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<!doctype article PUBLIC "-//Davenport//DTD DocBook V3.0//EN" [
<!entity Evolution "<application>Evolution</application>">
<!entity CUA "<acronym>CUA</acronym>">
<!entity PCS "<acronym>PCS</acronym>">
<!entity Bonobo "<application>Bonobo</application>">
<!entity CORBA "<acronym>CORBA</acronym>">
<!entity GTK "<acronym>GTK+</acronym>">
]>
<article class="whitepaper" id="calendar">
<artheader>
<title>&Evolution; Calendaring Framework</title>
<authorgroup>
<author>
<firstname>Federico</firstname>
<surname>Mena Quintero</surname>
<affiliation>
<address>
<email>federico@helixcode.com</email>
</address>
</affiliation>
</author>
</authorgroup>
<copyright>
<year>2000</year>
<holder>Helix Code, Inc.</holder>
</copyright>
<abstract>
<para>
The &Evolution; groupware suite provides a framework for
developing calendaring applications, as well as a graphical
calendar client and a personal calendar server. This white
paper describes the architecture of the &Evolution;
calendaring framework.
</para>
</abstract>
</artheader>
<!-- Introduction -->
<sect1 id="introduction">
<title>Introduction</title>
<para>
Calendaring is an important part of a groupware suite. A
calendaring framework will allow a user to keep a personal
calendar and have several applications use it. Such
applications could be a graphical calendar client that the user
employs to schedule appointments and keep track of his time, a
<productname>Palm Pilot</productname> synchronization client, or
a simple alarm or reminder utility. A comprehensive calendaring
framework will also allow multiple users to schedule
appointments between each other; for example, a project director
may want to schedule a weekly meeting with the rest of the
project members, or a person who owns a large house may want to
schedule a big party with his friends. The attendees will then
want to reply with messages such as, “I will
attend”, or “I will attend only if the proposed time
is changed”.
</para>
<para>
The &Evolution; groupware suite provides a framework for
developing calendaring applications, as well as a graphical
calendar client or calendar user agent (&CUA;) and a personal
calendar server (&PCS;).
</para>
<para>
The following sections explain the basic calendaring framework,
the functions of the calendar user agent and the personal
calendar server, and the relationship between the two.
</para>
</sect1>
<!-- Personal Calendar Server -->
<sect1 id="pcs">
<title>Personal Calendar Server</title>
<para>
The personal calendar server (&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>
Clients can ask the &PCS; for a list of the appointments that
occur within a specified time range; for example a graphical
client that has a per-week view could ask the &PCS; for all
the appointments that occur in a particular week. This
includes multiple occurrences of a single recurring event; for
example, the object for “a 1-hour meeting that occurs on
every Tuesday and Thursday” is represented inside the
&PCS; as a single event with a recurrence rule. Similarly,
clients can ask the &PCS; for a list of events that have
alarms that trigger within a specified time range.
</para>
</formalpara>
<formalpara>
<title>Notification of Changes</title>
<para>
This is the most important function of the &PCS;, as it allows
multiple calendar clients to maintain a unified view of the
calendar between the server and themselves. When a client
asks the &PCS; to modify or remove an event, the &PCS;
notifies all the clients that are connected to it about the
change. The policy is that “the server is always
right”; clients can act as dumb views onto the
calendar's data and they will be notified by the &PCS; when
something changes.
</para>
</formalpara>
</sect1>
<!-- Calenar User Agent -->
<sect1 id="cua">
<title>Calendar User Agent</title>
<para>
A calendar user agent (&CUA;) is a program that lets a user
manipulate a calendar. &Evolution; provides an attractive,
graphical calendar client that communicates with the &Evolution;
personal calendar server.
</para>
<para>
The &Evolution; calendar client just provides a view onto the
data that is stored and managed by the personal calendar server.
The calendar client does not perform direct manipulations on a
calendar's data; instead it offloads those requests to the
calendar server, which takes care of making the appropriate
modifications in the calendar and then notifies all the clients
about the changes.
</para>
</sect1>
<!-- Calendar Client Library -->
<sect1 id="client-lib">
<title>Calendar Client Library</title>
<para>
Communication between the personal calendar server and calendar
clients is defined by a set of &Bonobo; &CORBA; interfaces.
Clients can be written by implementing the client-side
<classname>Listener</classname> interface, which defines the
notification callbacks that the PCS uses to inform clients about
changes to the calendar.
</para>
<para>
As a convenience for >K; programmers, &Evolution; also
includes a library which provides a
<classname>CalClient</classname> class which can be used for
communication with the personal calendar server. Objects of
this class automatically contact the PCS when they are created.
<classname>CalClient</classname> provides functions to request
changes in the calendar, and it also emits signals when it gets
notification about changes from the PCS. This makes it easy and
convenient to write calendar clients for &Evolution; using
>K;.
</para>
<para>
The implementation of the <classname>CalClient</classname> class
simply wraps the &Evolution; &CORBA; interfaces for calendaring
with a familiar-looking >K; object. Calls to the
<classname>Listener</classname> interface get translated to
signal emissions from the <classname>CalClient</classname>, thus
shielding programmers from the details of the &CORBA;
interfaces.
</para>
</sect1>
</article>
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