The Stetsons are a family of four. David, the father, is a sales professional. He often visits clients out of the office. Mary, the mother, works from home. She processes claims for an insurance company. Susan, the sixteen-year old daughter, attends high school. She is also an aspiring ballerina. Mark, the seventeen-year old son, also attends high school. He is an aspiring soccer player. While all of them are busy with their work and activities, they remain a close-knit family. They often schedule activities that involve two or more family members. Such activities include family dinners, going together to the theater, cheering at Mark's soccer games, and attending Susan's ballet performances. The family uses a Web-based calendar service provided by a service provider to schedule several family activities.
Figure 3-2 shows the setting for the Web-based family calendar management application. Different family members use the Client parts of the same calendar application implemented on different devices. Mary uses her home PC to view and make entries in the family calendar. She synchronizes her calendar with the Server's calendar when she dials in to the network. Mark uses a PDA. He uses the PDA's calendar to view his schedule and make changes, and occasionally synchronizes with the Server using the PDA's wireless dial-up connection. David and Susan both use a mobile phone. They operate in the same mode as Mary and Mark but with two key distinctions. They do not have to use dial-up, as the mobile phones are mostly connected. They also have the additional capability of being alerted by the Server's calendar. David also uses a laptop version of the family calendar application.
Figure 3-2. The setting for a Web-based family calendar management application.
The laptop and the PC versions of the application additionally allow management of application preferences using a Web browser. Clearly, although the different Client calendars operate on the same data, the individual user experience may be different. The user interfaces are different, the manners of connection are different, and the application features are different. The Web version of the family calendar application runs on a server-class machine operated by a service provider. The service provider supports multiple underlying communication protocols. The Server calendar application implements most of the application logic discussed below. The family calendar also occasionally "consults" David's work calendar before scheduling personal appointments during work hours. David's company values work-life balance tremendously and has allowed limited access to his business calendar from his family calendar via a Web service interface.
The family calendar application has certain associated logic and semantics. The "rules" by which the application is guided include the following:
Family events may include only some and not necessarily all members of the family.
Events with overlapping times can be scheduled, provided that both events do not include the same family member.
Scheduling conflicts are resolved by the Server calendar application using configurable policies, such as priorities of meeting types (e.g. ballet performances have priority over lunches) or priorities of meeting originator, (e.g. events scheduled by Mary have priority over ones scheduled by David).
Certain events can be marked urgent. When an event is marked urgent, other family members are alerted to synchronize (in cases where they have Client devices that can be alerted), thus becoming aware of the urgent event.
When family events are scheduled during normal work hours, the Server's calendar must request clearance from David's business calendar.
Clearly, a family calendar can have associated rules that are simpler or more complex than the above. The rules above are only illustrative.
The following explores a few usage instances of this application and their possible realizations using different Client and Server application parts and SyncML as the underlying synchronization protocol.
Mary schedules a family dinner
Mary opens up the family calendar application on her PC. She enters a family dinner event in the calendar from 6:30 PM to 9:30 PM on the coming Friday. She then dials in to do some online shopping. The PC calendar application detects a connection and initiates a SyncML synchronization session with the Server calendar using the Hyper Text Transfer Protocol (HTTP). The Server calendar application accepts Mary's new entry, as it generates no conflict based on the rules above. Later, David, Susan, and Mark synchronize with the Server calendar and the calendars on their respective devices get updated with Mary's new entry.
Mark reschedules a soccer date
Mark just learned that his soccer game got moved from 3 PM Friday to 4 PM Thursday. David was scheduled to attend the game, taking off from work early on Friday. Mark updates the soccer game entry on his PDA application, marks it urgent for David to take notice, and synchronizes. The PDA application uses a SyncML Client to synchronize, using HTTP over a wireless dial-up connection. The Server calendar application receives the update and determines to check David's business calendar. It checks the business calendar and finds that David is available during the desired time. The Server calendar accepts Mark's change. Since Mark designated the update as urgent, the Server calendar uses the SyncML Server-alerted synchronization and alerts David to synchronize using the WAP Push [WPU01] feature on his mobile phone. David synchronizes upon the alert, becomes aware of the change, and is able to attend his son's soccer game.
 The business calendar is accessible as a Web service.
Mary chooses ballet over lunch
Susan wants Mary to be present during her final rehearsal for an upcoming ballet performance. Susan therefore makes a new entry in the calendar application on her mobile phone, indicating the event from noon to 1:30 PM Wednesday. She then synchronizes with the Server calendar. Her mobile phone calendar application uses SyncML with the underlying WAP transport protocol. The Server calendar accepts Susan's entry.
David and Mary sometimes get together for lunch during workdays. Unaware of Susan's new entry, Mary makes a new entry in the PC calendar application for lunch with David on Wednesday. When she synchronizes the PC calendar, the Server calendar application detects a conflict and, using a set of conflict resolution rules, resolves the conflict in favor of the ballet performance instead of the lunch appointment. The result of the conflict resolution is communicated in the same synchronization session. The PC calendar application processes the status message and indicates the ballet appointment in Mary's PC calendar. The application also chooses to communicate the conflict and its disposition to Mary, using a dialog box or other means.
The Benefits of SyncML
The use of SyncML to enable the above applications affords a number of benefits. We discuss the benefits from the perspective of the different parties involved in the realization and use of the application.
The consumer perspective
Imagine how you would feel if the hammer you have determines the nails you could use. Unfortunately, without SyncML, that is more or less an accurate characterization of mobile applications that require data synchronization. SyncML provides the user with the freedom to choose devices and service providers relatively independently of each other. Different members of the Stetson family use different devices according to their preference. The users are not compelled to use one particular device because a service provider only interoperates with that device. The user can also choose to change devices at some future time.
Consider a few examples of consumer flexibility. Mark may choose to use a mobile phone instead of a PDA if he feels the need for more spontaneous connectivity. SyncML also allows the Stetson family to choose service providers. In the future, if a different family calendar service provider offers a feature that they like, such as selective viewing of calendar entries to enable organization of surprise birthday parties, the Stetsons are free to switch to that service provider without having to buy a set of new devices. If another service provider offers a shared family picture gallery, the Stetsons are free to add that service provider as well. SyncML enables the user to break free from artificial and cumbersome restrictions imposed by proprietary synchronization technology.
The device manufacturer perspective
For a company selling nails it is important that any hammer be able to drive those nails. If the nails can only be driven by one kind of hammer, that severely restricts the market for the nails. Similarly, for the manufacturer of David's mobile phone, it is important that the applications on the phone interoperate with the Server counterparts provided by various service providers. For the calendar application, the device manufacturer (or the application developer for a PC or PDA) can focus on user interaction and minimal required logic, such as keeping a record of local changes. The SyncML software on the device can handle the remaining mechanics of data synchronization by using standard data formats such as vCard [VCARD21], the SyncML Representation Protocol, the SyncML Synchronization Protocol, and HTTP or WSP [WSP01] transport protocols. Since the SyncML software on the device will interoperate with the corresponding SyncML software on the Server, the application on the device can work with applications on diverse Servers.
This is not necessarily true if there is a high level of semantic coupling between Client and Server application parts. In the family calendar application, most of the application logic is implemented on the Server. The Client part of the application is purposely generic and simple, enabling the Client application to work with diverse Servers. Having a common synchronization stack and simpler Client applications also reduces memory requirements on mobile devices.
The service provider perspective
For a company selling hammers it is important that their hammers be able to drive any nail. Similarly, it is important for service provider applications to interoperate with any device. By using the SyncML Synchronization Protocol underneath, the service provider's application can work with SyncML compliant applications on diverse Client devices. This enables broader market reach and penetration, driving revenue for the service provider.
The service provider can focus primarily on the application logic and semantics of data rather than trying to offer multiple synchronization protocols to suit the needs of many Clients. Using the specified SyncML transport bindings, the service provider can support Clients accessing the application via multiple transport protocols. The Stetsons use HTTP and WSP protocols from different devices. The service provider is also able to exploit certain characteristics of mobile devices and associated transport protocols by using additional features of SyncML, such as Server-alerted synchronization. The Server-alerted synchronization capability allows David to be quickly aware of the rescheduling of Mark's soccer game.
SyncML affords many advantages to the service provider. A family calendar application that correlates with the business calendar of a family member is much more valuable than one that cannot. Such functionality is orthogonal to SyncML and can be freely implemented by service providers.