Irving L. Traiger

The notions of consistency and predicate locks in a database system

In database systems, users access shared data under the assumption that the data satisfies certain consistency constraints. This paper defines the concepts of transaction, consistency and schedule and shows that consistency requires that a transaction cannot request new locks after releasing a lock. Then it is argued that a transaction needs to lock a logical rather than a physical subset of the database. These subsets may be specified by predicates. An implementation of predicate locks which satisfies the consistency condition is suggested.

System R: relational approach to database management

System R is a database management system which provides a high level relational data interface. The systems provides a high level of data independence by isolating the end user as much as possible from underlying storage structures. The system permits definition of a variety of relational views on common underlying data. Data control features are provided, including authorization, integrity assertions, triggered transactions, a logging and recovery subsystem, and facilities for maintaining data consistency in a shared-update environment. This paper contains a description of the overall architecture and design of the system. At the present time the system is being implemented and the design evaluated. We emphasize that System R is a vehicle for research in database architecture, and is not planned as a product.

The Recovery Manager of the System R Database Manager

The recovery subsystem of an experimental data management system is described and evaluated. The transactmn concept allows application programs to commit, abort, or partially undo their effects. The DO-UNDO-REDO protocol allows new recoverable types and operations to be added to the recovery system Apphcation programs can record data m the transaction log to facilitate application-specific recovery. Transaction undo and redo are based on records kept in a transaction log. The checkpoint mechanism is based on differential fries (shadows). The recovery log is recorded on disk rather than tape.

A history and evaluation of System R

System R, an experimental database system, was constructed to demonstrate that the usability advantages of the relational data model can be realized in a system with the complete function and high performance required for everyday production use. This paper describes the three principal phases of the System R project and discusses some of the lessons learned from System R about the design of relational systems and database systems in general.

Transactions and consistency in distributed database systems

The concepts of transaction and of data consistency are defined for a distributed system. The cases of partitioned data, where fragments of a file are stored at multiple nodes, and replicated data, where a file is replicated at several nodes, are discussed. It is argued that the distribution and replication of data should be transparent to the programs which use the data. That is, the programming interface should provide location transparency, replica transparency, concurrency transparency, and failure transparency. Techniques for providing such transparencies are abstracted and discussed. By extending the notions of system schedule and system clock to handle multiple nodes, it is shown that a distributed system can be modeled as a single sequential execution sequence. This model is then used to discuss simple techniques for implementing the various forms of transparency.

Views, authorization, and locking in a relational data base system

In the interest of brevity we assume that the reader is familiar with the notion of a relational data base. In particular, we assume a familiarity with the work of Codd or Boyce and Chamberlin. The examples in this paper will be drawn from a data base which describes a department store and consists of three relations: EMP(NAME, SAL, MGR, DEPT) SALES(DEPT, ITEM, VOL) LOC(DEPT, FLOOR)

Virtual memory management for database systems

Over the last several years, a number of hardware and software systems have been developed which map entire files directly into the virtual memory address spaces used by programs. Since all file contents are directly addressable, there is no need for a programmer to issue explicit file system actions, such as Read or Write. In addition, all of the buffer management problems are eliminated, since programmers do not have to squeeze pieces of large files into small virtual spaces. Although these advantages are tempting, we find that database systems have gone their own way. In this paper, we will look at two particular approaches to database system design, and see how (and why) they interface to file systems as they do. We will then look at the potential advantages and implications of working more closely with virtual memory management, and describe some of the functions and constraints that would have to be supported by a generalized page manager.

System R: an architectural overview

We have described the architecture of System R, including the Relational Data System and the Research Storage System. The RDS supports a flexible spectrum of binding times, ranging from precompilation of “canned transactions” to on-line execution of ad hoc queries. The advantages of this approach may be summarized as follows: 1. For repetitive transactions, all the work of parsing, name binding, and access path selection is done once at precompilation time and need not be repeated. 2. Ad hoc queries are compiled on line into small machine-language routines that execute more efficiently than an interpreter. 3. Users are given a single language, SQL, for use in ad hoc queries as well as in writing PL/I and COBOL transaction programs. 4. The SQL parser, access path selection routines, and machine language code generator are used in common between query processing and precompilation of transaction programs. 5. When an index used by a transaction program is dropped, a new access path is automatically selected for the transaction without user intervention.

A note on the calculation of average working set size

Finite-length reference string of arbitrary structure are considered, and an exact expression for average working set size in terms of “corrected” interreference interval statistics is derived. An example is discussed; upper and lower bounds are obtained; and the average working set size function is shown to be efficiently obtained for a set of page sizes, in a single pass of the reference string. This work follows the developments of a paper by Denning and Schwartz, who consider infinite-length reference strings which satisfy certain statistical properties and who derive an expression relating the asymptotic average working set size to the asymptotic missing page rate function under working set replacement.

System R: A Relational Data Base Management System

A relational approach makes this experimental data base management system unusually easy to install and use. Some of the decisions made in System R design in order to enhance usability also offer major bonuses in other areas.