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<chapter id="calendar-architecture">
<title>Architecture of the Calendar</title>
<para>
This chapter gives an overview of the &Evolution; Calendar
architecture. It describes the model/view split of the calendar
into a personal calendar server, or &PCS;, and the GUI clients
that appear inside the &Evolution; Shell.
</para>
<!-- Model/View Separation -->
<sect1 id="calendar-model-view">
<title>Model/View Separation</title>
<para>
Like other base components in &Evolution;, the calendar
separates the data model from the views or clients. This is
done so that multiple clients can access the same calendar
data in an orderly fashion and without clashes. For example,
the user may be running a graphical calendar client. If he
then wants to synchronize his calendar with a handheld device,
then the corresponding synchronization program (e.g. a conduit
for the <application>gnome-pilot</application> package) will
also need to access the calendar storage. It is important
that both the GUI client and the synchronization program keep
a consistent view of the calendar at all times, otherwise one
of them will be left in an inconsistent state if the
calendar's data changes unexpectedly.
</para>
<para>
&Evolution; puts the calendar storage in a daemon called the
&Wombat; and completely separates it from clients who wants to
access calendar data. This part of the &Wombat; is called the
personal calendar server, or &PCS;. Clients must contact the
&PCS; and ask it to open an existing calendar or create a new
one. When a calendar component object (e.g. an appointment or
to-do item) changes in the &PCS; it will notify all the
clients that are using the component's parent calendar.
</para>
</sect1>
<!-- Personal Calendar Server -->
<sect1>
<title>Personal Calendar Server</title>
<para>
The personal calendar server, or &PCS;, provides centralized
management and storage of a user's personal calendar.
Multiple clients can connect to the &PCS; simultaneously to
query and modify the user's calendar in a synchronized
fashion. The main features of the &PCS; are as follows:
</para>
<formalpara>
<title>Storage</title>
<para>
The &PCS; is responsible for loading and saving calendars.
Centralizing the loading and saving functionality allows
multiple clients to use the same calendar at the same time
without having to worry about each other.
</para>
</formalpara>
<formalpara>
<title>Basic Queries</title>
<para>
The &PCS; provides functions to do basic queries on a
calendar, for example, a client can ask the server for a
list of all the appointments in the calendar, or for all the
data for a specific appointment.
</para>
</formalpara>
<formalpara>
<title>Recurrence and Alarm Queries</title>
<para>
Looking for the events that recur or have alarm triggers in
a specific period of time involves scanning all the
appointments in a calendar. To keep clients from having to
load whole calendars at once, the &PCS; can do these
computations and send the results to clients.
</para>
</formalpara>
<formalpara>
<title>Modification Log</title>
<para>
To allow multiple handheld devices to be synchronized
against a calendar, the &PCS; keeps a log of all the
modifications that are done to the calendar. When an
appointment is updated or removed, the &PCS; logs this
action in the modification log. Synchronization conduit
programs can then use this information to do their work.
</para>
</formalpara>
</sect1>
<!-- Data Views -->
<sect1>
<title>Data Views</title>
<para>
&Evolution; provides a graphical calendar client inside the
shell that is just a view onto the data stored in the personal
calendar server. You can launch as many views of a calendar
as you like and they will all receive notification from the
&PCS; when changes occur. The views are then responsible for
updating their respective displays.
</para>
<para>
Even within a single calendar view in the &Evolution; shell
there can be multiple clients of a single calendar. For
example, in the day view of the &Evolution; calendar there are
three widgets that act as three different clients of the
&PCS;: the multi-day view, the busy days calendar, and the
task list.
</para>
</sect1>
<!-- Non-graphical Clients -->
<sect1>
<title>Non-graphical Clients</title>
<para>
Clients of the personal calendar server can be non-graphical,
that is, they do not have to provide views of the data to the
user. Examples of such clients are the synchronization
conduit programs for handheld devices. These usually run with
no user interface as a result of being invoked by a daemon
that watches the connection to a handheld device. For
example, the calendar synchronization conduit in &Evolution;
gets run when the <application>gpilotd</application> daemon
from the <application>gnome-pilot</application> package
detects that the HotSync button has been pressed on a Palm
Pilot device.
</para>
<para>
Such clients simply take advantage of the centralized storage
in the &PCS; without presenting any graphical display of the
data; they just act as middlemen between the &PCS; and other
applications.
</para>
</sect1>
</chapter>
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