Computing devices have become increasingly mobile at the client end, and the

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1 Pronto: Mobie Gateway With Pubish- Subscribe Paradigm Over Wireess Network Eiko Yoneki University of Cambridge Pronto, a messaging middeware system for mobie appications, provides both centraized and decentraized modes. It addresses design issues in mobie computing, incuding data optimization and disconnected operation. Pronto consists of a ightweight message-oriented middeware (MOM) cient and an inteigent gateway serving as a message hub with store-and-forward messaging. Computing devices have become increasingy mobie at the cient end, and the diversity of cients and networks creates compex environments for mobie appications. In a mobie/wireess environment, devices have a sma ROM/RAM footprint, high atency, and ow bandwidth. Connections are frequenty interrupted. Middeware communications service is especiay important for integrating such hybrid environments into coherent distributed systems. We deveoped Pronto, a messaging middeware system for mobie appications over mobie/wireess networks, to address these issues. Evoution of message-oriented middeware Intercommunication commony uses directed inks between tighty couped senders and receivers such as in RPC or CORBA. A message s destination must be known at the time of sending, which is difficut when destinations change or the number of recipients varies. By contrast, MOM (message-oriented middeware) encourages oose couping between message senders and receivers with a high degree of anonymity, thereby removing static dependencies in distributed environments. MOM s characteristics (an intuitive programming mode, atency hiding, guaranteed deivery, and store-and-forward messaging) are highy appeaing for mobie appications. The business domain acknowedges MOM through the provision of Sun s Java Message Service ( as a common interface. IEEE Distributed Systems Onine Pubished by the IEEE Computer Society 2003 IEEE

2 JMS offers the pubish-subscribe paradigm and expands previous messaging capabiities. Existing MOM software, such as IBM s MQSeries ( Softwired s ibus ( and BEA s WebLogic ( has been rapidy integrated under the JMS API. A compementary distributed system based on a peer-to-peer network emerged independenty of the enterprise domain. The peer-to-peer interaction mode faciitates sophisticated resource sharing between peers over the Internet s edges. There have been research efforts to buid messaging systems over peerto-peer networks. Microsoft s Scribe ( is a topic-centric messaging system based on Pastry. 1 The Siena project created a distributed event service, 2 whie Hermes offers an event-broker system. 3 Gyphon ( is a distributed topic- and content-based message-brokering system. Narada ( is based on a grid event service, and it impements JMS. Integrating messaging systems and mobie computing In the enterprise domain, messaging is increasingy popuar for communication because it supports decouped interactions. However, intranets with centraized administration sti dominate. Recent extensions to mobie computing add an edge server to manage mobie devices. This approach is not fuy scaabe to dynamicay extended businesses over Internet services or mobie computing. Because of the pubish-subscribe paradigm s power, peer-to-peer network-based messaging offers the most promise for estabishing communication patforms over wide-area networks. However, support for mobie computing is not yet sufficient. Most research focuses on integrating ad hoc networks into peer-topeer networks, but this approach wi not support mobie computing in its fu generaity. Decentraized messaging systems are sti immature and fragmented. Most JMS products impement centraized server modes. To provide rich JMS functionaity, especiay persistent message deivery, servers require databases for message storage. So far, none of the commercia products has successfuy impemented JMS in a decentraized mode with enterprise-eve messaging functionaity. 2

3 Integrating messaging systems in both centraized and decentraized modes, incuding a mobie environment, requires architecting a messaging system from a unified viewpoint. MOM design issues in mobie computing Given the characteristics of mobie devices and wireess networks, more work is required for optima performance. Important design issues are described beow: Wireess networks are becoming increasingy packet oriented. With a bearer such as GPRS (Genera Packet Radio Service) or UMTS (Universa Mobie Teecommunications System), users typicay pay for ony the data they communicate. Reducing data size for transmission is crucia. Middeware shoud provide an interface to appications that aows communication to be maintained during disconnected operation. Dependabe caching is essentia. A data source can be interpreted in different formats and semantics, depending on the specifications of mobie devices and wireess networks. Semantic transcoding technoogy 4 wi aow a more efficient data fow. Bearers incude 2G, 2.5G, 3G, Buetooth, and IEEE , and many devices are nonprogrammabe. Middeware must offer an interface that provides a communication abstraction. An ad hoc network is a dynamicay reconfigurabe wireess network without a fixed infrastructure and with no need of intervention by a centraized access point. What Pronto offers We deveoped Pronto, a messaging middeware system for mobie appications. 5 It integrates mobie environments over MOM on the basis of JMS, and it provides soutions for mobie-appication-specific probems such as resource constraints, network characteristics, and data optimization. Pronto consists of two main components, a ightweight MOM cient (MobieJMS Cient) and an inteigent gateway (Gateway, deveoped at the University of Cambridge 6 ). A MobieJMS Cient adapts JMS to mobie environments in both centraized and 3

4 decentraized forms. JMS specifications and interfaces are compex, but not a functions are required for mobie environments. For exampe, Pronto impements durabe subscription but not a message types. As an extension of the JMS API, Message Seector is added to support content-based subscription. Message Seector is a fiter for a topic and as such provides greater fexibiity for appications with couping between producers and consumers. Serveress JMS is a nove decentraized version of MobieJMS Cient that uses IP muticast 7 as a transport mechanism. This mode performs best in two network environments: tempora group communication over ad hoc networks, and highspeed transmission of many messages for workoad distribution. IP muticast permits efficient transmission of messages from one pubisher to many subscribers, and the number of subscribers can increase without impacting network traffic. A pubisher acts as a temporary server and keeps a subscription ist. However, some JMS features such as durabe subscription were omitted, and Serveress JMS requires an IP environment in the current impementation, given the nature of the network mode and IP muticast protoco. We introduced the inteigent Gateway 6 in Pronto to serve as a message hub using store-and-forward messaging. Gateway provides powerfu optimization of data reduction and transformation. It is aso a MobieJMS Cient to serve as a message controer proxy. This aows construction of a distributed messaging system over JMS servers through cascading gateways. Gateway is a framework within which to perform pug-in functions for caching, semantic transcoding, device-specific transport, and message transformation. It aso supports disconnected operations. Semantic transcoding offers more than simpe data reduction: the information is made more abstract (providing compaction), and data shoud be evauated by the appications whenever necessary. Here, the data are inked to an annotation, which can be a document summary, a inguistic description of the content for voice synthesis, or grayscae/owresoution image data. A caching component, SmartCaching, provides generic caching, a centra function for message storage in Gateway. SmartCaching offers pu, subscribe, and snapshot services. The subscribe service provides asynchronous notification of cached data to cient appications so that appications needn t pu previousy requested data. The subscribe service cient permits appications to be event driven and active. The snapshot service provides a specia period during which mobie appications can obtain the ast cache image after disconnection. Gateway and SmartCaching are key technoogies for improved messaging among mixed mobie-tier environments in dynamic connectivity scenarios. Their 4

5 APIs are independent from the JMS API. Depoyment in a centraized mode Figure 1, an overview of a distributed system based on Pronto, iustrates different depoyment possibiities: Appication with MobieJMS Cient: An appication on a mobie device uses MobieJMS Cient and communicates directy with the JMS server. Appication with a oca Gateway: An appication on a mobie device uses a oca Gateway API. The oca Gateway, running either as a separate thread or within the appication, performs caching and transcoding through pugged-in components. Appication with a remote Gateway: An appication on a mobie device uses a remote Gateway API. The remote Gateway runs as a separate process, and transport based on the remote method invocation is impemented. Nonprogrammabe devices with a remote Gateway: Nonprogrammabe devices require a remote Gateway to perform proper transportation and message transformation for the target device. The remote Gateway represents every subscriber and pubisher for the nonprogrammabe device. 5

6 Figure 1. System overview showing depoyment possibiities with Pronto in a centraized mode. Disconnected operation Pronto incorporates these approaches for disconnected operation: Durabe subscription in JMS: Nondurabe subscriptions ast for the subscriber object s ifetime. A subscriber can become durabe by registering a durabe subscription with a unique identity. Gateway cache: Gateway maintains the cache even if appications are inactive. Appications can use puing, subscribe, and snapshot services on appropriate occasions. For exampe, an appication spawns a background activity that synchronizes the on-device messages when connected, and the appication can use cached messages ocay when disconnected. Depoyment in a decentraized mode Figure 2 describes two depoyment exampes in a decentraized mode: 6

7 A chat system over an ad hoc network: Figure 2a shows a chat system using Serveress JMS over a tempora network. Serveress JMS in the pubisher uses the autodiscovery function to detect subscribers. It maintains subscriptions and doesn t require a centra access point. Gateway cascade: Mutipe Gateways distribute messages to the target Gateways; from there, messages are sent to the devices. In Figure 2b, a Serveress JMS over a high-speed bus (JMS bus) can be LAN or WAN based if the routers aow IP muticast. The Gateway and JMS bus combination offers powerfu message fow contro for optimization. Figure 2. Depoyment exampes with Serveress JMS: (a) a chat system over an ad hoc network; (b) gateway cascade. 7

8 Appication exampe: A mixed-media chat system Figure 3 shows an appication in which a cients subscribe to the topic chat. A Gateway residing in the mobie aptop contains voice synthesizer and Short Message Service pug-in components. ipaq devices monitor the chat through the gateway cache. Additiona devices can join the chat anytime without additiona component changes. Figure 3. A mixed-media chat appication. Future of Pronto Pronto aims to extend messaging over wireess networks and sove inherent probems. We wi further deveop Pronto in severa directions. Topics for pubish-subscribe have to register pubicity and are used by other appications as remotey accessed distributed objects. A standard API for pubishing, managing, and accessing pubic references to distribute functionaity over a mobie environment wi be critica. This API wi incude security features 8

9 such as encryption, authentication, and access contro on distributed objects. We are working on an event-based distributed system over a peer-to-peer network using a mutievent-broker mode. The event broker approach is attractive for integrating generic event-based systems with mobie computing because it aows heterogeneous messaging systems to be combined under a unified interface. In particuar, constrained mobie devices can expoit resource sharing from middeware systems. We wi integrate Pronto with the eventbroker system. Future research wi focus on ways to abstract messaging in hybrid environments. Acknowedgments My thanks to Jean Bacon and Jon Crowcroft, University of Cambridge, for constructive comments and suggestions. References 1. A. Rowstron and P. Drusche, Pastry: Scaabe, Decentraized Object Location and Routing for Large-Scae Peer-to-Peer Systems, Proc. IFIP/ACM Int Conf. Distributed Systems Patforms (Middeware), LNCS 2218, Springer-Verag, 2001, pp A. Carzaniga et a., Content-Based Addressing and Routing: A Genera Mode and Its Appication, tech. report CU-CS , Dept. of Computer Science, Univ. of Coorado, P. Pietzuch and J. Bacon, Hermes: A Distributed Event-Based Middeware Architecture, Proc. 22nd Int Conf. Distributed Computing Systems Workshops (ICDCSW 02), IEEE CS Press, 2002, pp K. Nagano et a., Semantic Annotation and Transcoding: Making Web Content More Accessibe, IEEE Mutimedia, vo. 8, no. 2, 2001, pp E. Yoneki and J. Bacon, Pronto: MobieGateway with Pubish-Subscribe Paradigm over Wireess Network, tech. report UCAM-CL-TR559, Univ. of Cambridge, 2003, 6. E. Yoneki and J. Bacon, Gateway: A Message Hub with Store-and-Forward Messaging in Mobie Networks, Proc. 1st Int ICDCS Workshop Mobie Computing Middeware (MCM 03), IEEE CS Press, RFC1112, Host Extensions for IP Muticasting, Internet Eng. Task Force; Eiko Yoneki is a PhD candidate in the Computer Laboratory at the University of Cambridge, UK. Her research interests incude event-based distributed systems over mobie/wireess networks, incuding security, data synchronization, and distributed functionaity. She is a member of ACM and the IEEE Computer Society. Contact her at eiko.yoneki@c.cam.ac.uk. 9