This chapter surveys extended transaction models proposed to support long duration, interactive and/or cooperative activities in the context of multi-user software development and CAD/CAM environments. Many of these are variants of the checkout model, which addresses the long duration and interactive nature of the activities supported by environments but still isolates environment users, making it difficult for them to collaborate while their activities are in progress. However, a few cooperative transaction models have been proposed to facilitate collaboration, usually while maintaining some guarantees of consistency.

In a distributed storage system, client caches managed on the basis of small granularity objects can provide better memory utilization then page-based caches. However, object servers, unlike page servers, must perform additional disk reads. These installation reads are required to install modified objects onto their corresponding disk pages. The opportunistic log is a new technique that significantly reduces the cost of installation reads. It defers the installation reads, removing them from the modification commit path, and manages a large pool of pending installation reads that can be scheduled efficiently. Using

We propose a technique for maintaining coherency of a transactional distributed shared memory, used by applications accessing a shared persistent store. Our goal is to improve support for fine-grained distributed data sharing in collaborative design applications, such as CAD systems and software development environments. In contrast, traditional research in distributed shared memory has focused on supporting parallel programs; in this paper, we show how distributed programs can benefit from this shared-memory abstraction as well. Our approach, called log-based coherency, integrates coherency support with a standard mechanism for ensuring recoverability of persistent data. In our system, transaction logs are the basis of both recoverability and coherency. We have prototyped log-based coherency as a set of extensions to RVM [Satyanarayanan et al. 94], a runtime package supporting recoverable virtual memory. Our prototype adds coherency support to RVM in a simple way that does not requir...

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...

The principal tenet of the persistence model is that it abstracts over all the physical properties of data such as how long it is stored, where it is stored, how it is stored, what form it is kept in and who is using it. Experience with programming systems which support orthogonal persistence has shown that the simpler semantics and reduced complexity can often lead to a significant reduction in software production costs. Persistent systems are relatively new and it is not yet clear which of the many models of concurrency and distribution best suit the persistence paradigm. Previous work in this area has tended to build one chosen model into the system which may then only be applicable to a particular set of problems. This thesis challenges the orthodoxy by designing a persistent framework in which all models of concurrency and distribution can be integrated in an add-on fashion. The provision of such a framework is complicated by a tension between the conceptual ideas of persistence...

We classify clustering algorithms into sequence-based techniques---which transform the object net into a linear sequence---and partition-based clustering algorithms. Tsangaris and Naughton [TN91, TN92] have shown that the partition-based techniques are superior. However, their work is based on a single partitioning algorithm, the Kernighan and Lin heuristics, which is not applicable to realistically large object bases because of its high running-time complexity. The contribution of this paper is two-fold: (1) we devise a new class of greedy object graph partitioning algorithms (GGP) whose running-time complexity is moderate while still yielding good quality results. For large object graphs GGP is the best known heuristics with an acceptable running-time. (2) We carry out an extensive quantitative analysis of all well-known partitioning algorithms for clustering object graphs. Our analysis yields that no one algorithm performs superior for all object net characteristics. Therefore, we d...

In this thesis we address the problem of providing efficient processing of queries in the extensible environment induced by object-oriented databases. We define a framework for query processing in an object-oriented database and develop designs for major components of this framework. The framework encompasses an object-oriented data model, an algebra to query over that model, transformation rules for the algebra, an internal representation for queries expressed in the algebra, a cost model for analyzing query expressions, and an architecture for an extensible query optimizer. The major contributions of this thesis are an algebra and transformation rules, a representation, and an architecture for extensible query optimization. We show how these components fit into the framework and interact with each other. The EQUAL query algebra presented in this thesis is the first query algebra for object-oriented database systems to be completely consistent with data abstraction, and one of the few...

The generation of execution plans for object-oriented database queries is a new and challenging area of study. Unlike the relational algebra, a common set of object algebra operators has not been defined. Similarly, a standardized object manager interface analogous to storage manager interfaces of relational subsystems does not exist. We define the interface to an object manager whose operations are the executable elements of query execution plans. Parameters to the object manager interface are streams of tuples of object identifiers. The object manager can apply methods and simple predicates to the objects identified in a tuple. Two algorithms for generating such execution plans for queries expressed in an object algebra are presented. The first algorithm runs quickly but may produce inefficient plans. The second algorithm enumerates all possible execution plans and presents them in an efficient, compact representation. Keywords: object-oriented databases, query processing, query opt...

We consider how a persistent programming language might offer performance competitive with that of non-persistent languages, at least on memory resident data. We are concerned with object-oriented languages, and with implementing persistence via object faulting, where the system detects uses of non-resident objects and fetches them on demand. We present some background on object faulting and means for implementing it, and describe a specific language we are developing, namely Persistent Modula-3. Then we explore approaches to optimising persistence aspects of Persistent Modula-3, and outline techniques under consideration in our compiler development effort.

Object-oriented databases (OODBs) are believed to more naturally reflect the behavior and organization of complex application domains. The schema consists of a collection of classes, organized into hierarchies which nicely organize abstractions over the domain. Objects are created as instances of classes, encapsulating data and interpretation of data together. An important characteristic is the support for evolutionary programming, and so that existing programs may be extended with new classes without affecting other parts of the system.