1AbstractWith the advent of hand-held computing devices,the need for collaborative applications has becomemore urgent. In order to allow for the design of collabo-rative applications, a common infrastructure for thesharing of data is required for mobile devices. Such aninfrastructure should be built for devices that have lowcompute capabilities and have low bandwidth, highlatency network connections. The infrastructure shouldalso allow for such devices to periodically disconnectfrom the network and reconnect, as is typical of the useof hand-held computing devices. Typical softwareimplementations of distributed shared memory are notsuitable for mobile devices. Thus this study implementsand explores a software based shared object system formobile devices. An application was built on top of thisinfrastructure. The design, implementation, results andlimitations of the whole system are described.1. IntroductionWith the advent of hand-held computing devices,the need for collaborative applications has become moreurgent. Essential functionality such as being able toschedule meetings, read up to date address book entries,describe concepts on shared whiteboard spaces and todisseminate data is now required by the users of suchdevices.In order to allow for the design of such collabora-tive applications, a common infrastructure for the shar-ing of data is required for mobile devices. Such aninfrastructure should allow any application to createshared data that any mobile device can read and modify,irrespective of the mobile devices’ locations in the inter-connection framework. Such an infrastructure should bebuilt for devices that have low compute capabilities andhave low bandwidth, high latency network connections.The infrastructure should also allow for such devices toperiodically disconnect from the network and reconnect,as is typical of the use of hand-held computing devices.Typical software based DSMs, distributed sharedmemory, are not suitable for mobile devices becausethey were not designed to meet these constraints. Thusthis study implements and explores a software basedshared object system for mobile devices.The rest of this paper is organized as follows. Sec-tion 2 discusses existing software based DSMs. Section3 describes the design requirements of shared memoryfor such devices. Section 4 describes our implementa-tion. Section 5 presents an application built on this infra-structure. Section 6 describes the limitations of thisstudy and its implementation while section 7 concludesthe paper.2. Related WorkSeveral software based shared memory systemshave been explored by researchers in the past. This sec-tion will describe the main published works in this area.Treadmarks [K94], [K95], [KCZ92], [D93] is anexcellent example of a software implementation of dis-tributed shared memory. This system was designed forsharing data in an application running across multipleworkstations connected by a high speed network. Amodel of release consistency is implemented on a multi-ple writer update protocol. This implementation is notsuitable for sharing data across mobile devices. Firstly,the system is not light weight enough. It has heavy pro-cessing and bandwidth requirements due to the complexprotocol that the system implements. Also, data isshared at the granularity of pages. There is no uniformconcept of pages across different mobile devices. Per-formance and ease of use can be improved by allowingthe applications programmer to pick the right granular-ity of sharing.Orca [B98], [HAB96] is a software based DSM thatallows the applications programmer to pick the granu-larity of sharing. However, it is based on an update pro-tocol system, and updates are implemented by functionand argument shipping where each sharer executes thefunctions shipped by the multiple writers to modify theUNIVERSITY OF CALIFORNIA, BERKELEYSANTA BARBARA • SANTA CRUZBERKELEY • DAVIS • IRVINE • LOS ANGELES • RIVERSIDE • SAN DIEGO • SAN FRANCISCOObject Sharing Over Mobile DevicesSharad Agarwal, Ioannis Mavroidis, Alec WooDepartment of Electrical Engineering & Computer Science{sagarwal, maurog, awoo}@cs.berkeley.eduMay 19992data. This system would not be ideal for mobile devicesgiven both the limited bandwidth and the lack of com-pute power.CRL [JKW95] is a software based DSM systemthat also allows the applications programmer to pick thegranularity of sharing. However, it may not be portableacross all mobile devices. Sharing is achieved at thegranularity of programmer specified regions of bytes,rather than programmer specified objects with typeinformation which is a much more useful abstraction forapplications programmers.TSpaces [TSPACES] is a network communicationbuffer with database capabilities. It enables communica-tion between applications and devices in a network ofheterogeneous computers and operating systems. TSpaces provides group communication services, data-base services, URL-based file transfer services, andevent notification services. However, TSpaces does notprovide coherency and consistency guarantees, requir-ing applications to build in their own coherency mecha-nisms.The inapplicability of the existing software basedDSMs described above can be formulated into a set ofdesign requirements for the ideal system.3. Design RequirementsThere are several requirements that need to be satis-fied by a system that will allow data to be shared acrossmobile devices.3.1 Software SolutionOne of the main goals of this study is to exploresystems that will permit the sharing of data across allmobile devices. There does not appear to be anythinginherently special about any one set of mobile devicesthat warrants a specialized implementation of data shar-ing. Furthermore, the interconnection between mobiledevices is extremely diverse. Some devices may be con-nected via wireless modems, wireless pager systems orvia immobile docking stations. However, one commoncommunication medium that is accessible across alldevices and their interconnections is the Internet.Given this goal, it is apparent that a software solu-tion is called for. A hardware solution would require anenormous effort to support the variation in interconnec-tion and devices that share data with little added benefit.3.2 Shared Data StorageThe persistent storage of shared data can occur atdifferent levels in a shared data system. This data can bestored in the mobile devices that create them. This is anunsatisfactory solution given both the limited