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The DDS service provides tools for building distributed data structures to run on workstation clusters<br />

[12]. The DDS service must handle many of the problems of distributed systems in general, including<br />

performance and reliability; but by limiting its requirements to supporting particular types of carefully<br />

chosen data structures, the system can solve these hard problems and provide the necessary services.<br />

Porcupine uses a network of workstations to handle mail service, which requires significant amounts of<br />

writing and thus demands more sophisticated mechanisms to leverage the power of the distributed system<br />

[13]. Porcupine uses a set of nodes that can provide interchangeable services and replication techniques<br />

to build a high-performance mail server out of a network of workstation machines.<br />

Distributed Systems for Ubiquitous Computing<br />

Many researchers predict that homes, offices, and other buildings of the future will contain large numbers<br />

of objects that have embedded processors and communications devices. For example, all appliances in a<br />

house might contain processing and communications capabilities. Also, humans may carry several computing<br />

and communications devices on their bodies, in much the same way that most people today wear<br />

a watch. The purposes and uses of machines in these environments are not yet clear, but they seem likely<br />

to differ from the way office workstations in a LAN or machines browsing the Web behave. Plausibly,<br />

they will require different sets of system services than today’s distributed system services. Various researchers<br />

have started examining the requirements of these systems and designed distributed services for them.<br />

One area of interest is naming in such systems. The publish/subscribe model of naming has been<br />

proposed and implemented in systems like the Jini service discovery system. In this model, devices on<br />

the network publish their capabilities, and other devices that hear the publications subscribe to the<br />

services they are interested in. Another proposed model of naming for this environment is intentional<br />

naming, where the system provides name resolution and request routing via a self-configured overlay<br />

network [14]. In such systems, users name objects by what they want to do with them, and the naming<br />

service takes responsibility for forwarding the request to some entity capable of fulfilling their need.<br />

Ubiquitous computing systems also require access to persistent storage, both to access standard persistent<br />

data (e.g., files) and to allow the system to keep track of the state required to give users a consistent<br />

view of the world. Mobility, limited communications links, security concerns, and varying capabilities<br />

of participant machines make providing persistent data harder. One approach to solving the problem is<br />

provided by OceanStore, which postulates a utility-like model for providing persistent data handling in<br />

a ubiquitous environment [15]. OceanStore uses replication aggressively and relies on a combination of<br />

fast (but not always successful) search techniques and slow (but reliable) lookup algorithms.<br />

Security in Distributed Operating Systems<br />

Some recent research has tried to tackle the problems of running a secure distributed operating system<br />

when not all participants are fully trusted. For example, recent enhancements to SFS have produced a secure<br />

read-only file system that allows remote clients to independently verify that the data provided to them by<br />

a server is indeed the true version of the data [16]. Another difficult security problem for distributed systems<br />

that span multiple administrative domains is proper authentication across those boundaries, particularly<br />

when the different domains use different authentication mechanisms. One recent approach to solving this<br />

problem is to build a logic of authentication and mechanisms for applying that logic to authentication<br />

information attached to remote requests [17]. The system can then determine if it should regard incoming<br />

requests as being legitimately made by the putative requestor.<br />

35.6 Resources for Studying Distributed Operating Systems<br />

Research appears frequently in the field of distributed operating systems. Some of the work is commercial,<br />

but much of it is described in the research literature. The principal conferences where distributed<br />

operating system research appears most often are the bi-annual Symposium on Operating Systems<br />

(SOSP), the bi-annual Conference on Operating Systems Design and Implementation (OSDI, held in<br />

© 2002 by CRC Press LLC

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