13277150
Mutual consistency in decentralized distributed systems
In this paper we set forth a simple and efficient algorithm for managing replicated data in a decentralized distributed system, which allows for inserts, deletes, updates, and synonyms and which achieves a high degree of availability in the face of node or communication failures. We focus on the approach developed recently by Fischer and Michael and exploit the knowledge of the semantics of the database operations.
24298643
Translation of entity-relationship diagrams into relational structures
Abstract An entity-relationship diagram (ERD) is considered to have two conceptual domains: its upper and lower conceptual domains. The semantic properties intrinsic in the upper conceptual domain were studied recently by Lien. In this paper, we discuss the properties inherent in the lower conceptual domain. We show that the latter contains a significant portion of the semantic information of an ERD and helps determine its data representation power. We develop the notions of fundamental functional dependencies (FDs) which can be readily recognized for the lower conceptual domain of an ERD, and regular ERDs which are ERDs such that all the FDs present among their attributes can be derived from their fundamental FDs. Then we give conditions under which a large number of ERDs are equivalent to their underlying relational structures.
45139817
Alternative methods for the reconstruction of trees from their traversals
It is well-known that given the inorder traversal of a binary tree's nodes, along with either one of its preorder or postorder traversals, the original binary tree can be reconstructed using a recursive algorithm. In this short note we provide a short, elegent, iterative solution to this classical problem.
17317346
Representative instances and γ-acyclic relational schemes
In this paper, we study under what conditions will a pairwise inconsistent relational database ≪R,r≫ have a universal/representative instance L. If R is γ-acyclic and r satisfies all existence constraints, then it is possible to construct a universal instance L, using unmarked nulls, whose total projections onto R yield exactly the relations in r. We show that L would actually be a representative instance under a set of functional dependencies if R satisfies the additional mild condition: for any functional dependency X → A where A is a single attribute, whenever XA is contained in two relation schemes R and R' of R, it follows that R ∩R' is a relation scheme of R, having X as one of its keys.
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.
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.
10196984
Propagation of authorizations in distributed database systems
We consider the propagation of authorizations in distributed database systems. If no constraints are imposed on the propagation of authorization changes, then the authorization states at different sites may evolve inconsistently. A standard solution is to suppress the distributed aspect and make all changes appear as if they had occurred in some serial order at a single site, perhaps via an atomic commit protocol. However, rigid insistence on consistency may result in authorization changes being needlessly delayed, a problem exacerbated in the context of site or communication failures. We propose an optimistic authorization propagation algorithm. We specify an authorization table and a set of operations for altering the authorization table. Each site maintains a log of authorization operations. We exploit the semantics of authorization operations to avoid relying on an undo-redo mechanism for processing out of order operations. Instead we give efficient, direct algorithms to scan the log and update the authorization table. Any inconsistencies in replicas of the authorization table are transient and are eliminated by further communication between sites. We discuss pruning the authorization log.
3985567
Metadata Management in Outsourced Encrypted Databases
Database outsourcing is becoming increasingly popular introducing a new paradigm, called database-as-a-service, where a client’s database is stored at an external service provider. Outsourcing databases to external providers promises higher availability and more effective disaster protection than in-house operations. This scenario presents new research challenges on which the usability of the system is based. In particular, one important aspect is the metadata that must be provided to support the proper working of the system. 
 
In this paper, we illustrate the metadata that are needed, at the client and server, to store and retrieve mapping information for processing a query issued by a client application to the server storing the outsourced database. We also present an approach to develop an efficient access control technique and the corresponding metadata needed for its enforcement.
6981251
A propositional policy algebra for access control
Security-sensitive environments protect their information resources against unauthorized use by enforcing access control mechanisms driven by access control policies. Due to the need to compare, contrast, and compose such protected information resources, access control policies regulating their manipulation need to be compared, contrasted, and composed. An algebra for manipulating such access control policies at a higher (propositional) level, where the operations of the algebra are abstracted from their specification details, is the subject of this paper. This algebra is applicable to policies that have controlled nondeterminism and all or nothing assignments of access privileges in their specification. These requirements reflect current practices in discretionary and role-based access control models. Therefore, the proposed algebra can be used to reason about role-based access control policies combined with other forms of discretionary policies. We show how to use algebraic identities to reason about consistency, completeness, and determinacy of composed policies using similar properties of their constituents.
1697150
Distributed algorithms for dynamic replication of data
We present two distributed algorithms for dynamic replication of a data-item in communication networks. The algorithms are adaptive in the sense that they change the replication scheme of the item (i.e. the set of processors at which the data-item is replicated), as the read-write pattern of the processors in the network changes. Each algorithm continuously moves the replication scheme towards an optimal one, where optimality is defined with respect to different objective functions. One algorithm optimizes the communication cost objective function, and the other optimizes the communication time. We also provide a lower bound on the performance of any dynamic replication algorithm.
13090929
Implementation of a Storage Mechanism for Untrusted DBMSs
Several architectures have been recently proposed that store relational data in encrypted form on untrusted relational databases. Such architectures permit the creation of novel Internet services and also offer an opportunity for a better construction of ASP solutions. Environments where there are limited resources that do not permit an efficient management of databases or where it is critical to offer a robust Internet access to private data may all benefit from the above architectures. In this paper we analyze the impact that this architecture has on the typical services of a database. The analysis is based on the experience gained in the construction of a prototype of a complete architecture for the management of encrypted databases. Specifically, we illustrate the impact on query translation and optimization, and the main components of the software architecture of the prototype.