Visit the new documentation for updated content. DB4O. Table of contents. 1 Info; 2 The Basic Principle of db4o; 3 Downloads; 4 Current Status. This tutorial was written to get you started with db4o as quickly as possible. Before you The API documentation for db4o is supplied as a compiled Help file . db4o provides sources of documentation: tutorial, reference documentation, API documenation, online paircasts and.
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A properly tuned activation depth is the best way to optimize retrieval of data from a db4o database. Or, again, you could tweak the db4l to return the instance found, so that the Person instance had its OID appropriately associated, and so on. The db4o Java distribution includes several jar files, including a core db4o implementation for each of the JDK 1.
MaxValue will cause all related objects to be instantiated as deeply as possible, restoring the entire object graph to memory.
DefragmentConfig (db4o – database for objects – documentation)
Queries, updates, and identity. Astute readers of Joshua Bloch’s Effective Java will notice that I’ve left out the hashCode implementation, a clear violation of Rule 8. Please help improve it by removing promotional content and inappropriate external linksand by adding encyclopedic content written from a neutral point of view.
In this second article in The busy Java developer’s guide to db4oI used the OODBMS notion of identity as a doocumentation point for explaining how db4o stores and retrieves objects, as well as introducing its native query facility.
You can use the same Store object command to update an object. For strings, matches will end with the supplied value.
Using Person in a QBE query for just last name, however, mandates that the first name be allowed to be vb4o, which effectively means we either have to choose the domain constraint or the query capability, neither of which is entirely acceptable. LINQ allows the creation of object-oriented queries of any complexity with the benefit of compile-time checking, IDE Intellisense integration and automated refactoring.
Regardless of when it is run, query constraints against non-indexed data are always performed on the currently stored object state. Ddb4o to a feature referred to as “Generic Reflection”, db4o can work without implementing persistent classes on the server.
These handlers are great for selectively activating, refreshing, or cascading db operations. For strings, matches will start with the supplied value.
A list of object ID matches is generated completely when the query is executed and held in memory Lazy Best for limited resource environments.
Since the index snapshot is held in memory, the memory required varies greatly depending on what is being queried. Retrieved March 17, This multi-faceted approach to querying databases may strike you as complex and confusing and entirely different from how an RDBMS works. Views Read Edit View history. Accessing the Count or Length properties will cause full evaluation.
Queries, updates, and identity
Using Port 0 for your server creates an “Embedded” server which will not be available remotely. This gives me a basic method outline that looks like the one shown in Listing Too many indexed fields can cause poor insert performance. Comments Sign in or register to add and subscribe to comments.
And for those situations where db4o’s native query facility fails to meet the need, the SODA API which originally appeared as a standalone query system for any object system, and still lives on SourceForge allows you to tune the query down to its tiniest detail, at the expense of simplicity. In this worst-case scenario, db4o would have to instantiate every object of the queried type in the database and pass each one through the match implementation.
Global Update Depth We said earlier that when calling Store to update an object graph, that db4o will not by default descend into child objects to detect changes to the graph.
Because db4o uses the native object or referential identity, it’s better to open the database or connection when your application begins, do all your work, then close the database when your program is shutting down.
It can be any application class including third-party classes contained in referenced libraries. For string values, behaves as Like. Using tables to store objects is like driving your car home and then disassembling it to put it into the garage.
You can find complete instructions in the db4o reference documentation. Matching values will be the same object instance as the supplied value. A graph is a network of objects which represent a segment of data. Indexing fields is a great way to increase query performance, but each index table is one more place where a field’s value is stored.
Information storage and retrieval has been nearly synonymous with RDBMS for about a decade now, but recently that has begun to change. It will not undo changes to any currently loaded objects. From Wikipedia, the free encyclopedia. The default ActivationDepth in db4o is 5. One difference, however, is that db4o will for performance reasons not automatically check child objects for changes.
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Developers are encouraged to use the Native Query interfaces when working with db4o. Intuition isn’t always sufficient for anticipating a failure to optimize because the reasons can be entirely different from what a code review would imply. The ODB is primarily used as an application specific database in either extreme scale systems or embedded systems where typical DBA activities are automated. Walking the results is a simple exercise in using the Iterator interface implemented by ObjectSet.
To close the transaction, use the Commit method:. Free and open-source software portal.