1 Introduction

In the Mjølner System, objects generated by a BETA program execution may be saved on secondary storage and restored in another BETA program execution. Usually this property of a programming language or system is referred to as object persistence. Persistence in the Mjølner System is based on a reachability model, meaning that default behaviour when saving an object on secondary storage is to save everything reachable from the object in question.

Mjølner supports a single-user persistent store model as well as a multi-user shared persistent store. The multi-user persistent store makes it possible to build an object-oriented database (OODB) using a clienter/server model. Persistence in Mjølner is based on a number of frameworks:

Basic persistent object management. A program may saves its objects in a persistent store and these may later be re-fetched from another or the same program. This framework supports single-user persistence.
Concurrency control. If two or more programs (clients) need to acces the same persistent store at the same time, it is possible to synchronize acces to the persistent store through a server. The server implements a concurrency control mechanism that mediates the access to the persistent store. This framework supports multi-user persistent store.
Transaction management. Transaction management deals with the problem of keeping a persistent store in a consistent state when two or more clients concurrently access the store or when failures happen during a program execution. This framework adds functionality to a multi-user
Notification control. When two or more clients use the same persistent store, a notification mechanism is available for communication between the clients. Notifications may be sent to cleints when e.g. objects are written to the store by another client. This framework adds functionality to a multi-user persistent store.

In addition to the above main features, the following features are also supported:

The basic object management supports reference between objects in different persistent stores.
Recognizing that saving the full transitive object closure may at times be too coarse grained, the Mjølner System allows the programmer to control what part of the reachable object graph is actually saved along. This allows pointers from persistent objects to non-persistent objects despite the reachability based persistence model.

The persistence frameworks have gone through a number of iterations:

The first ideas for persistence in Mjølner were presented in [Agesen 89] and a first implementation of single-user persistence was made by Claus Nørgaard.
The first product-quality implementation of single-user persistence was made by Søren Brandt. Design considerations, experience and recommendations for a re-design are documented in the PhD Thesis of Søren Brandt [Brandt94]. This version of basic object management has been part of the Mjølner System for many years and is referred to as the old basic object management.
A multi-user shared persistence store with concurrency control, transaction management and notifications has recently been developed by Johanna Wiederhold Tind [Tind97].
Recently a new and more efficient implementation of the basic object management has been implemented by Stephan Erbs Korsholm [Korsholm99]. This work a.o. benefits from the work of Søren Brandt. This version of the basic object management is referred to as the new basic object management.

The new basic object management will eventually replace the old basic object management. In release 5.0 of the Mjølner System the old- as well as the new basic object management are included. We recommed to use the new basic object management since it is much more efficient than the old one. However, as this is the first release of the new basic object management, there may still be some errors or problems that have not yet been discovered.

The old and the new basic object management both work with the shared persistent store.

The directory

~beta/persiststore

contains the new basic object management as well as the shared persistent store.

The directory

~beta/persiststore/OLD

contains the old persistent store.

The interface to the old and the new persistent store is the same. That is, it is possible to alternate between the old and the new store by including either

~beta/persiststore/persistentstore

~beta/persiststore/OLD/persistentstore

Persistence in BETA - Reference Manual

[Modified: Monday October 23^rd 2000 at 10:43]