Richard Hull

A framework for supporting data integration using the materialized and virtual approaches

This paper presents a framework for data integration currently under development in the Squirrel project. The framework is based on a special class of mediators, called Squirrel integration mediators. These mediators can support the traditional virtual and materialized approaches, and also hybrids of them.In the Squirrel mediators, a relation in the integrated view can be supported as (a) fully materialized, (b) fully virtual, or (c) partially materialized (i.e., with some attributes materialized and other attributes virtual). In general, (partially) materialized relations of the integrated view are maintained by incremental updates from the source databases. Squirrel mediators provide two approaches for doing this: (1) materialize all needed auxiliary data, so that data sources do not have to be queried when processing the incremental updates; or (2) leave some or all of the auxiliary data virtual, and query selected source databases when processing incremental updates.The paper presents formal notions of consistency and freshness for integrated views defined over multiple autonomous source databases. It is shown that Squirrel mediators satisfy these properties.

Generating data integration mediators that use materialization

This paper presents a framework for data integration that is based on using “Squirrel integration mediators” that use materialization to support integrated views over multiple databases. These mediators generalize techniques from active databases to provide incremental propagation of updates to the materialized views. A framework based on “View Decomposition Plans” for optimizing the support of materialized integrated views is introduced. The paper describes the Squirrel mediator generator currently under development, which can generate the mediators based on high-level specifications.The integration of information by Squirrel mediators is expressed primarily through an extended version of a standard query language, that can refer to data from multiple information sources. In addition to materializing an integrated view of data, these mediators can monitor conditions that span multiple sources. The Squirrel framework also provides efficient support for the problem of “object matching”, that is, determining when object representations (e.g., OIDs) in different databases correspond to the same object-in-the-world, even if a universal key is not available.To establish a context for the research, the paper presents a taxonomy that surveys a broad variety of approaches to supporting and maintaining integrated views.

Heraclitus: elevating deltas to be first-class citizens in a database programming language

Traditional database systems provide a user with the ability to query and manipulate one database state, namely the current database state. However, in several emerging applications, the ability to analyze “what-if” scenarios in order to reason about the impact of an update (before committing that update) is of paramount importance. Example applications include hypothetical database access, active database management systems, and version management, to name a few. The central thesis of the Heraclitus paradigm is to provide flexible support for applications such as these by elevating deltas, which represent updates proposed against the current database state, to be first-class citizens. Heraclitus[Alg,C] is a database programming language that extends C to incorporate the relational algebra and deltas. Operators are provided that enable the programmer to explicitly construct, combine, and access deltas. Most interesting is the when operator, that supports hypothetical access to a delta: the expression E when &sgr; yields the value that side effect free expression E would have if the value of delta expression &sgr; were applied to the current database state. This article presents a broad overview of the philosophy underlying the Heraclitus paradigm, and describes the design and prototype implementation of Heraclitus[Alg, C]. A model-independent formalism for the Heraclitus paradigm is also presented. To illustrate the utility of Heraclitus, the article presents an in-depth discussion of how Heraclitus[Alg, C] can be used to specify, and thereby implement, a wide range of execution models for rule application in active databases; this includes both prominent execution models presented in the literature, and more recent “customized” execution models with novel features.

Structures for manipulating proposed updates in object-oriented databases

Support for virtual states and deltas between them is useful for a variety of database applications, including hypothetical database access, version management, simulation, and active databases. The Heraclitus paradigm elevates delta values to be first-class citizens in database programming languages, so that they can be explicitly created, accessed and manipulated.A fundamental issue concerns the trade-off between the accuracy or robustness of a form of delta representation, and the ease of access and manipulation of that form. At one end of the spectrum, code-blocks could be used to represent delta values, resulting in a more accurate capture of the intended meaning of a proposed update, at the cost of more expensive access and manipulation. In the context of object-oriented databases, another point on the spectrum is attribute-granularity deltas which store the net changes to each modified attribute value of modified objects.This paper introduces a comprehensive framework for specifying a broad array of forms for representing deltas for complex value types (tuple, set, bag, list, o-set and dictionary). In general, the granularity of such deltas can be arbitrarily deep within the complex value structure. Applications of this framework in connection with hypothetical access to, and merging of, proposed updates are discussed.

An execution model for limited ambiguity rules and its application to derived data update

A novel execution model for rule application in active databases is developed and applied to the problem of updating derived data in a database represented using a semantic, object-based database model. The execution model is based on the use of “limited ambiguity rules” (LARs), which permit disjunction in rule actions. The execution model essentially performs a breadth-first exploration of alternative extensions of a user-requested update. Given an object-based database schema, both integrity constraints and specifications of derived classes and attributes are compiled into a family of limited ambiguity rules. A theoretical analysis shows that the approach is sound: the execution model returns all valid “completions” of a user-requested update, or terminates with an appropriate error notification. The complexity of the approach in connection with derived data update is considered.

Using witness generators to support bi-directional update between object-based databases (extended abstract)

‘I’his paper considers the issue of incrementally maintaining materialized replicated information in the context of objectbased databases. Uni-directional update concerns propagating updates in one direction, and is closely related to the materialized view update problem. Bi-directional update arises when two databases hold overlapping information, and users of either database have the authority to make updates to that information. The focus here is on the impact of object identifiers (OIDS). A restricted form of ILOG is used, that corresponds essentially to select-project-join queries extended to include OID invention. If auxiliary information is not maintained, then the uniand hi-directional update problems are shown to be essentially equivalent to the graph isomorphism problem, and thus probably intractable. A form of auxiliary information, called “witnesses”, can be used to bring the uni-directional update problem into LOGSPACE. For the hi-directional case a subtlety arises in connection with incremental maintanence of the witnesses. This paper presents a non-trivial procedure that can be used in a limited context for creating programs that maintain witnesses. If such “witness generator” programs are available, then the hi-directional update problem is also in LOCL5PACE.

Towards a Framework for Efficient Management of Potentially Conflicting Database Updates

A new approach to identifying and managing potential conflict between long-duration, cooperative database updates is presented. A loose taxonomy of mechanisms for identifying potential conflict is sketched, and a family of mechanisms based on the use of ‘delta values’ (in the sense of the Heraclitus paradigm) is described. The paper is quite preliminary and philosophical — while we present a position, we expect that a primary contribution of the paper is to stimulate discussion about this important area.

Local ambiguity and derived data update

A novel execution model for rule application in active databases is developed and applied to the problem of updating derived data in the context of semantic, object-based database models. The execution model is based on the use of limited ambiguity rules (LARs), which permit disjunction in rule actions. The execution model essentially performs a breadth-first exploration of alternative extensions of a user-proposed update, and returns all completions of that update, where a completion is defined to be an extension of a user-proposed update that satisfies a family of natural conditions (e.g., that no constraints are violated). Given a semantic, object-based database model schema, integrity constraints as well as specifications of derived classes and attributes are compiled into a family of LARs. A formal proof of the correctness of the system is described.<<ETX>>

Response to “A close look at the IFO data model”

Several statements in [1] concerning the IFO data model (speci cally Section 3.1, item 3, and Section 3.2, items 1, 2, and 3) are false because in IFO, the Cartesian product construct (represented using the crossvertex) is used to represent a subset of a Cartesian product. Thus, it can explicitly represent binary and n-ary relationships, very similar to the relationship construct of the Entity-Relationship model. Further, as with the Entity-Relationship model, Cartesian product classes in IFO can have attributes.