Ana Paula Afonso, Mário J. Silva, João P. Campos, Francisco S. Regateiro. Departamento de Informática. Faculdade de Ciências da Universidade de Lisboa.
The Design and Implementation of the Ubidata Information Dissemination Framework Ana Paula Afonso, Mário J. Silva, João P. Campos, Francisco S. Regateiro Departamento de Informática Faculdade de Ciências da Universidade de Lisboa Phone: +351.1.7500153, Fax: +351.1.7500084 {apa, mjs, jcampos, fasr}@di.fc.ul.pt
Abstract. The dissemination model for information delivery has motivated an increasing interest, both in wired and wireless environments. In this model, the user subscribes the desired information and then passively receives new and filtered information. We propose a model for dissemination of information to mobile users, called dynamic channel model. It is a set of extensions to the conceptual models developed for information channels, enabling the capture of mobility-specific requirements. These are used for filtering and setting update order priorities for the information that users intend to monitor, which depends on their location and changes as they roam. This model is supported in Ubidata, an adaptable framework for information dissemination in mobile computing systems.
1. Introduction Mobile computing environments are characterized by frequent disconnections, resource limitations, bandwidth restrictions and fast-changing locations. A requirement of any information system for such a computing environment is the efficient access to critical data regardless of location. One key constraint is that data should only be transferred between the stationary network and the mobile computer if absolutely necessary. These characteristics impose flexibility and adaptability as a key criteria in the design of mobile information systems [8]. The need for alternatives to the traditional form of data distribution (client-server model) has been identified in modern large-scale networks, and information dissemination models were proposed to support the requirements/constraints of these environments [5]. In this model, a set of publishers uses channels to disseminate information to a large set of subscribers. Recently, push-based techniques [7] has been proposed as a means of disseminating information to a large set of users in wireless and wired environments. To address the intrinsic needs of mobile environments, we have developed Ubidata, a general and adaptable framework for mobile computing dissemination systems. This framework has been specifically designed for providing a solution to information distribution to a large set of mobile users, based on extensions to the conceptual models developed for information channels [4], and a hybrid scheme (combining push and pull) of dissemination of information [6].
2. Dynamic Channel Model Our conceptual model, called Dynamic Channel Model, is inspired on the notions introduced by D. Cheriton [5]. An information channel, shortly channel, is defined as an abstraction of communication and system resources necessary to distribute information to a set of users. Our application implements the publisher/subscriber model [6]. The contents carried by a channel are grouped into logical and atomic units called items. An item may be a source file or a database object represented on a SQL statement in the case of a database source information. When a subscriber adds a channel to his reception list, and before starting receiving data on that channel, he must select and configure the items he is interested on. Our work is based on a hybrid schema of data delivery [6]. We provide two different types of data delivery that may be combined to optimize application requirements: periodic pull and aperiodic push. The subscriber uses polling (periodic pull approach) to obtain data from publishers, defining individual schedules for each of the items subscribed. We use aperiodic push to broadcast information to reachable subscribers from the publisher. Publishers can associate notifications to the items. Push delivery is event-driven - a subscriber of an item with a notification receives an attention message triggered by an event such as a data modification in the item contents. A channel item of a channel has a region to model a geographical area, represented as a composite shape. We have augmented the channel concept with this characteristic to enable the adaptation of the items’ visualization behavior to the location of the subscribers. We coined the term dynamic channels to designate a channel with the basic adaptation properties we have just introduced. The class diagram representing our dissemination model and the detailed description of the dynamic channel model is presented in [1].
3. Ubidata The goal of Ubidata is to provide a flexible and general framework for mobile computing information dissemination systems. Ubidata can serve as the basis for development of information systems that have in common some characteristics, such as the distribution of information sources to a potentially large set of users. The architecture of Ubidata follows the three-tiered scheme common to information systems. The Presentation Layer, which includes the graphical interface, enables the user to view data sent by the publisher. The Application Logic Layer encloses the objects representing domain concepts that fulfil application requirements (Domain Objects) and the supporting services (Service Objects). The Storage Layer encloses databases or files that contain the information to be disseminated by the publisher and received by subscribers. The fundamental domain objects are Transmitter and Tuner. The framework also includes a set of service objects: Positioning System Manager, Replication Manager, Web Server, Event Listener and Event Manager. The detailed description of each domain and service object is presented in [1].
Development Process In our perspective, the elaboration and construction of information dissemination applications consists in the personalization of our dissemination model and customizing the software components of Ubidata. The development process we propose comprises the major steps of analysis, design, and implementation. We have introduced new steps to capture channel-specific information. We organized these steps in a methodology that uses UML [3]. To provide automatic support for the development process of mobile applications, Ubidata includes GIL, an open and portable tool for editing and generating dynamic channel specifications [2]. GIL is a multi-user, collaborative, editing tool. GIL is a tool that guides its users through the development of information dissemination applications, allowing them to create and characterize channel representations that automatically configure a software framework to disseminate third party contents.
4. Application We have applied the concepts presented in Section 2 and 3 to the development of a prototype mobile information system for firefighters of the National Civil Protection Service. We developed a first version of the fire-fighters system that runs on laptops with Microsoft database and Web software (for implementation details see [1]). We intend to evaluate GIL and Ubidata usability by monitoring its use in the development of application prototypes and comparing the software metrics of these prototypes against those of identical applications built using generic software development environments. This information will be used to improve a second version of GIL and to drive further refinements to the Ubidata architecture. We are working on a second version of Ubidata to run on much smaller devices, PDAs and smart mobile phones.
References [1] A.P. Afonso, F.S. Regateiro, and M.J. Silva, Dynamic Channels: A New Methodology for Mobile Computing Applications, Technical Report DI/FCUL TR-98-4, Department of Computer Science, University of Lisbon, April 1998. [2] A.P. Afonso, J.P. Campos, and M.J. Silva, GIL: a Software Tool for the Construction of Information Dissemination Systems, Technical Report DI/FCUL TR-99-4, Department of Computer Science, University of Lisbon, June 1999. [3] G. Booch, J. Rumbaugh, and I. Jacobson, The Unified Modeling Language User Guide, Addison Wesley, 1998. [4] CDF documentation, http://www.w3.org/TR/NOTE-CDFsubmit.html. [5] D. Cheriton, Dissemination-Oriented Communication Systems, Technical Report, Stanford University, 1992. [6] M. Franklin and S. Zdonik, A Framework to Scalable Dissemination-Based Systems, ACM OOPSLA Conference (Invited Paper), October 1997. [7] Push Publishing Technologies, http://www.strom.com/imc/t4a.html. [8] M. Satyanarayanan, Fundamental Challenges in Mobile Computing, 15th ACM Symposia on Principles of Distributed Computing, 1996.
