Daniel R. Ries

The implementation of an integrated concurrency control and recovery scheme

This paper describes the implementation level design of an integrated concurrency control and recovery scheme based on the maintenance of multiple versions of data objects in a database. The concurrency control mechanism enhances parallelism by eliminating interference between retrieval and update transactions. The recovery mechanism permits efficient transaction and system recovery by keeping before-images of data objects at the page (block) level. This paper addresses the key technical problems in the implementation of such an integrated scheme. We present an efficient garbage collection algorithm for reclaiming storage space used by old versions of data objects that will no longer be accessed. We also propose an on-line backup algorithm that will permit the backup procedure to run in parallel with regular transactions. This integrated concurrency control and recovery scheme is being implemented in the LDM: the local database manager component of a distributed database management system, now being developed by Computer Corporation of America, that will support the ADAPLEX database application programming language [Chan81a, Smith81].

Effects of locking granularity in a database management system

Many database systems guarantee some form of integrity control upon multiple concurrent updates by some form of locking. Some “granule” of the database is chosen as the unit which is individually locked, and a lock management algorithm is used to ensure integrity. Using a simulation model, this paper explores the desired size of a granule. Under a wide variety of seemingly realistic conditions, surprisingly coarse granularity is called for. The paper concludes with some implications of these results concerning the viability of so-called “predicate locking”.

Locking granularity revisited

Locking granularity refers to the size and hence the number of locks used to ensure the consistency of a database during multiple concurrent updates. In an earlier simulation study we concluded that coarse granularity, such as area or file locking, is to be preferred to fine granularity such as individual page or record locking. However, alternate assumptions than those used in the original paper can change that conclusion. First, we modified the assumptions concerning the placement of the locks on the database with respect to the accessing transactions. In the original model the locks were assumed to be well placed. Under worse case and random placement assumptions when only very small transactions access the database, fine granularity is preferable. Second, we extended the simulation to model a lock hierarchy where large transactions use large locks and small transactions use small locks. In this scenario, again under the random and worse case lock placement assumptions, fine granularity is preferable if all transactions accessing more than 1 percent of the database use large locks. Finally, the simulation was extended to model a “claim as needed” locking strategy together with the resultant possibility of deadlock. In the original study all locks were claimed in one atomic operation at the beginning of a transaction. The claim as needed strategy does not change the conclusions concerning the desired granularity.

A recovery algorithm for a distributed database system

We describe a reliability algorithm being considered for DDM, a distributed database system under development at Computer Corporation of America. The algorithm is designed to tolerate clean site failures in which sites simply stop running. The algorithm allows the system to reconfigure itself to run correctly as sites fail and recover. The algorithm solves the subproblems of atomic commit and replicated data handling in an integrated manner.

Storage and access structures to support a semantic data model

ABSTRACT This paper describes the design of storage and access structures for a high performance Ada * compatible database management system. This system supports the database application programming language ADAPLEX [Smith81, Smitb82], which is the result of embedding the database sublanguage DAPLEX [Shipman81] in the general purpose language Ada [DoD80]. A prominent feature of the underlying data model is its support for generalization hierarchies [Smith77] which are intended to simplify the mapping from conceptual entities to database objects. An in-depth discussion of the rationale behind our choice of storage and access structures to support semantics intrinsic to the data model and to permit physical database organization tuning is provided in this paper.

Office procedures as a distributed database application

An office proedure specification system is described. The system is to be implemented as a database application in a distributed database management system being developed at Computer Corporation of America. Because of its data-base application characteristic, the system treats form copying and routing differently from other office information systems. This differences makes controlling and tracing forms easier than it is in other systems. We also introduce a generalized trigger, called trigger-lock, for synchronizing independently created procedures and for exception handling.