Object, Page, and File Servers in Java

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whereas ODBMSs lack this commonly agreed model (recognized by many researchers, for example, [Atkinson 1989]; [TM-Lang uage 1996]; and [Cattell 1997]).
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Object, Page, and File Servers
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ODBMSs have originated from completely different camps through history, and the origination has affected the actual design of the database engines. Three major camps exist: Computer-Aided Design (CAD) and Computer-Aided Manufacturing (CAM) Programming language Database None of the conceptual designs being presented should be considered superior: The most effective solution depends on the environment in which it runs, the application domain, and the applications used. Figure 9.1 shows conceptual designs of the ODBMS servers from the different camps. The gray regions mark elements of the ODBMS. Object-servers have been developed from a database point of view, and accordingly the server parts of the ODBMSs are conceptually different than those originating from the other camps. The database camp wanted to create a professional object-oriented database system with object-oriented properties for handling complex objects and structures as found in Office Information Systems and other systems with similar requirements. Vendors having their origin in this camp include Versant, Itasca, and Poet, among others. In the object-server, the client application, and the server exchange objects, methods may be run at the client side or at the server side,1 and concurrency control is done by the server on the server side. Page servers originated from the programming language camp. This camp was in need of an object-oriented database to support them in the development of applications. Traditional DBMSs were not adequate for this purpose. ODBMSs originating from this camp include ObjectStore (Object Design), O2 (Ardent), ONTOS, and GemStone. The page server exchanges pages of data (containing objects), methods are run on clients only,2 queries are run on clients, and they support distributed concurrency control. File servers originated from the CAD/CAM camp. A need to be able to handle complex data and structures was the driving force no traditional databases were able to deliver with acceptable performance and concurrency control. In this camp, Objectivity has been the major contributor. The file server is actually a special kind of page server, with one major difference in a file server you are not able to request a page and lock a page in the same message.
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Performance Issues
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Database systems should offer good performance to the application. To accomplish this, different kinds of optimizations and adaptations have to be performed. This section presents some of the most important issues related to performance of an ODBMS in this context.
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An object server may execute methods on the server side, but this is not a requirement. Some page servers can run methods on the server as well.
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Object Databases and Java Architectural Issues
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Application Object Cache Object Manager
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File/Index Manager Page Cache Manager
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Page Cache
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a) Object Server Architecture
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Application Object Cache Object Manager File/Index Manager Page Cache Manager Log/Lock Manager Page Cache Manager Page Cache
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b) Page Server Architecture
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c) File Server Architecture
Figure 9.1 Examples of Object DBMS architecture.
Figures 9.1 a, b, and c appear in a slightly different version in DeWitt et al. [DeWitt 1990J. The copyrighted figures are used with permission of the VLDB Endowment. Other versions of these figures exists as well, e.g. in Manola [Manola 1994].
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Transfer Units and Caching
The different kinds of servers exchange information with the client differently. Object servers exchange objects, and each object usually is transferred as a separate network package. If the objects exchanged are smaller than a page, an object server will use the communication channel heavier than a page or file server. On the other hand, if the objects span more than one page, the object server will transfer fewer packages than the page server, thus the page/file server will use the communication channel more heavily than the object server. When an ODBMS accesses the disk, it will, as all other systems using disks, access one or more pages of the file system. The size of a page may vary, but is equal for all pages on a disk. When reading a page from disk, the page may contain several objects. An object-server would have both a page cache and an object cache on the server side, whereas a page or file server usually contains only a page cache (the object cache is bound to the client application). Figure 9.1 gives an overview of the different caches. The difference in transfer units results in another difference between the ODBMSs. A client application connected to an object server is thinner than an identical client using a page or file server. The client of an object server contains an object cache and an object manager that manage the cache, in addition to the application. This is sufficient since the client and the server exchange objects. The page and file clients on the other hand, receive and send pages (and files). Accordingly, they need additional packages in the client to be able to separate the objects from the pages or files being exchanged.