18022126
Construction of Universal Instances for Loop-Free Network Databases Using a Join-Like Operation
In this paper, we give a polynomial-time method to construct effectively the unique universal instance, using as few nulls as possible, from any loop-free network database, via a "minimal information" extension of natural join. Our results can be seen as concretely and quickly implementing the universal relation view for databases which are not pairwise consistent.
8520486
Local Area Networks: Software and Related Issues
In this paper, we present a review of the issues that affect the software requirements for a local area network. We introduce protocols for the local area networks and characterize their software needs. Two approaches to operating systems are outlined and examples of each approach are presented. Various applications which use local area networks and performance issues are also discussed.
45839880
Universal and Representative Instances Using Unmarked Nulls
Representative instances are important since in addition to the universal relation view they permit updates in the individual relations; if these updates are locally valid, then they do not violate global consistency. In this paper, we first give conditions under which we can construct a universal instance I which uses only one type of nulls (unmarked nulls) and whose total projections onto the relation schemes yield exactly the database relations. Then, we give a necessary and sufficient condition under which this I is actually a representative instance. Our conditions are simple and easily maintained.
21910023
On Universal and Representative Instances for Inconsistent Databases
120699
Enhancements to the Voting Algorithm
There are several consistency control algorithms for managing replicated files in the face of network partitioning due to site or communication link failures. In this paper, we consider the popular voting scheme along with three enhancements: voting with a primary site, dynamic voting, and dynamic voting with linearly ordered copiee. We develop a stochastic model which compares the file availabilities afforded by each of these schemes. We show that in this model dynamic voting with linearly ordered copies provides the greatest availability.
14430828
An Extension of "Representative Instances and &#947;-Acyclic Relational Schemes"
Let R be a γ-acyclic relational scheme, and let F be the set of functional dependencies (FD's) embodied in R. Given an existence constrained database r over R, it was shown in [1] that it is possible to connect tuples from different relations in r and construct a universal instance L, possibly containing null values δ, such that the total projection of L onto R yields exactly the set r. Moreover, conditions were given which guarantee that this L would satisfy the functional dependency with nulls (NFD) counterparts of FD's, in F. The purpose of this note is to generalize the latter result and show that under the same conditions, L actually satisfies NFD counterparts of FD's in the closure F+ of F.
14768987
Distributed timestamp generation in planar lattice networks
Timestamps are considered for distributed environments in which information flow is restricted to one direction through a planar lattice imposed on a network. For applications in such networks, existing timestamping algorithms require extension and modification. For example, in secure environments, typical timestamps provide a potential signaling channel between incomparable levels. In hierarchical databases, typical timestamps cause peripheral sites to unnecessarily affect the behavior at main sites.
Algorithms are presented by which a network node may generate and compare timestamps using timestamp components maintained at dominated nodes in the network. The comparison relation is shown to be acyclic for timestamps produced by the generation algorithm. We discuss ways to safely relax the requirement that the network be a lattice. By example, we show how to modify a simple nonplanar lattice so that the generation algorithm can be applied. Uses of the timestamp generation algorithm in the motivating applications are outlined.
8753336
Using semantic correctness in multidatabases to achieve local autonomy, distribute coordination, and maintain global integrity
Abstract A multidatabase poses the following four, often contradictory, requirements. First, local databases require both design autonomy to accommodate the diverse legacy nature of the local databases, and execution autonomy to ensure that local transactions are not unduly blocked by global transactions. Second, management of global transactions must be distributed to avoid bottlenecks and to tolerate failure in the global database. Third, both local and global integrity constraints must be maintained. Finally, concurrent processing of transactions requires that execution histories be correct, an objective traditionally achieved with serializability. Although alternate forms of correctness have been proposed, none of the solutions advanced to date has simultaneously achieved all four requirements. We propose a transaction processing model that uses a semantics-based notion of correctness to achieve all four requirements simultaneously for applications that satisfy a certain set of properties. The support required for global transactions forms a separate layer at each site that respects both design autonomy and execution autonomy. Global transactions are managed at each site via a successor set mechanism so that site and communication failures do not impede transactions at operating sites. Semantic correctness encompasses three properties: ensuring that local and global integrity constraints are maintained, that transactions output consistent data and that all partially executed transactions can complete. A fourth property ensures that the successor set description is a valid refinement of the specification. These four properties must be proved for applications executed by our model. We show how model checking can automate, in part, the verification of the properties.
8106419
Integrity principles and mechanisms in database management systems
Our objective in this paper is to answer the following question: what mechanisms are required in a general-purpose multiuser database management system (DBMS) to facilitate the integrity objectives of information systems? In a nutshell our conclusion is that realistic mechanisms do exist. Although existing commercial products fall far short of providing the requisite mechanisms, they can be easily extended to incorporate these mechanisms. Our principal contribution is to identify these mechanisms and to point out where gaps still remain. We have also bridged the terminology and concepts of database and security specialists in a coherent manner. In the more detailed considerations the focus of this paper is on relational DBMSs.
4991589
Consistent policy enforcement in distributed systems using mobile policies
This paper briefly traces the evolution of information system architectures from mainframe-connected terminals to distributed multi-tier architectures. It presents the challenges facing developers of multitier information systems in providing effective consistent data policy enforcement, such as access control in these architectures. Finally, it introduces "Mobile Policy" (MoP) as a potential solution and presents a framework for using mobile policy in the business logic tier of multi-tier information systems.
15255057
A comparative performance analysis of reliable group rekey transport protocols for secure multicast
In this paper, we present a new scalable and reliable key distribution protocol for group key management schemes that use logical key hierarchies (LKH) for scalable group rekeying. Our protocol called WKA-BKR is based upon two ideas--weighted key assignment and batched key retransmission--both of which exploit the special properties of LKH and the group rekey transport payload to reduce the bandwidth overhead of the reliable key delivery protocol. Using both analytic modeling and simulation, we compare the performance of WKA-BKR with that of other rekey transport protocols, including a recently proposed protocol based on proactive FEC. Our results show that for most network loss scenarios, the bandwidth used by WKA-BKR is lower than that of the other protocols.