Smart Client Architecture

Smart Client Architecture

By now you are probably aware that smart client applications differ significantly from thin client, browser-based wireless solutions. As a general rule, smart client applications enable the user to access data when disconnected from the network. There are many ways that this can be accomplished, but for the purposes of this section, we are going to focus on building applications that incorporate a persistent data store, rather than just using simple caching mechanisms.

The smart client architecture is illustrated in Figure 7.1. On the client, you have the user interface, the business logic, as well as a persistent data store. This application communicates with a back-end data source, often through an intermediate synchronization server. The communication stream itself can run either wirelessly or over a wireline connection. Depending on the technology being used, the connection may require an IP-based network or an additional communication layer for the synchronization process.

Click To expand Figure 7.1: Smart client architecture.

When this type of solution is implemented, it quickly becomes clear why it is called a smart client solution. By deploying an application to the device itself, you have the ability to give the client application some "smarts," or logic. This logic dictates many aspects of the application. It determines where the application gets it data from (either locally or from a round-trip to the server), how the data is presented and stored, as well as the set of data that needs to be communicated back to the enterprise systems via a synchronization process. In the wireless space, the impact of having business logic on the device is often overlooked as many vendors and developers focus on low-level technical features of a solution rather than satisfying the requirements of the mobile user.

The Client

Smart client applications provide many attractive features for end users. Many of these features reside in the client application itself. By providing a rich user interface with persistent data storage, smart client applications are suitable for a large variety of corporate applications. We are going to take a closer look at each of the components that comprise the client of smart client applications.

User Interface

Initially, the user interface may not seem important when developing mobile applications, as most people feel that there is always going to be a trade-off between size and usability. But this trade-off does not always have to take place. Smart client applications can have very sophisticated user interfaces. In most cases, they are programmed using development tools that have extensive component frameworks. Incorporating tables, drop-down lists, radio buttons, and graphs are straightforward operations. Contrast this with the capabilities available for most wireless Internet applications, where the user interface is often text-based with limited support for graphical components.

Whether you are developing for the Palm OS, Windows CE, Symbian OS, or a Java-based system, you will find that user interface development is a very important part of a successful application. When the screen size is limited, as it is for handheld devices, the developer must take full advantage of the space that is available. In addition, putting special focus on the navigation through an application is essential. Mobile application users are often the same people who have been using advanced Windows-based application for years; hence, they have certain usability expectations that need to be met. One of the main reasons given by consumers for their slow acceptance of wireless applications is the complexity of the available solutions. This complexity is usually due to poor application design, rather than technical limitations. Obtaining end-user feedback during the development phase via a prototype can help you address usability concerns at an early stage in the application development process. In the Mobile Operating Systems section later in this chapter, we will take a look at the most popular mobile operating systems and review the types of applications that each operating system can support.

Data Storage

When you look at the requirements of typical mobile enterprise customers, you see some trends emerge. First and foremost, data access is a requirement. Users do not really care how the data is retrieved; they care only about its availability. Theoretically, they understand that wireless access is not pervasive; nevertheless, they quickly become frustrated if they can access their data only periodically, depending on the network coverage and/or penetration. By providing data storage capabilities on the device, you can ensure that users will be able to access important data when they need it. (Note: Data storage mechanisms are not covered here, as they are covered in depth in Chapter 9, "Persistent Data on the Client.")


In addition to providing timely data access, smart client applications also provide performance benefits. Consider how computer technicians work: They travel from location to location repairing a variety of computer systems. As often as possible they carry some inventory with them for common repairs; but just as often, they need to order a special part from the warehouse and schedule a follow-up visit. Before they can schedule the next visit, they have to make sure that the part in question will be available at a time that fits into their schedule. By utilizing a smart client application that contains a persistent data store, technicians can have immediate access to inventory information, as well as their schedule. Imagine if they had to search a database for a specific part number. This task could take a considerable amount of time if a wireless browser-based application were being used. The task could even be impossible if a technician happened to be at a location that was outside of the wireless network's coverage. By implementing a mobile data store, technicians can perform a quick search of the inventory right on the device to provide them with the information that they require.

The one concern with this approach is data freshness. If the data on the device is not updated regularly, then the technician may end up, for example, believing that a part is available, when in fact it is out of stock, leading him or her to make decisions based on inaccurate data. This leads to the data synchronization aspect of smart client applications.

Data Synchronization

In most smart client architectures, the majority of the data synchronization work is executed, not on the client, but on a synchronization server. Nevertheless, the client application is still required to have a certain amount of synchronization knowledge. Minimally, the client has to know the location of the synchronization server, details about the communication stream to the server, what data has to be synchronized, and how to handle incoming data from the server.

If you are developing the synchronization layer yourself, you will have to keep these issues in mind when designing the application. If you are going to use commercially available software, most vendors provide a synchronization client module that incorporates the required functionality with various levels of sophistication. (Chapter 10, "Enterprise Integration through Synchronization" provides a closer look at the options available for data synchronization for both the client and the server.)


Smart client applications can also take advantage of intelligent application-to-application messaging systems to communicate data. To do so, a client component has to be able to both send and receive these messages. When receiving a message, it has to be smart enough to relay the information appropriately. This communication may involve notifying the user directly or, possibly, updating a set of the data in the client data store. Application-to-application messaging can be important for applications that require frequent communication with enterprise systems. (For more information on all of the leading forms of mobile and wireless messaging, refer back to Chapter 5, "Mobile and Wireless Messaging.")

The Server

Though the server component of smart client applications is invisible to the end user, it is still very important. The server component is responsible for data synchronization, data storage, and messaging.

Data Synchronization

When we refer to data synchronization, we are talking about how enterprise data is moved from the back-end enterprise system to the mobile device, and vice versa. This data movement can be accomplished in a number of ways, depending on the solution that is chosen. For a general overview, we will look only at the major components that are required in a synchronization server, as shown in Figure 7.2. These components include the interface to the client application, the synchronization logic, and the integration layer to the back-end data source.

Click To expand
Figure 7.2: Synchronization architecture.

Data can be sent between the client application and the synchronization server in a variety of formats. Transferring data over a wired connection is a fairly straightforward process. Transferring it over a wireless network can be a different story. Depending on the requirements of the synchronization software, some wireless networks may work better than others, and some may not work at all. Discussions about wireless data synchronization are saved for Chapter 10; for the purpose of this overview, we will assume a valid connection exists between the client and the server.

If you are developing an in-house solution, you may choose to create a proprietary data stream format, possibly basing it on XML, or you may decide to use an industry-specified format such as SyncML. The same options are available in commercial synchronization software. In both cases, a stream of data is being transferred that both the client and the server know how to decipher. When the server receives data, it needs to know how to process it. This capability involves understanding the data layout of the enterprise data source. In the most basic form, the synchronization logic will simply take the updates and apply them to the data store, without considering whether they should be applied or not. In more complex cases, the synchronization logic will perform a variety of duties, including conflict detection and resolution.

The integration with the back-end data source is often a simple process, using Open Database Connectivity (ODBC) or Java Database Connectivity (JDBC) drivers for direct database access. At the same time, integration can be difficult, requiring complex logic to interface with more complicated systems such as Enterprise Resource Planning (ERP) systems. In either case, providing the ability to access enterprise data is critical for an enterprise synchronization solution. Rarely does a company use only one enterprise data storage mechanism, so, ideally, the synchronization server will be flexible enough to integrate with a variety of back-end sources.

Enterprise Data Source

The final part of the smart client solution is the enterprise data itself. This data will vary in formats ranging from enterprise databases from vendors, such as Oracle, Sybase, Microsoft, or IBM, to flat-file systems and everything in between. For the more common storage systems, you should be able to find a driver or adapter that will provide an integration layer for your synchronization server. If you are using something that is uncommon or something developed in-house, you will most likely have to develop this integration layer yourself.

Keep in mind that one of the most common reasons for implementing a mobile solution is to provide enterprise data access to the mobile worker, so the enterprise integration is an essential part of the overall solution.


The term messaging can be used to describe many different types of systems. Depending on whom you are talking to and the type of system being discussed, messaging can mean email, paging, SMS, voice, text, data, or a variety of other things. For the purpose of this section on smart client applications, messaging will be referred to in the context of application-to-application messaging with store-and-forward capabilities. (If you are interested in text messaging as it refers to thin client applications, refer to Chapter 5, "Mobile and Wireless Messaging," and Chapter 11, "Thin Client Overview.")

As seen in Figure 7.3, the basic form of smart client messaging is composed of a message queue on the client and another queue on the server. These message queues allow you to communicate with messaging system without a wireless network connection. When a message is sent from the server, it is simply placed in the queue that corresponds to a particular client alias. When the client matching that alias connects to the server, the messages in the server-side queue are sent to the client. In the same way, when a client sends a message to the server, it goes into a queue on the client device. When the client connects to the server, the messages from the client queue are sent to the server. This is commonly called store-and-forward messaging, since messages are stored when a connection is not available and forwarded when the connection becomes available.

Click To expand
Figure 7.3: Store-and-forward messaging.

When building smart client applications you have three options:

  • Create a client that uses a persistent data store and synchronization.

  • Create a client that uses store-and-forward messaging.

  • Create a client that combines both a persistent data store and store-and-forward messaging.

Depending on the type of solution being implemented, any one of these three options may meet your needs.