Presence Service

Overview

The Presence Service is a D-Bus service on the XO that coordinates activity sharing. It uses Telepathy to find other XOs on the network, and to provide information about their owners and the activities that they are participating in. It provides a D-Bus API for activities. Sugar provides a wrapper interface to the D-Bus API in the Python module sugar.presence.

The Presence Service is responsible for initiating Telepathy connections when the XO is turned on, and reconnecting Telepathy connections when they are disconnected. It is also responsible for registering the laptop with the Jabber server.

Sugar's “neighborhood view” (or “mesh view”) uses the Presence Service to show who is online (“present”). This performs a function similar to that of a buddy list in a conventional instant messaging application.

See Shared Sugar Activities for an overview of collaboration, or Tubes Tutorial for how to add collaboration to an activity.

Presence stack

Python activities (and Sugar) make use of sugar.presence to interact with Presence Service. sugar.presence provides an API for accessing Presence Service over the D-Bus session bus.

Presence Service in turn interacts with telepathy-gabble and telepathy-salut over the session bus, and once everything is set up (or in the case of non-Python activities) the activity can use Telepathy directly (for example, to use Tubes).

Connection types

Presence Service supports two Telepathy backends (connection managers):

• Telepathy Gabble, that provides server-based Jabber/XMPP functionality. It talks to a Jabber server (e.g. olpc.collabora.co.uk) and sets up different chat rooms through which activities talk to each other.

• Telepathy Salut, a link-local Connection Manager using reliable multicast over the local network

Although Telepathy hides the differences between the protocols, the Presence Services varies its behaviour a little according to which connection it is using. To achieve this, it internally uses a plugin structure, where each type of connection used has an associated plugin.

Only one connection manager is used at a time. The Presence Service will use Gabble when it has Internet access (a valid IPv4 address) and has connected to a Jabber server, otherwise Salut.

Active connection

Though Presence Service is designed to run both connection managers simultaneously, at this stage only one is in effect at a time, since there is no UI support for showing which activities are shared on which connection.

Presence Service will use Gabble when it has Internet access (a valid IPv4 address) and has connected to a Jabber server, otherwise Salut (but not both at the same time). Here is the sequence of events:

• Both plugins are started when Presence Service starts, unless PRESENCE_SERVICE_DEBUG=disable-gabble or disable-salut are set.
• Salut succeeds faster than Gabble, so link local buddies are shown first.
• PS's server plugin watches Network Manager signals on the D-Bus system bus.
• When NM indicates that we have a valid IPv4 address, we run the _init_connection method of the ServerPlugin instance.

• Gabble is automatically run by the session bus (dbus-daemon) via service activation, the first time the Presence Service uses it, if it isn't already running.
• If the Gabble connection fails, we schedule a timer (starting at 5 seconds with exponential backoff to a maximum of 5 minutes) and retry running _init_connection when the timer runs out.

(classes TelepathyPlugin and ServerPlugin, methods _init_connection, _reconnect_cb, _could_connect, _handle_connection_status_change.)

• If the server plugin has connected to a jabber server, PS stops the link local plugin (salut). Link local buddies disappear.
• If NM loses the IP address, the server plugin stops itself.
• When the server plugin stops, PS starts the link local plugin.

API

Python code can use the sugar.presence library to talk to Presence Service. Non-python code can use the API that Presence Service provides over the D-Bus session bus.

See Low-level Activity API#Presence for the basic operations, and Presence Service DBus API for a more comprehensive, but out of date, API listing.

Source

Browse: http://dev.laptop.org/git?p=projects/presence-service;a=summary

git clone git://dev.laptop.org/projects/presence-service