This paper describes an efficient optimistic concurrency control scheme for use in distributed database systems in which objects are cached and manipulated at client machines while persistent storage and transactional support are provided by servers. The scheme provides both serializability and external consistency for committed transactions; it uses loosely synchronized clocks to achieve global serialization. It stores only a single version of each object, and avoids maintaining any concurrency control information on a perobject basis; instead, it tracks recent invalidations on a per-client basis, an approach that has low in-memory space overhead and no per-object disk overhead. In addition to its low space overheads, the scheme also performs well. The paper presents a simulation study that compares the scheme to adaptive callback locking, the best concurrency control scheme for client-server object-oriented database systems studied to date. The study shows that our scheme outperforms adaptive callback locking for low to moderate contention workloads, and scales better with the number of clients. For high contention workloads, optimism can result in a high abort rate; the scheme presented here is a first step toward a hybrid scheme that we expect to perform well across the full range of workloads. 1

Thor is an object-oriented database system designed for use in a heterogeneous distributed environment. It provides highly-reliable and highly-available persistent storage for objects, and supports safe sharing of these objects by applications written in different programming languages. Safe heterogeneous sharing of long-lived objects requires encapsulation: the system must guarantee that applications interact with objects only by invoking methods. Although safety concerns are important, most object-oriented databases forgo safety to avoid paying the associated performance costs. This paper gives an overview of Thor's design and implementation. We focus on two areas that set Thor apart from other objectoriented databases. First, we discuss safe sharing and techniques for ensuring it; we also discuss ways of improving application performance without sacrificing safety. Second, we describe our approach to cache management at client machines, including a novel adaptive prefetching strat...

Thor is a new object-oriented database management system (OODBMS), intended to be used in heterogeneous distributed systems to allow programs written in different programming languages to share objects in a convenient manner. Thor objects are persistent in spite of failures, are highly likely to be accessible whenever they are needed, and can be structured to reflect the kinds of information of interest to users. Thor combines the advantages of the object-oriented approach with those of database systems: users can store and manipulate objects that capture the semantics of their applications, and can also access objects via queries. Thor is an ongoing project, and this paper is a snapshot: we describe our first design and a partial implementation of that design. This design is primarily concerned with issues related to the implementation of an OODBMS as a distributed system. 1 INTRODUCTION Distributed systems contain different kinds of computers, and users write programs in different...

THOR is a persistent object store that provides a powerful programming model. THOR ensures that persistent objects are accessed only by calling their methods and it supports atomic transactions. The result is a system that allows applications to share objects safely across both space and time. The paper

Object-oriented databases store many small objects on disks. Disks perform poorly when reading and writing individual small objects. This thesis presents a new storage management architecture that substantially improves disk performance of a distributed object-oriented database system. The storage architecture is built around a large modified object buffer (MOB) that is stored in primary memory. The MOB provides volatile storage for modified objects. Modified objects are placed in the MOB instead of being immediately written out to disk. Modifications are written to disk lazily as the MOB fills up and space is required for new modifications. The MOB improves performance because even if an object is modified many times in a short period of time, the object has to be written out to disk only once. Furthermore, by the time an object modification has to be flushed from the MOB, many modifications to other objects on the same page may have accumulated. All of these modifications can be writ...

This paper discusses issues raised by providing disconnected operation in the Thor object-oriented database system. Disconnected operation in such a system poses new challenges because of the small size of objects, the richness and complexity of their interconnections, the huge number of them, and the fact that they are accessed within atomic transactions. We propose three techniques to address these challenges: (1) using the database query language for hoarding; (2) using dependent commits to tentatively commit transactions at the disconnected client; (3) using the high-level semantic of objects to avoid transaction aborts.

In a client-server system, it may be possible for the client to corrupt server data through unsafe access methods or programming errors. A common method for protecting the server data is to separate the client and server into distinct protection domains, e.g. Unix processes. Communication between client and server is restricted to a well-defined set of crossdomain calls and results are passed as opaque pointers to server data. Unfortunately, cross-domain calls are expensive and seriously affect the performance of the system. In his work on Batched Futures, Bogle presents a mechanism for batching cross-domain calls, thus amortizing the overhead of the domain crossing across the batch. Bogle showed that the improvement in performance is directly proportional to the average size of a batch (batching factor). This thesis describes two mechanisms that can be used to improve the batching factor: basic value promises and batched control structures. Basic value promises allow the batching of calls that return basic values (e.g. integer, boolean, character and real). Batched control structures allow the batching of simple control flow statements such as if statements, while loops and for loops. Both mechanisms have been implemented in Thor, an object-oriented database system. This thesis presents performance results showing the e#ectiveness of both mechanisms in increasing the performance of cross-domain calls.

We present a new prefetching technique for objectoriented databases which exploits the availability of multiprocessor client workstations. The prefetching information is obtained from the object relationships on the database pages and is stored in a Prefetch Object Table. This prefetching algorithm is implemented using multithreading. In the results we show the theoretical and empirical benefits of prefetching. The benchmark tests show that multithreaded prefetching can improve performance significantly for applications where the object access is reasonably predictable. Keywords: prefetching, distribution, object-oriented databases, performance analysis, multithreading, application access pattern, storage management 1 Introduction Two industry trends in the performance/price ratio of hardware systems have implications for the efficient implementation of object-oriented database management systems (OODBMSs) in a client/server computing environment. Firstly, the continuing fall in pric...

The performance of many object-oriented database applications suffers from the page fetch latency which is determined by the expense of disk access. In this work we suggest several prefetching techniques to avoid, or at least to reduce, page fetch latency. In practice no prediction technique is perfect and no prefetching technique can entirely eliminate delay due to page fetch latency. Therefore we are interested in the trade-off between the level of accuracy required for obtaining good results in terms of elapsed time reduction and the processing overhead needed to achieve this level of accuracy. If prefetching accuracy is high then the total elapsed time of an application can be reduced significantly otherwise if the prefetching accuracy is low, many incorrect pages are prefetched and the extra load on the client, network, server and disks decreases the whole system performance. Access pattern of object-oriented databases are often complex and usually hard to predict accurately. The ...

The main-memory management of an object cache that provides an orthogonally persistent platform for Java is described. Features of the architecture include: two levels of buffering between the disk and virtual machine, a consistent representation of transient and active objects, an efficient residency checking algorithm in the presence of multiple threads of execution, and well developed object-cache management technology. We present some of the challenges of providing persistence for the Java virtual machine, which may be typical of any attempt at an industrial-strength orthogonally persistent programming system. Some of these might have been avoided by sacrificing persistence independence, but that is far too valuable to sacrifice. We report on some detailed investigations of pinning and cache replacement techniques applicable in this context. The information available for eviction-victim selection is inevitably limited by imprecise information about the state of the machine and the ...