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Improve the modules doc (#10758) 

* Split up the documentation in several files rather than one huge one
* Add examples for each callback category
* Other niceties like fixing https://github.com/matrix-org/synapse/issues/10632
* Add titles to callbacks so they're easier to find in the navigation panels and link to
pull/10733/head
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Split up the modules documentation and add examples for module developers.

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- [URL Previews](development/url_previews.md)
- [User Directory](user_directory.md)
- [Message Retention Policies](message_retention_policies.md)
- [Pluggable Modules](modules.md)
- [Third Party Rules]()
- [Spam Checker](spam_checker.md)
- [Presence Router](presence_router_module.md)
- [Media Storage Providers]()
- [Pluggable Modules](modules/index.md)
- [Writing a module](modules/writing_a_module.md)
- [Spam checker callbacks](modules/spam_checker_callbacks.md)
- [Third-party rules callbacks](modules/third_party_rules_callbacks.md)
- [Presence router callbacks](modules/presence_router_callbacks.md)
- [Account validity callbacks](modules/account_validity_callbacks.md)
- [Porting a legacy module to the new interface](modules/porting_legacy_module.md)
- [Workers](workers.md)
- [Using `synctl` with Workers](synctl_workers.md)
- [Systemd](systemd-with-workers/README.md)

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# Modules
Synapse supports extending its functionality by configuring external modules.
## Using modules
To use a module on Synapse, add it to the `modules` section of the configuration file:
```yaml
modules:
- module: my_super_module.MySuperClass
config:
do_thing: true
- module: my_other_super_module.SomeClass
config: {}
```
Each module is defined by a path to a Python class as well as a configuration. This
information for a given module should be available in the module's own documentation.
**Note**: When using third-party modules, you effectively allow someone else to run
custom code on your Synapse homeserver. Server admins are encouraged to verify the
provenance of the modules they use on their homeserver and make sure the modules aren't
running malicious code on their instance.
Also note that we are currently in the process of migrating module interfaces to this
system. While some interfaces might be compatible with it, others still require
configuring modules in another part of Synapse's configuration file. Currently, only the
spam checker interface is compatible with this new system.
## Writing a module
A module is a Python class that uses Synapse's module API to interact with the
homeserver. It can register callbacks that Synapse will call on specific operations, as
well as web resources to attach to Synapse's web server.
When instantiated, a module is given its parsed configuration as well as an instance of
the `synapse.module_api.ModuleApi` class. The configuration is a dictionary, and is
either the output of the module's `parse_config` static method (see below), or the
configuration associated with the module in Synapse's configuration file.
See the documentation for the `ModuleApi` class
[here](https://github.com/matrix-org/synapse/blob/master/synapse/module_api/__init__.py).
### Handling the module's configuration
A module can implement the following static method:
```python
@staticmethod
def parse_config(config: dict) -> dict
```
This method is given a dictionary resulting from parsing the YAML configuration for the
module. It may modify it (for example by parsing durations expressed as strings (e.g.
"5d") into milliseconds, etc.), and return the modified dictionary. It may also verify
that the configuration is correct, and raise an instance of
`synapse.module_api.errors.ConfigError` if not.
### Registering a web resource
Modules can register web resources onto Synapse's web server using the following module
API method:
```python
def ModuleApi.register_web_resource(path: str, resource: IResource) -> None
```
The path is the full absolute path to register the resource at. For example, if you
register a resource for the path `/_synapse/client/my_super_module/say_hello`, Synapse
will serve it at `http(s)://[HS_URL]/_synapse/client/my_super_module/say_hello`. Note
that Synapse does not allow registering resources for several sub-paths in the `/_matrix`
namespace (such as anything under `/_matrix/client` for example). It is strongly
recommended that modules register their web resources under the `/_synapse/client`
namespace.
The provided resource is a Python class that implements Twisted's [IResource](https://twistedmatrix.com/documents/current/api/twisted.web.resource.IResource.html)
interface (such as [Resource](https://twistedmatrix.com/documents/current/api/twisted.web.resource.Resource.html)).
Only one resource can be registered for a given path. If several modules attempt to
register a resource for the same path, the module that appears first in Synapse's
configuration file takes priority.
Modules **must** register their web resources in their `__init__` method.
### Registering a callback
Modules can use Synapse's module API to register callbacks. Callbacks are functions that
Synapse will call when performing specific actions. Callbacks must be asynchronous, and
are split in categories. A single module may implement callbacks from multiple categories,
and is under no obligation to implement all callbacks from the categories it registers
callbacks for.
Modules can register callbacks using one of the module API's `register_[...]_callbacks`
methods. The callback functions are passed to these methods as keyword arguments, with
the callback name as the argument name and the function as its value. This is demonstrated
in the example below. A `register_[...]_callbacks` method exists for each module type
documented in this section.
#### Spam checker callbacks
Spam checker callbacks allow module developers to implement spam mitigation actions for
Synapse instances. Spam checker callbacks can be registered using the module API's
`register_spam_checker_callbacks` method.
The available spam checker callbacks are:
```python
async def check_event_for_spam(event: "synapse.events.EventBase") -> Union[bool, str]
```
Called when receiving an event from a client or via federation. The module can return
either a `bool` to indicate whether the event must be rejected because of spam, or a `str`
to indicate the event must be rejected because of spam and to give a rejection reason to
forward to clients.
```python
async def user_may_invite(inviter: str, invitee: str, room_id: str) -> bool
```
Called when processing an invitation. The module must return a `bool` indicating whether
the inviter can invite the invitee to the given room. Both inviter and invitee are
represented by their Matrix user ID (e.g. `@alice:example.com`).
```python
async def user_may_create_room(user: str) -> bool
```
Called when processing a room creation request. The module must return a `bool` indicating
whether the given user (represented by their Matrix user ID) is allowed to create a room.
```python
async def user_may_create_room_alias(user: str, room_alias: "synapse.types.RoomAlias") -> bool
```
Called when trying to associate an alias with an existing room. The module must return a
`bool` indicating whether the given user (represented by their Matrix user ID) is allowed
to set the given alias.
```python
async def user_may_publish_room(user: str, room_id: str) -> bool
```
Called when trying to publish a room to the homeserver's public rooms directory. The
module must return a `bool` indicating whether the given user (represented by their
Matrix user ID) is allowed to publish the given room.
```python
async def check_username_for_spam(user_profile: Dict[str, str]) -> bool
```
Called when computing search results in the user directory. The module must return a
`bool` indicating whether the given user profile can appear in search results. The profile
is represented as a dictionary with the following keys:
* `user_id`: The Matrix ID for this user.
* `display_name`: The user's display name.
* `avatar_url`: The `mxc://` URL to the user's avatar.
The module is given a copy of the original dictionary, so modifying it from within the
module cannot modify a user's profile when included in user directory search results.
```python
async def check_registration_for_spam(
email_threepid: Optional[dict],
username: Optional[str],
request_info: Collection[Tuple[str, str]],
auth_provider_id: Optional[str] = None,
) -> "synapse.spam_checker_api.RegistrationBehaviour"
```
Called when registering a new user. The module must return a `RegistrationBehaviour`
indicating whether the registration can go through or must be denied, or whether the user
may be allowed to register but will be shadow banned.
The arguments passed to this callback are:
* `email_threepid`: The email address used for registering, if any.
* `username`: The username the user would like to register. Can be `None`, meaning that
Synapse will generate one later.
* `request_info`: A collection of tuples, which first item is a user agent, and which
second item is an IP address. These user agents and IP addresses are the ones that were
used during the registration process.
* `auth_provider_id`: The identifier of the SSO authentication provider, if any.
```python
async def check_media_file_for_spam(
file_wrapper: "synapse.rest.media.v1.media_storage.ReadableFileWrapper",
file_info: "synapse.rest.media.v1._base.FileInfo",
) -> bool
```
Called when storing a local or remote file. The module must return a boolean indicating
whether the given file can be stored in the homeserver's media store.
#### Account validity callbacks
Account validity callbacks allow module developers to add extra steps to verify the
validity on an account, i.e. see if a user can be granted access to their account on the
Synapse instance. Account validity callbacks can be registered using the module API's
`register_account_validity_callbacks` method.
The available account validity callbacks are:
```python
async def is_user_expired(user: str) -> Optional[bool]
```
Called when processing any authenticated request (except for logout requests). The module
can return a `bool` to indicate whether the user has expired and should be locked out of
their account, or `None` if the module wasn't able to figure it out. The user is
represented by their Matrix user ID (e.g. `@alice:example.com`).
If the module returns `True`, the current request will be denied with the error code
`ORG_MATRIX_EXPIRED_ACCOUNT` and the HTTP status code 403. Note that this doesn't
invalidate the user's access token.
```python
async def on_user_registration(user: str) -> None
```
Called after successfully registering a user, in case the module needs to perform extra
operations to keep track of them. (e.g. add them to a database table). The user is
represented by their Matrix user ID.
#### Third party rules callbacks
Third party rules callbacks allow module developers to add extra checks to verify the
validity of incoming events. Third party event rules callbacks can be registered using
the module API's `register_third_party_rules_callbacks` method.
The available third party rules callbacks are:
```python
async def check_event_allowed(
event: "synapse.events.EventBase",
state_events: "synapse.types.StateMap",
) -> Tuple[bool, Optional[dict]]
```
**<span style="color:red">
This callback is very experimental and can and will break without notice. Module developers
are encouraged to implement `check_event_for_spam` from the spam checker category instead.
</span>**
Called when processing any incoming event, with the event and a `StateMap`
representing the current state of the room the event is being sent into. A `StateMap` is
a dictionary that maps tuples containing an event type and a state key to the
corresponding state event. For example retrieving the room's `m.room.create` event from
the `state_events` argument would look like this: `state_events.get(("m.room.create", ""))`.
The module must return a boolean indicating whether the event can be allowed.
Note that this callback function processes incoming events coming via federation
traffic (on top of client traffic). This means denying an event might cause the local
copy of the room's history to diverge from that of remote servers. This may cause
federation issues in the room. It is strongly recommended to only deny events using this
callback function if the sender is a local user, or in a private federation in which all
servers are using the same module, with the same configuration.
If the boolean returned by the module is `True`, it may also tell Synapse to replace the
event with new data by returning the new event's data as a dictionary. In order to do
that, it is recommended the module calls `event.get_dict()` to get the current event as a
dictionary, and modify the returned dictionary accordingly.
Note that replacing the event only works for events sent by local users, not for events
received over federation.
```python
async def on_create_room(
requester: "synapse.types.Requester",
request_content: dict,
is_requester_admin: bool,
) -> None
```
Called when processing a room creation request, with the `Requester` object for the user
performing the request, a dictionary representing the room creation request's JSON body
(see [the spec](https://matrix.org/docs/spec/client_server/latest#post-matrix-client-r0-createroom)
for a list of possible parameters), and a boolean indicating whether the user performing
the request is a server admin.
Modules can modify the `request_content` (by e.g. adding events to its `initial_state`),
or deny the room's creation by raising a `module_api.errors.SynapseError`.
#### 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.
The available presence router callbacks are:
```python
async def get_users_for_states(
self,
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.
```python
async def get_interested_users(
self,
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.
For example, 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.
### Porting an existing module that uses the old interface
In order to port a module that uses Synapse's old module interface, its author needs to:
* ensure the module's callbacks are all asynchronous.
* register their callbacks using one or more of the `register_[...]_callbacks` methods
from the `ModuleApi` class in the module's `__init__` method (see [this section](#registering-a-callback)
for more info).
Additionally, if the module is packaged with an additional web resource, the module
should register this resource in its `__init__` method using the `register_web_resource`
method from the `ModuleApi` class (see [this section](#registering-a-web-resource) for
more info).
The module's author should also update any example in the module's configuration to only
use the new `modules` section in Synapse's configuration file (see [this section](#using-modules)
for more info).
### Example
The example below is a module that implements the spam checker callback
`user_may_create_room` to deny room creation to user `@evilguy:example.com`, and registers
a web resource to the path `/_synapse/client/demo/hello` that returns a JSON object.
```python
import json
from twisted.web.resource import Resource
from twisted.web.server import Request
from synapse.module_api import ModuleApi
class DemoResource(Resource):
def __init__(self, config):
super(DemoResource, self).__init__()
self.config = config
def render_GET(self, request: Request):
name = request.args.get(b"name")[0]
request.setHeader(b"Content-Type", b"application/json")
return json.dumps({"hello": name})
class DemoModule:
def __init__(self, config: dict, api: ModuleApi):
self.config = config
self.api = api
self.api.register_web_resource(
path="/_synapse/client/demo/hello",
resource=DemoResource(self.config),
)
self.api.register_spam_checker_callbacks(
user_may_create_room=self.user_may_create_room,
)
@staticmethod
def parse_config(config):
return config
async def user_may_create_room(self, user: str) -> bool:
if user == "@evilguy:example.com":
return False
return True
```

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# Account validity callbacks
Account validity callbacks allow module developers to add extra steps to verify the
validity on an account, i.e. see if a user can be granted access to their account on the
Synapse instance. Account validity callbacks can be registered using the module API's
`register_account_validity_callbacks` method.
The available account validity callbacks are:
### `is_user_expired`
```python
async def is_user_expired(user: str) -> Optional[bool]
```
Called when processing any authenticated request (except for logout requests). The module
can return a `bool` to indicate whether the user has expired and should be locked out of
their account, or `None` if the module wasn't able to figure it out. The user is
represented by their Matrix user ID (e.g. `@alice:example.com`).
If the module returns `True`, the current request will be denied with the error code
`ORG_MATRIX_EXPIRED_ACCOUNT` and the HTTP status code 403. Note that this doesn't
invalidate the user's access token.
### `on_user_registration`
```python
async def on_user_registration(user: str) -> None
```
Called after successfully registering a user, in case the module needs to perform extra
operations to keep track of them. (e.g. add them to a database table). The user is
represented by their Matrix user ID.

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# Modules
Synapse supports extending its functionality by configuring external modules.
## Using modules
To use a module on Synapse, add it to the `modules` section of the configuration file:
```yaml
modules:
- module: my_super_module.MySuperClass
config:
do_thing: true
- module: my_other_super_module.SomeClass
config: {}
```
Each module is defined by a path to a Python class as well as a configuration. This
information for a given module should be available in the module's own documentation.
**Note**: When using third-party modules, you effectively allow someone else to run
custom code on your Synapse homeserver. Server admins are encouraged to verify the
provenance of the modules they use on their homeserver and make sure the modules aren't
running malicious code on their instance.
Also note that we are currently in the process of migrating module interfaces to this
system. While some interfaces might be compatible with it, others still require
configuring modules in another part of Synapse's configuration file.
Currently, only the following pre-existing interfaces are compatible with this new system:
* spam checker
* third-party rules
* presence router

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# Porting an existing module that uses the old interface
In order to port a module that uses Synapse's old module interface, its author needs to:
* ensure the module's callbacks are all asynchronous.
* register their callbacks using one or more of the `register_[...]_callbacks` methods
from the `ModuleApi` class in the module's `__init__` method (see [this section](writing_a_module.html#registering-a-callback)
for more info).
Additionally, if the module is packaged with an additional web resource, the module
should register this resource in its `__init__` method using the `register_web_resource`
method from the `ModuleApi` class (see [this section](writing_a_module.html#registering-a-web-resource) for
more info).
The module's author should also update any example in the module's configuration to only
use the new `modules` section in Synapse's configuration file (see [this section](index.html#using-modules)
for more info).

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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()
```

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# Spam checker callbacks
Spam checker callbacks allow module developers to implement spam mitigation actions for
Synapse instances. Spam checker callbacks can be registered using the module API's
`register_spam_checker_callbacks` method.
## Callbacks
The available spam checker callbacks are:
### `check_event_for_spam`
```python
async def check_event_for_spam(event: "synapse.events.EventBase") -> Union[bool, str]
```
Called when receiving an event from a client or via federation. The module can return
either a `bool` to indicate whether the event must be rejected because of spam, or a `str`
to indicate the event must be rejected because of spam and to give a rejection reason to
forward to clients.
### `user_may_invite`
```python
async def user_may_invite(inviter: str, invitee: str, room_id: str) -> bool
```
Called when processing an invitation. The module must return a `bool` indicating whether
the inviter can invite the invitee to the given room. Both inviter and invitee are
represented by their Matrix user ID (e.g. `@alice:example.com`).
### `user_may_create_room`
```python
async def user_may_create_room(user: str) -> bool
```
Called when processing a room creation request. The module must return a `bool` indicating
whether the given user (represented by their Matrix user ID) is allowed to create a room.
### `user_may_create_room_alias`
```python
async def user_may_create_room_alias(user: str, room_alias: "synapse.types.RoomAlias") -> bool
```
Called when trying to associate an alias with an existing room. The module must return a
`bool` indicating whether the given user (represented by their Matrix user ID) is allowed
to set the given alias.
### `user_may_publish_room`
```python
async def user_may_publish_room(user: str, room_id: str) -> bool
```
Called when trying to publish a room to the homeserver's public rooms directory. The
module must return a `bool` indicating whether the given user (represented by their
Matrix user ID) is allowed to publish the given room.
### `check_username_for_spam`
```python
async def check_username_for_spam(user_profile: Dict[str, str]) -> bool
```
Called when computing search results in the user directory. The module must return a
`bool` indicating whether the given user profile can appear in search results. The profile
is represented as a dictionary with the following keys:
* `user_id`: The Matrix ID for this user.
* `display_name`: The user's display name.
* `avatar_url`: The `mxc://` URL to the user's avatar.
The module is given a copy of the original dictionary, so modifying it from within the
module cannot modify a user's profile when included in user directory search results.
### `check_registration_for_spam`
```python
async def check_registration_for_spam(
email_threepid: Optional[dict],
username: Optional[str],
request_info: Collection[Tuple[str, str]],
auth_provider_id: Optional[str] = None,
) -> "synapse.spam_checker_api.RegistrationBehaviour"
```
Called when registering a new user. The module must return a `RegistrationBehaviour`
indicating whether the registration can go through or must be denied, or whether the user
may be allowed to register but will be shadow banned.
The arguments passed to this callback are:
* `email_threepid`: The email address used for registering, if any.
* `username`: The username the user would like to register. Can be `None`, meaning that
Synapse will generate one later.
* `request_info`: A collection of tuples, which first item is a user agent, and which
second item is an IP address. These user agents and IP addresses are the ones that were
used during the registration process.
* `auth_provider_id`: The identifier of the SSO authentication provider, if any.
### `check_media_file_for_spam`
```python
async def check_media_file_for_spam(
file_wrapper: "synapse.rest.media.v1.media_storage.ReadableFileWrapper",
file_info: "synapse.rest.media.v1._base.FileInfo",
) -> bool
```
Called when storing a local or remote file. The module must return a boolean indicating
whether the given file can be stored in the homeserver's media store.
## Example
The example below is a module that implements the spam checker callback
`check_event_for_spam` to deny any message sent by users whose Matrix user IDs are
mentioned in a configured list, and registers a web resource to the path
`/_synapse/client/list_spam_checker/is_evil` that returns a JSON object indicating
whether the provided user appears in that list.
```python
import json
from typing import Union
from twisted.web.resource import Resource
from twisted.web.server import Request
from synapse.module_api import ModuleApi
class IsUserEvilResource(Resource):
def __init__(self, config):
super(IsUserEvilResource, self).__init__()
self.evil_users = config.get("evil_users") or []
def render_GET(self, request: Request):
user = request.args.get(b"user")[0]
request.setHeader(b"Content-Type", b"application/json")
return json.dumps({"evil": user in self.evil_users})
class ListSpamChecker:
def __init__(self, config: dict, api: ModuleApi):
self.api = api
self.evil_users = config.get("evil_users") or []
self.api.register_spam_checker_callbacks(
check_event_for_spam=self.check_event_for_spam,
)
self.api.register_web_resource(
path="/_synapse/client/list_spam_checker/is_evil",
resource=IsUserEvilResource(config),
)
async def check_event_for_spam(self, event: "synapse.events.EventBase") -> Union[bool, str]:
return event.sender not in self.evil_users
```

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# Third party rules callbacks
Third party rules callbacks allow module developers to add extra checks to verify the
validity of incoming events. Third party event rules callbacks can be registered using
the module API's `register_third_party_rules_callbacks` method.
## Callbacks
The available third party rules callbacks are:
### `check_event_allowed`
```python
async def check_event_allowed(
event: "synapse.events.EventBase",
state_events: "synapse.types.StateMap",
) -> Tuple[bool, Optional[dict]]
```
**<span style="color:red">
This callback is very experimental and can and will break without notice. Module developers
are encouraged to implement `check_event_for_spam` from the spam checker category instead.
</span>**
Called when processing any incoming event, with the event and a `StateMap`
representing the current state of the room the event is being sent into. A `StateMap` is
a dictionary that maps tuples containing an event type and a state key to the
corresponding state event. For example retrieving the room's `m.room.create` event from
the `state_events` argument would look like this: `state_events.get(("m.room.create", ""))`.
The module must return a boolean indicating whether the event can be allowed.
Note that this callback function processes incoming events coming via federation
traffic (on top of client traffic). This means denying an event might cause the local
copy of the room's history to diverge from that of remote servers. This may cause
federation issues in the room. It is strongly recommended to only deny events using this
callback function if the sender is a local user, or in a private federation in which all
servers are using the same module, with the same configuration.
If the boolean returned by the module is `True`, it may also tell Synapse to replace the
event with new data by returning the new event's data as a dictionary. In order to do
that, it is recommended the module calls `event.get_dict()` to get the current event as a
dictionary, and modify the returned dictionary accordingly.
Note that replacing the event only works for events sent by local users, not for events
received over federation.
### `on_create_room`
```python
async def on_create_room(
requester: "synapse.types.Requester",
request_content: dict,
is_requester_admin: bool,
) -> None
```
Called when processing a room creation request, with the `Requester` object for the user
performing the request, a dictionary representing the room creation request's JSON body
(see [the spec](https://matrix.org/docs/spec/client_server/latest#post-matrix-client-r0-createroom)
for a list of possible parameters), and a boolean indicating whether the user performing
the request is a server admin.
Modules can modify the `request_content` (by e.g. adding events to its `initial_state`),
or deny the room's creation by raising a `module_api.errors.SynapseError`.
### `check_threepid_can_be_invited`
```python
async def check_threepid_can_be_invited(
medium: str,
address: str,
state_events: "synapse.types.StateMap",
) -> bool:
```
Called when processing an invite via a third-party identifier (i.e. email or phone number).
The module must return a boolean indicating whether the invite can go through.
### `check_visibility_can_be_modified`
```python
async def check_visibility_can_be_modified(
room_id: str,
state_events: "synapse.types.StateMap",
new_visibility: str,
) -> bool:
```
Called when changing the visibility of a room in the local public room directory. The
visibility is a string that's either "public" or "private". The module must return a
boolean indicating whether the change can go through.
## Example
The example below is a module that implements the third-party rules callback
`check_event_allowed` to censor incoming messages as dictated by a third-party service.
```python
from typing import Optional, Tuple
from synapse.module_api import ModuleApi
_DEFAULT_CENSOR_ENDPOINT = "https://my-internal-service.local/censor-event"
class EventCensorer:
def __init__(self, config: dict, api: ModuleApi):
self.api = api
self._endpoint = config.get("endpoint", _DEFAULT_CENSOR_ENDPOINT)
self.api.register_third_party_rules_callbacks(
check_event_allowed=self.check_event_allowed,
)
async def check_event_allowed(
self,
event: "synapse.events.EventBase",
state_events: "synapse.types.StateMap",
) -> Tuple[bool, Optional[dict]]:
event_dict = event.get_dict()
new_event_content = await self.api.http_client.post_json_get_json(
uri=self._endpoint, post_json=event_dict,
)
event_dict["content"] = new_event_content
return event_dict
```

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# Writing a module
A module is a Python class that uses Synapse's module API to interact with the
homeserver. It can register callbacks that Synapse will call on specific operations, as
well as web resources to attach to Synapse's web server.
When instantiated, a module is given its parsed configuration as well as an instance of
the `synapse.module_api.ModuleApi` class. The configuration is a dictionary, and is
either the output of the module's `parse_config` static method (see below), or the
configuration associated with the module in Synapse's configuration file.
See the documentation for the `ModuleApi` class
[here](https://github.com/matrix-org/synapse/blob/master/synapse/module_api/__init__.py).
## Handling the module's configuration
A module can implement the following static method:
```python
@staticmethod
def parse_config(config: dict) -> dict
```
This method is given a dictionary resulting from parsing the YAML configuration for the
module. It may modify it (for example by parsing durations expressed as strings (e.g.
"5d") into milliseconds, etc.), and return the modified dictionary. It may also verify
that the configuration is correct, and raise an instance of
`synapse.module_api.errors.ConfigError` if not.
## Registering a web resource
Modules can register web resources onto Synapse's web server using the following module
API method:
```python
def ModuleApi.register_web_resource(path: str, resource: IResource) -> None
```
The path is the full absolute path to register the resource at. For example, if you
register a resource for the path `/_synapse/client/my_super_module/say_hello`, Synapse
will serve it at `http(s)://[HS_URL]/_synapse/client/my_super_module/say_hello`. Note
that Synapse does not allow registering resources for several sub-paths in the `/_matrix`
namespace (such as anything under `/_matrix/client` for example). It is strongly
recommended that modules register their web resources under the `/_synapse/client`
namespace.
The provided resource is a Python class that implements Twisted's [IResource](https://twistedmatrix.com/documents/current/api/twisted.web.resource.IResource.html)
interface (such as [Resource](https://twistedmatrix.com/documents/current/api/twisted.web.resource.Resource.html)).
Only one resource can be registered for a given path. If several modules attempt to
register a resource for the same path, the module that appears first in Synapse's
configuration file takes priority.
Modules **must** register their web resources in their `__init__` method.
## Registering a callback
Modules can use Synapse's module API to register callbacks. Callbacks are functions that
Synapse will call when performing specific actions. Callbacks must be asynchronous, and
are split in categories. A single module may implement callbacks from multiple categories,
and is under no obligation to implement all callbacks from the categories it registers
callbacks for.
Modules can register callbacks using one of the module API's `register_[...]_callbacks`
methods. The callback functions are passed to these methods as keyword arguments, with
the callback name as the argument name and the function as its value. This is demonstrated
in the example below. A `register_[...]_callbacks` method exists for each category.
Callbacks for each category can be found on their respective page of the
[Synapse documentation website](https://matrix-org.github.io/synapse).