The user interface lets the user enter inputs into the system. The mobile client application will handle the events generated by the user's interaction. It will also make sure that the application works normally even if the device isn't connected (offline mode). This can require keeping the application state information and caching some content in the mobile device storage (depending on the scenario).

The application will call the Web Service residing in a remote server using the SOAP serializer and WSRM in Figure 3. The SOAP serializer will create a SOAP request message as required by the service and send the message to the WSRM component for reliable communication. The response is passed from the WSRM to the SOAP deserializer to convert the SOAP response to the application-specific format.

WSRM ensures that the reliability level (exactly once, at least once, etc.) of the communication during the Web Service invocation is attained. Depending on the requirements of the application the reliability level for the Web Service invocation can be set. In the case of a reliability level being set at 'at most once,' the application can switch over to offline mode if a response isn't available. It's also possible that the application may not have to have an offline execution mode. In that case the user is kept oblivious of the connection status of the device and the WSRM component will ensure that the Web service is called whenever the connection is available.

Figure 4 shows the flow of the request process for placing an order.

The first flowchart depicts the flow of placing an order with the server. Here two Web Services are used. The first one is the QueryInventory Web Service for checking the status of the inventory. This Web Service uses a "at most once" delivery assurance profile. So if the WSRM component tries to send it once and it fails, it informs the application. Depending on the whether the response has arrived from this Web Service, the application decides whether to use the inventory value or the quota. The second Web Service is for placing the order, i.e., the SubmitOrder Web Service. This Web Service uses the 'exactly once' delivery assurance profile. Once the application has created the request, the WSRM component tries to send it; if it fails, it tries twice more and then tries again after some specified period of time.

Implementation Details
In our current implementation we're using Microsoft's .NET Framework 2.0 running on IIS 5.1 to implement Web Services. To work with WSRM messages we're using Web Service Enhancements (WSE) 3.0. On the client side we're using a Symbian smartphone running J2ME and PocketPC running .NET Compact Framework. To persist the data on the client side, we're using files.

The WSRM component of the client application installed on these mobile devices creates the WSRM-compliant messages and stores them in files. The application resends messages in case of failure. The WSRM component takes care of the 'once and only once' and 'at most once' delivery assurance depending on the configuration file associated with each Web Service.

Conclusions and Future Work
We've shown that despite the inherent occasionally connected nature of mobile devices architectures can be devised to enable enterprise-level mobile applications. Two key contributing technologies to do so include at the core a Web Services-based framework that is predicated on the universal support of Web Services in most mobile platforms. Further, for reliability, we proposed a framework based on WSRM to handle the occasionally connected problem in mobile Web Services. We've used WSRM, which is not yet an Oasis standard. Alternatively, WS-Reliability can be used, which is an Oasis standard. We decided to go with WSRM because it's consistent with other WS-* standards.

Our framework stores data in files, which is not optimal in enterprise applications. Future work can include more optimal ways of storing data on the client side.

References

• Symbian technology page with overviews of its mobile operating systems: www.symbian.com/technology/technology.html
• Home page for Windows Mobile range of mobile operating systems: www.microsoft.com/windowsmobile/default.mspx
• Home page for the Palm operating system: www.palmsource.com/palmos/
• Blackberry OS developer page with a brief description of the OS framework: www.blackberry.net/developers/started/blackberry.shtml
• Sun's resource page for J2ME: http://java.sun.com/j2me/
• Microsoft's developer documentation and resources page for .NET CF: http://msdn.microsoft.com/netframework/programming/netcf/default.aspx
• The Java Community page for JSR-172: www.jcp.org/en/jsr/detail?id=172
• Ksoap, the Open Source J2ME SOAP API: http://ksoap.objectweb.org/
• Product page for MIC: www.agiledelta.com/MIC.htm
• IBM's WSRM page: www-128.ibm.com/developerworks/library/specification/ws-rm/
• Occasionally Connected Profile v0.1, an extension of the SOA Blueprints Reference Example to deal with occasionally connected rich client interfaces: http://xml.coverpages.org/SOA-BlueprintsOccConnPv01.pdf
• Information on WSE 3.0: http://msdn.microsoft.com/webservices/webservices/building/wse/default.aspx
• Web Services Reliable Messaging TC WS-Reliability 1.1: http://docs.oasis-open.org/wsrm/ws-reliability/v1.1/wsrm-ws_reliability-1.1-spec-os.pdf