Object Databases and Java Architectural Issues in Java

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Object Databases and Java Architectural Issues
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Implementing Persistence
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Implementing the persistence-by-reachability paradigm into the Java language binding of an ODBMS requires that the extra code needed to make objects persistent has to be inserted into the source or byte code of the program. The process of inserting the extra code may be done before compilation of the Java source using some kind of preprocessing, or after the Java-source has been compiled, by analyzing the byte code generated by the compiler and inserting other byte code sequences to perform transactional controls and database operations. The result of either strategy (pre- or postprocessing) is byte code that would make the program run only on those platforms that support the defined transactional and database framework. Another possibility would be to modify an existing Java Virtual Machine (JVM). This latter approach is taken mostly by research projects, for example, PJama a type-safe, object-oriented, orthogonally persistent programming language based on Java [Atkinson 1996]. Some ODBMSs do not support Java's object model directly in the database. Instead, the database includes an internal object model. One of the major causes for this situation is related to the difference in the internal representation of an object and the difference in the type and run-time systems of the different programming languages supported by an ODBMS. For example, both Java and C++ have their own Date class, but they do not have identical properties. For a Date object created in Java to be interpreted correctly by a C++ application, the ODBMS has to ensure that the state of the object is preserved in such a manner that both languages may access the object and interpret its contents correctly. The easiest solution to this problem is for the ODBMS to store the object using its own data model and to translate the object to the correct representation when retrieved from a C++ or Java application. Another option would be, of course, to ensure that all objects that are accessed from different programming languages used the ODBMSs data model instead. The latter approach would still comply with the ODMG's statement earlier, and might improve performance of the application in question.
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Every persistent object in an ODBMS has a unique identifier. The identifier is usually some kind logical reference (logical object ID) to the physical location of the object. When a client application accesses such an object, this logical reference is used to read in the object from the database into the client. The identifier then usually is converted from its external form (logical object ID) to an internal form of the actual programming language (direct memory pointer). This operation is known as pointer swizzling [Moss 1992], or just swizzling. The basic trade-off in swizzling is obvious: the conversion costs something up front (and at the end of the program, to convert direct memory pointers back to logical object IDs), but saves a little each time a reference is followed. The tradeoffs in swizzling give rise to two different schemes: Eager swizzling. Whenever a pointer to a persistent object is swizzled, all pointers to objects referenced from this object are (optionally read from the database, dependent on implementation) swizzled as well (if not already read and swizzled).
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Lazy swizzling. Objects are swizzled only when their reference is accessed (if not already swizzled). Consequently, if eager swizzling is used, whenever a persistent object is referenced, all objects contained (e.g., in an array) within the object referenced will be read in and swizzled. This kind of swizzling takes time, but if most of the contained objects are accessed, this may be a good approach. On the other hand, if only a few of the contained objects are accessed, lazy swizzling may be a far better approach. Another aspect of swizzling is whether or not a swizzled pointer can refer to an object that is not main memory resident. This gives rise to two other swizzling related schemes; indirect and direct swizzling [Kemper 1993]: Direct swizzling. Direct swizzling places the in-memory address of the referenced persistent object directly in the swizzled pointer itself. As a consequence, direct swizzling is concerned only with objects being main memory resident. Indirect swizzling. When indirect swizzling is used, the reference to the persistent object is replaced by a pointer to some intermediate data object (usually called fault block) that points to the targeted object when it is in memory. Consequently, the direct swizzling scheme is faster than the indirect swizzling scheme because no extra overhead is added concerning maintenance of fault blocks and the extra level of indirection. On the other hand, indirect swizzling is more flexible, which may improve performance. For example, the indirect approach makes it easier to support incremental uncaching of objects (instead of unswizzling all pointers that reference an object, only the fault block is set to null). This is exemplified by Figure 9.2(a) and (b), where solid lines indicate references based on direct memory pointers and dashed lines indicate inter-object references (the figure does not show cells related to chaining of the hash table). In Figure 9.2(a), object 4 may be removed from main memory without updating the references to it in the other objects (an update of object 4's descriptor is sufficient). In Figure 9.2(b), the Reverse Reference List (RRL in the figure) has to be traversed to update the references to object 4 in other objects (indicated in Figure 9.2(b) as dotted lines). Eager/lazy swizzling may be combined with the indirect /direct swizzling, which together constitute the four main schemes of swizzling (eager direct swizzling, eager indirect swizzling, lazy direct swizzling, and lazy indirect swizzling). Not surprisingly, a number of other swizzling techniques exist; for example, Texas uses swizzling at page-fault time [Singhal 1992]. White gives an overview of the different dimensions and techniques of pointer swizzling as well as a performance study of some of the techniques [White 1994]. Suppose that a language binding does not use swizzling to translate object identifiers between their external form (logical object identity LOID) and the internal form used in a programming language (OID). Instead it uses a hashed table (Figure 9.2(c)) that keeps an entry for each LOID being used. The hashed table is part of the client cache. Each entry in the table keeps information about the object; for example, the memory address of the object, if the object is dirty, and so on. A pointer to an object in the client cache would be nothing else but a pointer into the hashed table, and consequently no hashing is required when referencing an object, but one additional dereference is necessary to get to the physical address of the object.
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