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

9 files changed, 536 insertions, 404 deletions

diff --git a/docs/SUMMARY.md b/docs/SUMMARY.md
index 44338a78b3..fd0045e1ef 100644
--- a/docs/SUMMARY.md
+++ b/docs/SUMMARY.md
@@ -37,11 +37,13 @@
     - [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)
diff --git a/docs/modules.md b/docs/modules.md
deleted file mode 100644
index ae8d6f5b73..0000000000
--- a/docs/modules.md
+++ /dev/null
@@ -1,399 +0,0 @@
-# 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
-```
diff --git a/docs/modules/account_validity_callbacks.md b/docs/modules/account_validity_callbacks.md
new file mode 100644
index 0000000000..80684b7828
--- /dev/null
+++ b/docs/modules/account_validity_callbacks.md
@@ -0,0 +1,33 @@
+# 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.
diff --git a/docs/modules/index.md b/docs/modules/index.md
new file mode 100644
index 0000000000..3fda8cb7f0
--- /dev/null
+++ b/docs/modules/index.md
@@ -0,0 +1,34 @@
+# 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
diff --git a/docs/modules/porting_legacy_module.md b/docs/modules/porting_legacy_module.md
new file mode 100644
index 0000000000..a7a251e535
--- /dev/null
+++ b/docs/modules/porting_legacy_module.md
@@ -0,0 +1,17 @@
+# 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).
diff --git a/docs/modules/presence_router_callbacks.md b/docs/modules/presence_router_callbacks.md
new file mode 100644
index 0000000000..4abcc9af47
--- /dev/null
+++ b/docs/modules/presence_router_callbacks.md
@@ -0,0 +1,90 @@
+# 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()
+```
diff --git a/docs/modules/spam_checker_callbacks.md b/docs/modules/spam_checker_callbacks.md
new file mode 100644
index 0000000000..c45eafcc4b
--- /dev/null
+++ b/docs/modules/spam_checker_callbacks.md
@@ -0,0 +1,160 @@
+# 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
+```
diff --git a/docs/modules/third_party_rules_callbacks.md b/docs/modules/third_party_rules_callbacks.md
new file mode 100644
index 0000000000..2ba6f39453
--- /dev/null
+++ b/docs/modules/third_party_rules_callbacks.md
@@ -0,0 +1,125 @@
+# 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
+```
diff --git a/docs/modules/writing_a_module.md b/docs/modules/writing_a_module.md
new file mode 100644
index 0000000000..4f2fec8dc9
--- /dev/null
+++ b/docs/modules/writing_a_module.md
@@ -0,0 +1,70 @@
+# 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).
\ No newline at end of file