Richard A. McBride

Security considerations for active messages

Messages are generally regarded as passive since they merely act as a conveyance for information. An active message triggers processing activities at each of its recipients. A discussion is provided on the different classes of processing activities that can occur for active messages. A mechanism for ensuring the security of an active message and its associated processing operations is also outlined.

Query processing in a distributed data base

The current research on optimizing algorithms for queries in distributed data base networks is presented. An identification of additional factors which add cost and time to processing of a query both at a node and in the transporting of data along a link in the network is given. The idea of a user topology is presented as the basis for a query optimization algorithm and the effects of a users query constraints on the user topology are illustrated. With the influencing factors on a query request identified, a model of query processing is described.

Distributed search for cooperative applications

Applications at individual sites throughout a network can cooperate to find information re uested by a user. % These apphcations cooperate by exc angmg mqsaa es ? and are autonomous except that all have been Imp emented to respond to a common protocol. Every ap li‘F cation 1s responsible for mamtamlng Its own local m ormation. Further, each application haa limited knowledge of the other sites in the network, namely, those siteq i~med!ately connected to lt which run a, cooperating application. Thw collection of apphcatlons 1s referred to as a (loosely) distributed application. The emerging standard on distributed directories (X.500) is an example of such a distributed application. A distributed search algorithm is presented which is used by each ap Iication to initiate a se,arch or to ref spend to a searc request from another site. The cooperating applications are guaranteed to find and ~eturn an appropriate ~eply. to the ori inating site. It 1s also # shown that no site w1ll retain m o~matlon about a message for an indefinite amount of time. “Support for this research was provided in part by the National Science and Research Council of Canada. ACM-SAC ‘931WMIUUSA O 1993ACMCM9791—%&Z@~l..M~ Introduction The Iar e scale interconnection of many diffe~ent hosts J has m e applications, such as electromc mad and file transfer important tools m many different organizations. ~his large scale connectivity and diversity of hosts has also made it difficult for a user. to identify individuals or hosts of Interest without prior knowledge. This problem has motivated research mto the use of online sources of knowledge about users, hosts, and other objects within electronic mail systems[4, 6], work on attribute-baaed naming[l] and work on cooperating information sources[5, 7]. Such problems have motivated the work on X.500 [2] to support telecommunication applications such as electronic mail (X .400[3, 8]) and other telematic services (e.g., FAX, Telex). The emerging X.500 standard introduces the notion of a dtstm”buted directory ~ a single, global application distributed across many different, autonomous .sItes.. The standard specifies {rotoco’s ‘0 ~y directing queries to local directory agenLs. These ermlt other apphcatlons to make use of t e directory agents cooperate ; F “ rovide the designated directory serwces t , the% a p Icatlons. The dlrector~ information itself IS dlstrl uted throughout these sttes and is managed by the directory agents. On oing work on the f X.500 standard IS focu=d on protoco s and procedures for dwtrlbutmg and replicating directory information. A similar scenario has been reposed for networks of library systems. A user woul~ from his or her own home library, make a request for a particular book.. The distributed mfo~matlon agents would cooperate m Iocatin a hbrary which had a copy of the desired text. Suc 1! an information network could even tap into publishers’ databases (with suitable protection) and inform the user of an ordering source or publication date. Although these applications may differ in many ways, there 1s a common problem at the core of then behawour the retrieval of information from one or more sources dwtrlbuted throughout a distributed system. This problem has the following characteristics: . The applications are essentially autonomous except

Simulation of a state prison population

A simulation model of the SD prison system was constructed to act as a tool for forecasting future inmate growth and to predict the impact. on the prison population resulting from new legislation. An examination of the structure of this tool and some of the decisions surrounding its design and development are discussed. The model’s results and their validity is also addressed.

A front-end process for fault tolerant distributed systems

A Front-End Process (FEP) acts as an intermediary between clients and servers. All messages passed between the client and server pass through the FEP. Thus, the FEP can easily provide load balancing, location and concurrency transparency, and fault tolerance. Although the FEP adds operational overhead to RPCs, it allows robust applications to be constructed which operate correctly despite system failures.

Specification and verification of learning

This paper discusses the specification and verification of the leaxn.mg behaviors of objects. A proof procedure that can be automated is introduced.

Ada as an introductory language

The concepts of software engineering have become increasingly integrated into the first programming courses in Computer Science. Modularity, abstract data types, information hiding, and reusability are essential concepts in these beginning courses. The first two courses for Computer Science majors at the University of South Dakota were redesigned baaed on using an Ada environment on modern workstations. This paper describes these courses and compares the results with our previous traditional (Pascal-based) courses.