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diff --git a/synapse/events/presence_router.py b/synapse/events/presence_router.py
new file mode 100644
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+++ b/synapse/events/presence_router.py
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+# -*- coding: utf-8 -*-
+# Copyright 2021 The Matrix.org Foundation C.I.C.
+#
+# 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.
+
+from typing import TYPE_CHECKING, Dict, Iterable, Set, Union
+
+from typing_extensions import Literal
+
+from synapse.api.presence import UserPresenceState
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+
+class PresenceRouter:
+    """
+    A module that the homeserver will call upon to help route user presence updates to
+    additional destinations. If a custom presence router is configured, calls will be
+    passed to that instead.
+    """
+
+    def __init__(self, hs: "HomeServer"):
+        self.custom_presence_router = None
+
+        # Check whether a custom presence router module has been configured
+        if hs.config.presence_router_module_class:
+            # Initialise the module
+            self.custom_presence_router = hs.config.presence_router_module_class(
+                config=hs.config.presence_router_config, module_api=hs.get_module_api()
+            )
+
+    async def get_users_for_states(
+        self,
+        state_updates: Iterable[UserPresenceState],
+    ) -> Dict[str, Set[UserPresenceState]]:
+        """
+        Given an iterable of user presence updates, determine where each one
+        needs to go.
+
+        Args:
+            state_updates: An iterable of user presence state updates.
+
+        Returns:
+          A dictionary of user_id -> set of UserPresenceState, indicating which
+          presence updates each user should receive.
+        """
+        if self.custom_presence_router is not None and hasattr(
+            self.custom_presence_router, "get_users_for_states"
+        ):
+            # Ask the custom module
+            return await self.custom_presence_router.get_users_for_states(
+                state_updates=state_updates
+            )
+
+        # Don't include any extra destinations for presence updates
+        return {}
+
+    async def get_interested_users(
+        self, user_id: str
+    ) -> Union[Set[str], Literal["ALL"]]:
+        """
+        Retrieve a list of users that the provided user is interested in receiving the presence
+        of. Optionally, the str "ALL" can be returned to mean that this user should receive all
+        local and remote incoming presence.
+
+        Note that this method will only be called for local users.
+
+        Args:
+            user_id: A user requesting presence updates.
+
+        Returns:
+            A set of user IDs to return presence updates for, or ALL to return all
+            known updates.
+        """
+        if self.custom_presence_router is not None and hasattr(
+            self.custom_presence_router, "get_interested_users"
+        ):
+            # Ask the custom module for interested users
+            return await self.custom_presence_router.get_interested_users(
+                user_id=user_id
+            )
+
+        # A custom presence router is not defined, or doesn't implement any relevant function.
+        # Don't report any additional interested users.
+        return set()