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# Slixmpp Architecture

The core of Slixmpp is contained in four classes: `XMLStream`, `BaseXMPP`, `ClientXMPP`, and `ComponentXMPP`. Along side this stack is a library for working with XML objects that eliminates most of the tedium of creating/manipulating XML.

## The Foundation: XMLStream

`XMLStream` is a mostly XMPP-agnostic class whose purpose is to read and write from a bi-directional XML stream. It also allows for callback functions to execute when XML matching given patterns is received; these callbacks are also referred to as stream handlers. The class also provides a basic eventing system which can be triggered either manually or on a timed schedule.

### The event loop

`XMLStream` instances inherit the `asyncio.BaseProtocol` class, and therefore do not have to handle reads and writes directly, but receive data through `data_received()` and write data in the socket transport.

Upon receiving data, stream handlers are run immediately, except if they are coroutines, in which case they are scheduled using `asyncio.async()`.

Event handlers (which are called inside stream handlers) work the same way.

### How XML Text is Turned into Action

To demonstrate the flow of information, let’s consider what happens when this bit of XML is received (with an assumed namespace of `jabber:client`):

    <message to="user@example.com" from="friend@example.net">
      <body>Hej!</body>
    </message>

1. Convert XML strings into objects.

   Incoming text is parsed and converted into XML objects (using ElementTree) which are then wrapped into what are referred to as Stanza objects. The appropriate class for the new object is determined using a map of namespaced element names to classes.

   Our incoming XML is thus turned into a `Message` stanza object because the namespaced element name `{jabber:client}message` is associated with the class `Message`.

2. Match stanza objects to callbacks.

   These objects are then compared against the stored patterns associated with the registered callback handlers.

   Each handler matching our stanza object is then added to a list.

3. Processing callbacks

   Every handler in the list is then called with the stanza object as a parameter; if the handler is a `CoroutineCallback` then it will be scheduled in the event loop using `asyncio.async()` instead of run.

4. Raise Custom Events

   Since a stream handler shouldn’t block, if extensive processing for a stanza is required (such as needing to send and receive an `Iq` stanza), then custom events must be used. These events are not explicitly tied to the incoming XML stream and may be raised at any time.

   In contrast to stream handlers, these functions are referred to as event handlers.

   The code for `BaseXMPP._handle_message()` follows this pattern, and raises a `'message'` event

       self.event('message', msg)

5. Process Custom Events

   The event handlers are then executed, passing the stanza as the only argument.

   Note

   Events may be raised without needing stanza objects. For example, you could use `self.event('custom', {'a': 'b'})`. You don’t even need any arguments: `self.event('no_parameters')`. However, every event handler MUST accept at least one argument.

   Finally, after a long trek, our message is handed off to the user’s custom handler in order to do awesome stuff:

       reply = msg.reply()
       reply['body'] = "Hey! This is awesome!"
       reply.send()

## Raising XMPP Awareness: BaseXMPP

While `XMLStream` attempts to shy away from anything too XMPP specific, `BaseXMPP`’s sole purpose is to provide foundational support for sending and receiving XMPP stanzas. This support includes registering the basic message, presence, and iq stanzas, methods for creating and sending stanzas, and default handlers for incoming messages and keeping track of presence notifications.

The plugin system for adding new XEP support is also maintained by `BaseXMPP`.

## ClientXMPP

`ClientXMPP` extends `BaseXMPP` with additional logic for connecting to an XMPP server by performing DNS lookups. It also adds support for stream features such as STARTTLS and SASL.

## ComponentXMPP

`ComponentXMPP` is only a thin layer on top of `BaseXMPP` that implements the component handshake protocol.