# Presence Router Module

Synapse supports configuring a module that can specify additional users (local or remote) to should receive certain presence updates from local users.

Note that routing presence via Application Service transactions is not currently supported.

The presence routing module is implemented as a Python class, which will be imported by the running Synapse.

## Python Presence Router Class

The Python class is instantiated with two objects:

- A configuration object of some type (see below).
- An instance of `synapse.module_api.ModuleApi`.

It then implements methods related to presence routing.

Note that one method of `ModuleApi` that may be useful is:

    async def ModuleApi.send_local_online_presence_to(users: Iterable[str]) -> None

which can be given a list of local or remote MXIDs to broadcast known, online user presence to (for those users that the receiving user is considered interested in). It does not include state for users who are currently offline, and it can only be called on workers that support sending federation. Additionally, this method must only be called from the process that has been configured to write to the the presence stream. By default, this is the main process, but another worker can be configured to do so.

### Module structure

Below is a list of possible methods that can be implemented, and whether they are required.

#### `parse_config`

    def parse_config(config_dict: dict) -> Any

Required. A static method that is passed a dictionary of config options, and should return a validated config object. This method is described further in Configuration.

#### `get_users_for_states`

    async def get_users_for_states(
        self,
        state_updates: Iterable[UserPresenceState],
    ) -> Dict[str, Set[UserPresenceState]]:

Required. An asynchronous method that is passed an iterable of user presence state. This method can determine whether a given presence update should be sent to certain users. It does this by returning a dictionary with keys representing local or remote Matrix User IDs, and values being a python set of `synapse.handlers.presence.UserPresenceState` instances.

Synapse will then attempt to send the specified presence updates to each user when possible.

#### `get_interested_users`

    async def get_interested_users(self, user_id: str) -> Union[Set[str], str]

Required. An asynchronous method that is passed a single Matrix User ID. This method is expected to return the users that the passed in user may be interested in the presence of. Returned users may be local or remote. The presence routed as a result of what this method returns is sent in addition to the updates already sent between users that share a room together. Presence updates are deduplicated.

This method should return a python set of Matrix User IDs, or the object `synapse.events.presence_router.PresenceRouter.ALL_USERS` to indicate that the passed user should receive presence information for all known users.

For clarity, if the user `@alice:example.org` is passed to this method, and the Set `{"@bob:example.com", "@charlie:somewhere.org"}` is returned, this signifies that Alice should receive presence updates sent by Bob and Charlie, regardless of whether these users share a room.

### Example

Below is an example implementation of a presence router class.

    from typing import Dict, Iterable, Set, Union
    from synapse.events.presence_router import PresenceRouter
    from synapse.handlers.presence import UserPresenceState
    from synapse.module_api import ModuleApi

    class PresenceRouterConfig:
        def __init__(self):
            # Config options with their defaults
            # A list of users to always send all user presence updates to
            self.always_send_to_users = []  # type: List[str]

            # A list of users to ignore presence updates for. Does not affect
            # shared-room presence relationships
            self.blacklisted_users = []  # type: List[str]

    class ExamplePresenceRouter:
        """An example implementation of synapse.presence_router.PresenceRouter.
        Supports routing all presence to a configured set of users, or a subset
        of presence from certain users to members of certain rooms.

        Args:
            config: A configuration object.
            module_api: An instance of Synapse's ModuleApi.
        """
        def __init__(self, config: PresenceRouterConfig, module_api: ModuleApi):
            self._config = config
            self._module_api = module_api

        @staticmethod
        def parse_config(config_dict: dict) -> PresenceRouterConfig:
            """Parse a configuration dictionary from the homeserver config, do
            some validation and return a typed PresenceRouterConfig.

            Args:
                config_dict: The configuration dictionary.

            Returns:
                A validated config object.
            """
            # Initialise a typed config object
            config = PresenceRouterConfig()
            always_send_to_users = config_dict.get("always_send_to_users")
            blacklisted_users = config_dict.get("blacklisted_users")

            # Do some validation of config options... otherwise raise a
            # synapse.config.ConfigError.
            config.always_send_to_users = always_send_to_users
            config.blacklisted_users = blacklisted_users

            return config

        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. Returned results will not affect presence updates that are
            sent between users who share a room.

            Args:
                state_updates: An iterable of user presence state updates.

            Returns:
              A dictionary of user_id -> set of UserPresenceState that the user should
              receive.
            """
            destination_users = {}  # type: Dict[str, Set[UserPresenceState]

            # Ignore any updates for blacklisted users
            desired_updates = set()
            for update in state_updates:
                if update.state_key not in self._config.blacklisted_users:
                    desired_updates.add(update)

            # Send all presence updates to specific users
            for user_id in self._config.always_send_to_users:
                destination_users[user_id] = desired_updates

            return destination_users

        async def get_interested_users(
            self,
            user_id: str,
        ) -> Union[Set[str], PresenceRouter.ALL_USERS]:
            """
            Retrieve a list of users that `user_id` is interested in receiving the
            presence of. This will be in addition to those they share a room with.
            Optionally, the object PresenceRouter.ALL_USERS can be returned to indicate
            that this user should receive all incoming local and remote presence updates.

            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 additional presence updates for, or
              PresenceRouter.ALL_USERS to return presence updates for all other users.
            """
            if user_id in self._config.always_send_to_users:
                return PresenceRouter.ALL_USERS

            return set()

#### A note on `get_users_for_states` and `get_interested_users`

Both of these methods are effectively two different sides of the same coin. The logic regarding which users should receive updates for other users should be the same between them.

`get_users_for_states` is called when presence updates come in from either federation or local users, and is used to either direct local presence to remote users, or to wake up the sync streams of local users to collect remote presence.

In contrast, `get_interested_users` is used to determine the users that presence should be fetched for when a local user is syncing. This presence is then retrieved, before being fed through `get_users_for_states` once again, with only the syncing user's routing information pulled from the resulting dictionary.

Their routing logic should thus line up, else you may run into unintended behaviour.

## Configuration

Once you've crafted your module and installed it into the same Python environment as Synapse, amend your homeserver config file with the following.

    presence:
      routing_module:
        module: my_module.ExamplePresenceRouter
        config:
          # Any configuration options for your module. The below is an example.
          # of setting options for ExamplePresenceRouter.
          always_send_to_users: ["@presence_gobbler:example.org"]
          blacklisted_users:
            - "@alice:example.com"
            - "@bob:example.com"
          ...

The contents of `config` will be passed as a Python dictionary to the static `parse_config` method of your class. The object returned by this method will then be passed to the `__init__` method of your module as `config`.