Transparent Activation Db4objects. Optimization can also be done at buildtime using Db4oTool. Store new Person "Petra" ; db. OpenFile [filename] ; While a db4o database file is open, it is locked and cannot be accessed by another application at the same time.
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Transparent Activation Db4objects. Optimization can also be done at buildtime using Db4oTool. Store new Person "Petra" ; db. OpenFile [filename] ; While a db4o database file is open, it is locked and cannot be accessed by another application at the same time. Starting a db4o Server By default, the db4o server runs in-process within your application. OpenServer [filename], [port] ; server. GrantAccess [user], [password] ; To shut down the server: server.
Close ; The port parameter specifies the network port number. Acceptable values are any number above which are not already in use. Using Port 0 for your server creates an "Embedded" server which will not be available remotely.
This is useful for multi-threaded operations or web-server style environments where you wish to handle parallel operations in a single process. It is not required at all for embedded servers. OpenClient [serverAddress], [port], [user], [password] ; To close the client connection to the server: client. Close ; Storing an Object db. Store anObject ; db. Commit Just one line of code is all it takes. Updating an Object db.
Commit ; Looks familiar? You can use the same Store object command to update an object. One difference, however, is that db4o will for performance reasons not automatically check child objects for changes.
By default, db4o will NOT descend into child objects. Store must be called for each modified object unless you change the default UpdateDepth see the UpdateDepth parameter in the Configuration section, below or configure cascading update for the persisted class. This means that there is only ever 1 instance of an object in memory for each stored instance of the object.
Avoid Confusion: Always make sure that the object you are trying to update or delete was previously stored or retrieved in the database. Calling Store with 2 User objects both with an ID of "jack" will result in 2 separate instances. However, if you retrieve the user, and modify the first instance, then store it again, you will have only 1 updated instance in the database.
Deleting an Object db. Delete anObject ; db. Like updates, db4o will not automatically delete child objects unless you configure cascading deletes for your object will remain in memory until the objects are refreshed or garbage collected.
Database Transactions Whenever you start making changes to your database using the Store and Delete commands you are automatically in an open transaction.
To close the transaction, use the Commit method: db. Commit ; Your changes will be permanently saved. If you wish to cancel or roll back any uncommitted changes, use the Rollback method: db. Rollback ; Useful for beginners: Rollback only undoes uncommitted changes in the database. It will not undo changes to any currently loaded objects. So, when you call Rollback you will not see any difference to your objects. If concerned about consistency, use the Refresh object command to cause the objects to be refreshed with stored database values.
Closing a database cleanly will automatically call Commit for you, so any uncommitted transactions are committed automatically. If the database is not closed cleanly, or if the application crashes at any time and uncommitted or incomplete transactions are discarded.
To do this, db4o will reflect all of the properties of the object and assemble all non-default property values into a single query expression. QBE queries also cannot query for value ranges greater than, less than or stringbased expressions contains, starts with.
So QBE can be used only to retrieve exact-value matches. NQ can do all this because they are expressed as native. NET 3. NET 2. CompareTo u2. NET Framework version 3. LINQ namespace.
LINQ Queries have all the benefit of compiler checking and automated code refactorability that Native Queries have, but are expressed in syntax more familiar to SQL developers. Contains "Smith" orderby c. Use JOIN expressions to filter one dataset based on the contents of another.
Use Anonymous Types to get back only the fields you wish. Use an expression to process, filter, or format the data as it is returned from the database. Using combinations of SODA query and constraint keywords, you can build up what is called a query "graph". A graph is a network of objects which represent a segment of data.
Query Namespace. Query ; query. Constrain typeof Customer ; query. Descend "Name". Descend "Country". Constrain "USA" ; c1. Execute ; Notice how each of the calls to Constrain will return an IConstraint? You can keep references to those constraints and then use constraint keywords like And , Or , and Not to relate the constraints together, as we did at the end of that example.
Note that the Descend method returns an IQuery too. Query ; q1. Descend "Total". Constrain OrderDescending Order the result descending according to the current node. Execute Execute the query graph and return the objects at the current node.
Contains For collection nodes, matches will contain the specified value. For string values, behaves as Like. EndsWith bool For strings, matches will end with the supplied value. Optionally case sensitive. Equal Combine with Smaller and Greater to include the specified value. Identity Matching values will be the same object instance as the supplied value. Like For strings, matching values will contain the supplied value anywhere within the match.
Not Performs a negation comparison. Matching values will NOT equal the supplied value. Added to any other constraint keyword, this will reverse the result. Smaller Matching values will be smaller or less than the specified value. StartsWith bool For strings, matches will start with the supplied value.
Query Performance If you are experiencing poor NQ performance, then you probably forgot to either enable run-time optimization of Native Queries by including a reference to the assemblies listed in the "Required Libraries" section or running Db4oTool.
You can change the Query EvaluationMode configuration to control how and when a query should be evaluated. See below in the Configuration section You can index fields to aid query evaluation. Indexing fields causes db4o to store the values of the field in a separate index lookup table in the db.
As a result, when evaluating the query, db4o does not have to seek through all of the object data to resolve the query results. Too many indexed fields can cause poor insert performance. The application developer should tune the number of indexes with the desired Query and Insert performance. Section 5 Dealing With Object Activation When dealing with objects that may have relations to other objects quite deep think of the path of data from Customer to Order to OrderItem to Product with relations to Address objects for billing and shipping and then PO and payment transactions it would be quite expensive to have to pull all of that data into memory from the DB if all you wanted was the Customer object.
Modern object databases use the idea of activation to control the depth to which objects are instantiated and populated when retrieved from the database. The default ActivationDepth in db4o is 5. A properly tuned activation depth is the best way to optimize retrieval of data from a db4o database. See ActivationDepth in the Configuration section for more ideas With an ActivationDepth of 5, objects will be populated up to 5 levels deep. Properties of the 5th descendant object will have their values left as default or null.
If you encounter an object that is not yet activated, you can pass it to db4o for manual late activation: db. Activate [unactivatedObject], [depth] ; Objects can also be manually de-activated: db. Deactivate [activatedObject], [depth] ; Fine-grained activation depth can be configured per class. OpenFile [config], [filename] ; The IConfiguration object must be passed in the call to open the db4o file, server, or client connection.
Query Evaluation Mode This property controls when and how much of a query is executed.
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Kijora 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. This is useful for multi-threaded operations or web-server style environments where you wish to handle parallel operations in a single process. Please help improve this article if you can. Queries, updates, and identity The default ActivationDepth in db4o is 5.
DB4O DOCUMENTATION PDF
Fenrigal Walking the results is a simple exercise in using the Iterator interface implemented by ObjectSet. Please help improve this article if you can. This feature can be viewed as an advantage over relational model, where any change in the schema results in mostly manual code review and upgrade to match the schema changes. You need it to compile the examples.
db4o (database for objects)
Vudoshura Rollback only undoes uncommitted changes in the database. For strings, matches will end with the supplied value. Over a million developers have joined DZone. For collection nodes, matches will contain the specified value. In OctoberActian declined to continue to actively pursue and promote the commercial db4o product offering for new customers. From Wikipedia, the free encyclopedia.