Heterogeneous databases: some outstanding problems
Authors: S. Heiler
Venue: Proceedings of the 1987 Fall Joint Computer Conference on Exploring technology: today and tomorrow (1987)

Systems to manage heterogeneous databases, HDBMSs, are designed to insulate the user from the effects of heterogeneity in hardware, networks, data management software, data formats, naming conventions, and constraints. Generally, they do this by providing a layer between the user and the underlying databases that creates the illusion of a centralized, homogeneous database, without requiring changes to the actual databases and the systems that manage them. Many of the problems involved in managing heterogeneous databases are well-understool. Existing research prototypes, e.g., CCA's MULTIBASE, can support queries that span heterogeneous databases and resolve conflicts where data repositories overlap. In addition, commercial systems that address useful subsets of the problems of heterogeneity, including providing basic update facilities, are becoming available. However, some important problem areas associated with data management in heterogeneous environments have barely been touched: they include problems of optimization, transaction management, support for database design, and managing the metadatabase. The main issue for systems to manage heterogeneous databases is the degree to which they can make assumptions regarding the underlying systems and databases. In general, designs of HDBMSs reflect their level of control over, knowledge about, and trust in the underlying systems, as well as the types of heterogeneity they must address. In this paper we focus on two of these problems &mdash; support for database design and metadata management. Both problems need to be addressed early in the design of the HDBMS because they impact the architecture of other crucial components. For example, in order to process queries the HDBMS needs to know what metadata are available from each underlying system and whether one can trust their consistency with its own metadata. Powerful facilities for view integration are needed to support database design. Since the HDBMS does not itself provide data storage facilities, database design in a heterogeneous environment consists primarily of specifying external schemas as views over the underlying databases. The purpose is to provide &ldquo;integrating views&rdquo; that can be mapped onto the underlying heterogeneous databases. Queries to the heterogeneous databases, then, are specified as queries over these &ldquo;integrating views&rdquo;. Support for the definition of such external schemas requires tools for comparing schemas, suggesting alternative ways of reconciling them, interacting with the administrator to select one of the alternatives, and generating mappings in both directions between the original and integrated schemas. Tools are also needed to identify dependencies among schema contents and to provide for change propagation. Also, powerful languages are needed for expressing queries over the metadatabase and using their results in the definitions of other queries or integrating views. For example, queries over previously-defined external schemas and local database schemas may be of the form &ldquo;find all relations with a column named x or with domain x and use the result as a column name in a subsequent query&rdquo;. Managing the metadatabase is itself a problem in a heterogeneous environment Two aspects of metadata management raise particularly difficult issues. First, the HDBMS must ensure that its metadatabase remains consistent with the metadata of the underlying systems (e.g., that the integrated views are consistent with the local schemas from which they are derived). However, often the local systems operate autonomously. This means either that the HDBMS must ensure that changes to the underlying systems&rdquo;