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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).
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