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+# Presence router callbacks
+
+Presence router callbacks allow module developers to specify additional users (local or remote)
+to receive certain presence updates from local users. Presence router callbacks can be 
+registered using the module API's `register_presence_router_callbacks` method.
+
+## Callbacks
+
+The available presence router callbacks are:
+
+### `get_users_for_states`
+
+```python 
+async def get_users_for_states(
+    state_updates: Iterable["synapse.api.UserPresenceState"],
+) -> Dict[str, Set["synapse.api.UserPresenceState"]]
+```
+**Requires** `get_interested_users` to also be registered
+
+Called when processing updates to the presence state of one or more users. This callback can
+be used to instruct the server to forward that presence state to specific users. The module
+must return a dictionary that maps from Matrix user IDs (which can be local or remote) to the
+`UserPresenceState` changes that they should be forwarded.
+
+Synapse will then attempt to send the specified presence updates to each user when possible.
+
+### `get_interested_users`
+
+```python
+async def get_interested_users(
+    user_id: str
+) -> Union[Set[str], "synapse.module_api.PRESENCE_ALL_USERS"]
+```
+**Requires** `get_users_for_states` to also be registered
+
+Called when determining which users someone should be able to see the presence state of. This
+callback should return complementary results to `get_users_for_state` or the presence information 
+may not be properly forwarded.
+
+The callback is given the Matrix user ID for a local user that is requesting presence data and
+should return the Matrix user IDs of the users whose presence state they are allowed to
+query. The returned users can be local or remote. 
+
+Alternatively the callback can return `synapse.module_api.PRESENCE_ALL_USERS`
+to indicate that the user should receive updates from all known users.
+
+## Example
+
+The example below is a module that implements both presence router callbacks, and ensures
+that `@alice:example.org` receives all presence updates from `@bob:example.com` and
+`@charlie:somewhere.org`, regardless of whether Alice shares a room with any of them.
+
+```python
+from typing import Dict, Iterable, Set, Union
+
+from synapse.module_api import ModuleApi
+
+
+class CustomPresenceRouter:
+    def __init__(self, config: dict, api: ModuleApi):
+        self.api = api
+
+        self.api.register_presence_router_callbacks(
+            get_users_for_states=self.get_users_for_states,
+            get_interested_users=self.get_interested_users,
+        )
+
+    async def get_users_for_states(
+        self,
+        state_updates: Iterable["synapse.api.UserPresenceState"],
+    ) -> Dict[str, Set["synapse.api.UserPresenceState"]]:
+        res = {}
+        for update in state_updates:
+            if (
+                update.user_id == "@bob:example.com"
+                or update.user_id == "@charlie:somewhere.org"
+            ):
+                res.setdefault("@alice:example.com", set()).add(update)
+
+        return res
+
+    async def get_interested_users(
+        self,
+        user_id: str,
+    ) -> Union[Set[str], "synapse.module_api.PRESENCE_ALL_USERS"]:
+        if user_id == "@alice:example.com":
+            return {"@bob:example.com", "@charlie:somewhere.org"}
+
+        return set()
+```