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. 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 gnome-pilot 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. &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. 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: 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. 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. 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. 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. &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. 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. 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 gpilotd daemon from the gnome-pilot package detects that the HotSync button has been pressed on a Palm Pilot device. 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.