14539530
The Problem of Equivalence for Entity-Relationship Diagrams
We investigate the question of when two entity-relationship diagrams (ERD's) should be considered equivalent, in the sense of representing the same information. This question is very important for a database design process which uses the ERD model, and can be interpreted in various ways. We give three natural and increasingly stricter criteria for developing concepts of equivalence for ERD's. We first give a notion of "domain data compatibility" which ensures that the ERD's in question represent the same universe of data in an aggregate sense. Then we define the set of functional dependencies which are naturally embedded in each ERD, and use it to develop a concept of "data dependency equivalence" which ensures that the ERD's satisfy the same constraints (functional dependencies) among the represented data. Finally, we give our strongest criterion, instance data equivalence, which requires the ERD's to have the same power to represent instances of data. We develop several alternate forms of this third notion, including some giving efficient tableaux tests for its occurrence. Indeed, for each type of equivalence, we give a polynomial-time algorithm to test for it.
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.
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.
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.
497093
Selective Data Encryption in Outsourced Dynamic Environments
The amount of information held by organizations' databases is increasing very quickly. A recently proposed solution to the problem of data management, which is becoming increasingly popular, is represented by database outsourcing. Several approaches have been presented to database outsourcing management, investigating the application of data encryption together with indexing information to allow the execution of queries at the third party, without the need of decrypting the data. These proposals assume access control to be under the control of the data owner, who has to filter all the access requests to data. In this paper, we put forward the idea of outsourcing also the access control enforcement at the third party. Our approach combines cryptography together with authorizations, thus enforcing access control via selective encryption. The paper describes authorizations management investigating their specification and representation as well as their enforcement in a dynamic scenario.
28727734
A Novel Approach to Certificate Revocation Management
With the ever-increasing growth in electronic messaging and electronic commerce, the need for an infrastructure to provide confidentiality, security, and confidence for such exchanges to take place is quite evident [2]. Here, public keys and certificates are issued to users for authorization purposes. One of the primary concerns in these systems is the handling of certificate revocation prior to the expiration date. In this paper, we propose a new approach for managing certificate revocation. All existing schemes require that the information about revoked certificates be sent only periodically to the directories used for verification. This gives rise to the problem of obsolescence. To overcome this problem, we have introduced a new layer in the traditional architecture. Using a preliminary analysis, we show the impact of the new scheme on the up-to-datedness, robustness, load distribution, and response time of the system. Similarly, we show the additional costs incurred in terms of communication cost, processing cost, and hardware costs.
39447464
Temporal Mediators: Supporting Uniform Accesses to Heterogeneous Temporal Information
In order to support uniform access to heterogeneous temporal information, we introduce the concept of a temporal mediator. A temporal mediator consists of three components: (i) a repository for windowing functions and conversion functions, (ii) a time granularity thesaurus and (iii) a query interpreter. There are two types of windowing functions: one associates each time point to a set of tuples, and the other associates each tuple to a set of time points. A conversion function transforms information in terms of one time granularity into that in terms of another time granularity. The time granularity thesaurus stores the knowledge about time granularities (e.g., names of time granularities and relationships among them). Users pose queries using the windowing functions and in terms of desired time granularities. (A query language, which can be used to form such queries, is given in the paper.) To answer such a user query, the query interpreter first employs the windowing functions together with the time granularity thesaurus to retrieve needed temporal data from the underlying databases and then uses the time granularity thesaurus to select suitable conversion functions which convert the responses to the desired time granularities. Thus, a temporal mediator provides a simple interface that supports uniform accesses to heterogeneous temporal databases.
11728681
CARDS: A Distributed System for Detecting Coordinated Attacks
A major research problem in intrusion detection is the efficient Detection of coordinated attacks over large networks. Issues to be resolved include determining what data should be collected, which portion of the data should be analyzed, where the analysis of the data should take place, and how to correlate multi-source information. This paper proposes the architecture of a Coordinated Attack Response and Detection System (CARDS). CARDS uses a signature-based model for resolving these issues. It consists of signature managers, monitors, and directory services. The system collects data in a flexible, distributed manner, and the detection process is decentralized among various monitors and is event-driven. The paper also discusses related implementation issues.
6829877
Aggregation in Relational Databases: Controlled Disclosure of Sensitive Information
It has been observed that often the release of a limited part of an information resource poses no security risks, but the relase of a sufficiently large part of that resource might pose such risks. This problem of controlled disclosure of sensitive information is an example of what is known as the aggregation problem. In this paper we argue that it should be possible to articulate specific secrets within a database that should be protected against overdisclosure, and we provide a general framework in which such controlled disclosure can be achieved. Our methods foil any attempt to attack these predefined secrets by disguising queries as queries whose definitions do not resemble secrets, but whose answers nevertheless “nibble” at secrets. Our methods also foil attempts to attack secrets by breaking queries into sequences of smaller requests that extract information less conspicuously. The accounting methods we employ to thwart such attempts are shown to be both accurate and economical.