(request, "ts", required=True)
-        direction = parse_string(request, "dir", default="f", allowed_values=["f", "b"])
+        direction = parse_enum(request, "dir", Direction, default=Direction.FORWARDS)
 
         (
             event_id,
diff --git a/synapse/rest/admin/statistics.py b/synapse/rest/admin/statistics.py
index 3b142b8402..9c45f4650d 100644
--- a/synapse/rest/admin/statistics.py
+++ b/synapse/rest/admin/statistics.py
@@ -16,8 +16,9 @@ import logging
 from http import HTTPStatus
 from typing import TYPE_CHECKING, Tuple
 
+from synapse.api.constants import Direction
 from synapse.api.errors import Codes, SynapseError
-from synapse.http.servlet import RestServlet, parse_integer, parse_string
+from synapse.http.servlet import RestServlet, parse_enum, parse_integer, parse_string
 from synapse.http.site import SynapseRequest
 from synapse.rest.admin._base import admin_patterns, assert_requester_is_admin
 from synapse.storage.databases.main.stats import UserSortOrder
@@ -102,13 +103,7 @@ class UserMediaStatisticsRestServlet(RestServlet):
                 errcode=Codes.INVALID_PARAM,
             )
 
-        direction = parse_string(request, "dir", default="f")
-        if direction not in ("f", "b"):
-            raise SynapseError(
-                HTTPStatus.BAD_REQUEST,
-                "Unknown direction: %s" % (direction,),
-                errcode=Codes.INVALID_PARAM,
-            )
+        direction = parse_enum(request, "dir", Direction, default=Direction.FORWARDS)
 
         users_media, total = await self.store.get_users_media_usage_paginate(
             start, limit, from_ts, until_ts, order_by, direction, search_term
diff --git a/synapse/rest/admin/users.py b/synapse/rest/admin/users.py
index 0841b89c1a..b9dca8ef3a 100644
--- a/synapse/rest/admin/users.py
+++ b/synapse/rest/admin/users.py
@@ -18,12 +18,13 @@ import secrets
 from http import HTTPStatus
 from typing import TYPE_CHECKING, Dict, List, Optional, Tuple
 
-from synapse.api.constants import UserTypes
+from synapse.api.constants import Direction, UserTypes
 from synapse.api.errors import Codes, NotFoundError, SynapseError
 from synapse.http.servlet import (
     RestServlet,
     assert_params_in_dict,
     parse_boolean,
+    parse_enum,
     parse_integer,
     parse_json_object_from_request,
     parse_string,
@@ -120,7 +121,7 @@ class UsersRestServletV2(RestServlet):
             ),
         )
 
-        direction = parse_string(request, "dir", default="f", allowed_values=("f", "b"))
+        direction = parse_enum(request, "dir", Direction, default=Direction.FORWARDS)
 
         users, total = await self.store.get_users_paginate(
             start,
diff --git a/synapse/rest/client/relations.py b/synapse/rest/client/relations.py
index 9dd59196d9..7456d6f507 100644
--- a/synapse/rest/client/relations.py
+++ b/synapse/rest/client/relations.py
@@ -16,6 +16,7 @@ import logging
 import re
 from typing import TYPE_CHECKING, Optional, Tuple
 
+from synapse.api.constants import Direction
 from synapse.handlers.relations import ThreadsListInclude
 from synapse.http.server import HttpServer
 from synapse.http.servlet import RestServlet, parse_integer, parse_string
@@ -59,7 +60,7 @@ class RelationPaginationServlet(RestServlet):
         requester = await self.auth.get_user_by_req(request, allow_guest=True)
 
         pagination_config = await PaginationConfig.from_request(
-            self._store, request, default_limit=5, default_dir="b"
+            self._store, request, default_limit=5, default_dir=Direction.BACKWARDS
         )
 
         # The unstable version of this API returns an extra field for client
diff --git a/synapse/rest/client/room.py b/synapse/rest/client/room.py
index 790614d721..d0db85cca7 100644
--- a/synapse/rest/client/room.py
+++ b/synapse/rest/client/room.py
@@ -26,7 +26,7 @@ from prometheus_client.core import Histogram
 from twisted.web.server import Request
 
 from synapse import event_auth
-from synapse.api.constants import EventTypes, Membership
+from synapse.api.constants import Direction, EventTypes, Membership
 from synapse.api.errors import (
     AuthError,
     Codes,
@@ -44,6 +44,7 @@ from synapse.http.servlet import (
     RestServlet,
     assert_params_in_dict,
     parse_boolean,
+    parse_enum,
     parse_integer,
     parse_json_object_from_request,
     parse_string,
@@ -1297,7 +1298,7 @@ class TimestampLookupRestServlet(RestServlet):
         await self._auth.check_user_in_room_or_world_readable(room_id, requester)
 
         timestamp = parse_integer(request, "ts", required=True)
-        direction = parse_string(request, "dir", default="f", allowed_values=["f", "b"])
+        direction = parse_enum(request, "dir", Direction, default=Direction.FORWARDS)
 
         (
             event_id,
diff --git a/synapse/storage/databases/main/__init__.py b/synapse/storage/databases/main/__init__.py
index 0e47592be3..837dc7646e 100644
--- a/synapse/storage/databases/main/__init__.py
+++ b/synapse/storage/databases/main/__init__.py
@@ -17,6 +17,7 @@
 import logging
 from typing import TYPE_CHECKING, List, Optional, Tuple, cast
 
+from synapse.api.constants import Direction
 from synapse.config.homeserver import HomeServerConfig
 from synapse.storage.database import (
     DatabasePool,
@@ -167,7 +168,7 @@ class DataStore(
         guests: bool = True,
         deactivated: bool = False,
         order_by: str = UserSortOrder.NAME.value,
-        direction: str = "f",
+        direction: Direction = Direction.FORWARDS,
         approved: bool = True,
     ) -> Tuple[List[JsonDict], int]:
         """Function to retrieve a paginated list of users from
@@ -197,7 +198,7 @@ class DataStore(
             # Set ordering
             order_by_column = UserSortOrder(order_by).value
 
-            if direction == "b":
+            if direction == Direction.BACKWARDS:
                 order = "DESC"
             else:
                 order = "ASC"
diff --git a/synapse/storage/databases/main/events_worker.py b/synapse/storage/databases/main/events_worker.py
index f42af34a2f..d7d08369ca 100644
--- a/synapse/storage/databases/main/events_worker.py
+++ b/synapse/storage/databases/main/events_worker.py
@@ -38,7 +38,7 @@ from typing_extensions import Literal
 
 from twisted.internet import defer
 
-from synapse.api.constants import EventTypes
+from synapse.api.constants import Direction, EventTypes
 from synapse.api.errors import NotFoundError, SynapseError
 from synapse.api.room_versions import (
     KNOWN_ROOM_VERSIONS,
@@ -2240,7 +2240,7 @@ class EventsWorkerStore(SQLBaseStore):
         )
 
     async def get_event_id_for_timestamp(
-        self, room_id: str, timestamp: int, direction: str
+        self, room_id: str, timestamp: int, direction: Direction
     ) -> Optional[str]:
         """Find the closest event to the given timestamp in the given direction.
 
@@ -2248,14 +2248,14 @@ class EventsWorkerStore(SQLBaseStore):
             room_id: Room to fetch the event from
             timestamp: The point in time (inclusive) we should navigate from in
                 the given direction to find the closest event.
-            direction: ["f"|"b"] to indicate whether we should navigate forward
+            direction: indicates whether we should navigate forward
                 or backward from the given timestamp to find the closest event.
 
         Returns:
             The closest event_id otherwise None if we can't find any event in
             the given direction.
         """
-        if direction == "b":
+        if direction == Direction.BACKWARDS:
             # Find closest event *before* a given timestamp. We use descending
             # (which gives values largest to smallest) because we want the
             # largest possible timestamp *before* the given timestamp.
@@ -2307,9 +2307,6 @@ class EventsWorkerStore(SQLBaseStore):
 
             return None
 
-        if direction not in ("f", "b"):
-            raise ValueError("Unknown direction: %s" % (direction,))
-
         return await self.db_pool.runInteraction(
             "get_event_id_for_timestamp_txn",
             get_event_id_for_timestamp_txn,
diff --git a/synapse/storage/databases/main/media_repository.py b/synapse/storage/databases/main/media_repository.py
index 9b172a64d8..b202c5eb87 100644
--- a/synapse/storage/databases/main/media_repository.py
+++ b/synapse/storage/databases/main/media_repository.py
@@ -26,6 +26,7 @@ from typing import (
     cast,
 )
 
+from synapse.api.constants import Direction
 from synapse.storage._base import SQLBaseStore
 from synapse.storage.database import (
     DatabasePool,
@@ -176,7 +177,7 @@ class MediaRepositoryStore(MediaRepositoryBackgroundUpdateStore):
         limit: int,
         user_id: str,
         order_by: str = MediaSortOrder.CREATED_TS.value,
-        direction: str = "f",
+        direction: Direction = Direction.FORWARDS,
     ) -> Tuple[List[Dict[str, Any]], int]:
         """Get a paginated list of metadata for a local piece of media
         which an user_id has uploaded
@@ -199,7 +200,7 @@ class MediaRepositoryStore(MediaRepositoryBackgroundUpdateStore):
             # Set ordering
             order_by_column = MediaSortOrder(order_by).value
 
-            if direction == "b":
+            if direction == Direction.BACKWARDS:
                 order = "DESC"
             else:
                 order = "ASC"
diff --git a/synapse/storage/databases/main/room.py b/synapse/storage/databases/main/room.py
index fbbc018887..4ddb27f686 100644
--- a/synapse/storage/databases/main/room.py
+++ b/synapse/storage/databases/main/room.py
@@ -35,6 +35,7 @@ from typing import (
 import attr
 
 from synapse.api.constants import (
+    Direction,
     EventContentFields,
     EventTypes,
     JoinRules,
@@ -2204,7 +2205,7 @@ class RoomStore(RoomBackgroundUpdateStore, RoomWorkerStore):
         self,
         start: int,
         limit: int,
-        direction: str = "b",
+        direction: Direction = Direction.BACKWARDS,
         user_id: Optional[str] = None,
         room_id: Optional[str] = None,
     ) -> Tuple[List[Dict[str, Any]], int]:
@@ -2213,8 +2214,8 @@ class RoomStore(RoomBackgroundUpdateStore, RoomWorkerStore):
         Args:
             start: event offset to begin the query from
             limit: number of rows to retrieve
-            direction: Whether to fetch the most recent first (`"b"`) or the
-                oldest first (`"f"`)
+            direction: Whether to fetch the most recent first (backwards) or the
+                oldest first (forwards)
             user_id: search for user_id. Ignored if user_id is None
             room_id: search for room_id. Ignored if room_id is None
         Returns:
@@ -2236,7 +2237,7 @@ class RoomStore(RoomBackgroundUpdateStore, RoomWorkerStore):
                 filters.append("er.room_id LIKE ?")
                 args.extend(["%" + room_id + "%"])
 
-            if direction == "b":
+            if direction == Direction.BACKWARDS:
                 order = "DESC"
             else:
                 order = "ASC"
diff --git a/synapse/storage/databases/main/stats.py b/synapse/storage/databases/main/stats.py
index 0c1cbd540d..d7b7d0c3c9 100644
--- a/synapse/storage/databases/main/stats.py
+++ b/synapse/storage/databases/main/stats.py
@@ -22,7 +22,7 @@ from typing_extensions import Counter
 
 from twisted.internet.defer import DeferredLock
 
-from synapse.api.constants import EventContentFields, EventTypes, Membership
+from synapse.api.constants import Direction, EventContentFields, EventTypes, Membership
 from synapse.api.errors import StoreError
 from synapse.storage.database import (
     DatabasePool,
@@ -663,7 +663,7 @@ class StatsStore(StateDeltasStore):
         from_ts: Optional[int] = None,
         until_ts: Optional[int] = None,
         order_by: Optional[str] = UserSortOrder.USER_ID.value,
-        direction: Optional[str] = "f",
+        direction: Direction = Direction.FORWARDS,
         search_term: Optional[str] = None,
     ) -> Tuple[List[JsonDict], int]:
         """Function to retrieve a paginated list of users and their uploaded local media
@@ -714,7 +714,7 @@ class StatsStore(StateDeltasStore):
                     500, "Incorrect value for order_by provided: %s" % order_by
                 )
 
-            if direction == "b":
+            if direction == Direction.BACKWARDS:
                 order = "DESC"
             else:
                 order = "ASC"
diff --git a/synapse/storage/databases/main/transactions.py b/synapse/storage/databases/main/transactions.py
index f8c6877ee8..6b33d809b6 100644
--- a/synapse/storage/databases/main/transactions.py
+++ b/synapse/storage/databases/main/transactions.py
@@ -19,6 +19,7 @@ from typing import TYPE_CHECKING, Iterable, List, Optional, Tuple, cast
 import attr
 from canonicaljson import encode_canonical_json
 
+from synapse.api.constants import Direction
 from synapse.metrics.background_process_metrics import wrap_as_background_process
 from synapse.storage._base import db_to_json
 from synapse.storage.database import (
@@ -496,7 +497,7 @@ class TransactionWorkerStore(CacheInvalidationWorkerStore):
         limit: int,
         destination: Optional[str] = None,
         order_by: str = DestinationSortOrder.DESTINATION.value,
-        direction: str = "f",
+        direction: Direction = Direction.FORWARDS,
     ) -> Tuple[List[JsonDict], int]:
         """Function to retrieve a paginated list of destinations.
         This will return a json list of destinations and the
@@ -518,7 +519,7 @@ class TransactionWorkerStore(CacheInvalidationWorkerStore):
         ) -> Tuple[List[JsonDict], int]:
             order_by_column = DestinationSortOrder(order_by).value
 
-            if direction == "b":
+            if direction == Direction.BACKWARDS:
                 order = "DESC"
             else:
                 order = "ASC"
@@ -550,7 +551,11 @@ class TransactionWorkerStore(CacheInvalidationWorkerStore):
         )
 
     async def get_destination_rooms_paginate(
-        self, destination: str, start: int, limit: int, direction: str = "f"
+        self,
+        destination: str,
+        start: int,
+        limit: int,
+        direction: Direction = Direction.FORWARDS,
     ) -> Tuple[List[JsonDict], int]:
         """Function to retrieve a paginated list of destination's rooms.
         This will return a json list of rooms and the
@@ -569,7 +574,7 @@ class TransactionWorkerStore(CacheInvalidationWorkerStore):
             txn: LoggingTransaction,
         ) -> Tuple[List[JsonDict], int]:
 
-            if direction == "b":
+            if direction == Direction.BACKWARDS:
                 order = "DESC"
             else:
                 order = "ASC"
diff --git a/synapse/streams/config.py b/synapse/streams/config.py
index 5cb7875181..a044280410 100644
--- a/synapse/streams/config.py
+++ b/synapse/streams/config.py
@@ -18,7 +18,7 @@ import attr
 
 from synapse.api.constants import Direction
 from synapse.api.errors import SynapseError
-from synapse.http.servlet import parse_integer, parse_string
+from synapse.http.servlet import parse_enum, parse_integer, parse_string
 from synapse.http.site import SynapseRequest
 from synapse.storage.databases.main import DataStore
 from synapse.types import StreamToken
@@ -44,15 +44,9 @@ class PaginationConfig:
         store: "DataStore",
         request: SynapseRequest,
         default_limit: int,
-        default_dir: str = "f",
+        default_dir: Direction = Direction.FORWARDS,
     ) -> "PaginationConfig":
-        direction_str = parse_string(
-            request,
-            "dir",
-            default=default_dir,
-            allowed_values=[Direction.FORWARDS.value, Direction.BACKWARDS.value],
-        )
-        direction = Direction(direction_str)
+        direction = parse_enum(request, "dir", Direction, default=default_dir)
 
         from_tok_str = parse_string(request, "from")
         to_tok_str = parse_string(request, "to")