Transaction Models in Java

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Transaction Models
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Some ODBMSs support several kinds of transaction models. For example, Versant supports short transactions with the traditional ACID properties [Harder 1983], long transactions, and closed nested transactions. It is also possible to create your own lock models (optionally relaxing the ACID properties), or to use an optimistic locking scheme (timestamp ordering) instead of the traditional two-phase locking [Danielsen 1999]. Nested transactions may be implemented using several threads, which again may improve the performance of the transaction. On the other hand, programming nested transactions are more complex than the traditional short and flat transactions. Using an optimistic locking scheme postpones the concurrency problems until commit-time, which in some cases may improve performance. Some applications and domains may enjoy the benefits of different transaction models, both related to functionality and to performance. We should carefully consider whether the targeted application (and domain) requires such transactional properties.
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Multithreaded Database Clients
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Support for multithreaded database clients is important in multitier systems. In such systems a server (e.g., CORBA server) in the middle tier handles a number of different clients (CORBA clients). The CORBA server acts as a single database client to the database. To avoid blocking of the different CORBA clients due to concurrent requests issued by different CORBA clients, the ODBMS should support multithreaded clients.
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However, currently not all ODBMSs support multithreaded databases clients. Examples of ODBMSs that support multithreaded clients are Versant, Jasmine, and Objectivity.
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Storage Reclamation
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Data are inserted, changed, and deleted in a database as time goes by, resulting in unused or unreferenced pages. These kinds of pages may be reclaimed by the database system using some kind of reclaiming strategy. The strategy chosen by different ODBMSs differ, but generally two approaches are used: Explicit reclamation. The DBA of the ODBMS will make the system unavailable, and explicitly perform some kind of reclamation procedure restructuring the database; that is, a kind of offline garbage collection in the store or database. Implicit reclamation. The reclamation process is performed by the ODBMS at run time, preferably when the ODBMS is not used, and as a background activity. This kind of reclamation strategy may in some cases result in performance hiccups (temporary degradation of performance). This represents a kind of online garbage collection in the store or database. Inaccessible pages may degrade performance by forcing the ODBMS to perform disk I/Os that do not return data, and if the database does not reclaim these pages, the database files will continuously increase in size and performance will be degraded.
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Java Language Binding
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Offering persistence to objects in Java may be done in a number of ways. For example, a class may inherit persistence capabilities from a predefined persistent class. The object instantiated from this class may either be persistent or transient, but the operation of moving the data between transient and persistent areas must be performed explicitly [Lausen 1997]. This principle is generally known as persistence-by-inheritance. Another way of offering persistence to an object is to let the object obtain its persistent capabilities when instantiated. This principle is generally known as persistence-byinstantiation. A third way, and the most intriguing solution as well for Java, is to make an object persistent if the object is reachable (using object references) from another persistent object. This is generally referred to as persistence-by-reachability. In this section, we will focus on the Java language binding defined by the Object Data Management Group (ODMG) in The Object Database Standard: ODMG 2.0 [Cattell 1997], and how it may be implemented. ODMG defines the design principle of the binding as follows: "The ODMG Java binding is based on one fundamental principle: the programmer should perceive the binding as a single language for expressing both database and programming operations, not two separate languages with arbitrary boundaries between them." [Cattell 1997] Thus, all ODBMSs claiming ODMG-compliance of their Java language binding supports this binding based on the persistence-by-reachability paradigm outlined earlier.
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