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Object-relational Mapping with Cayenne ORM
by Benoy Jose
As databases and applications become increasingly complex, application architects see value in having system-generated classes, compared to hard written value objects and data access objects. This trend towards using system-generated classes has fueledthe market for JDO-based systems and object-relational mapping (ORM) tools. Cayenne falls into the same category and competes with the best of the tools in this arena. Cayenne is an open source, production-quality ORM tool that provides an object-based access to relational databases. Cayenne's stand-out features are the fact that it's open source and its ease-of-use.
This article will take you through some of the features that make Cayenne a powerful choice for your ORM needs. Cayenne comes with a GUI-based ORM tool that generates Java classes and also provides an API to access thosesame classes.
Features
- Automatic code generation
- Support for multiple databases
- Support for non-RBDMS data sources like flat files and XML schemas
- Automatically manages batched transactions with integrity constraints
- Expression language support
- In-memory sorting for quicker pagination
- Query pre-fetching
- A Java Query API
- Simultaneously commits to multiple databases
- Local and distributed caching
- DataViews for composite user defined objects
- Integrated ANT support
- Validation of data objects before committing them to the database
- User Interface (CayenneModeler) for modeling
- Support for container managed transactions
- Generic event handling support
- Transparent and automatic primary key generation
Architecture
The heart of a Cayenne application is the cayenne.xml file. This file contains all the meta-data information about the underlying database. The cayenne.xml file contains information about the DataContext, DataDomian, and the DataNode. It also has information on where the DataMap .xml file is stored. The DataMap has information about the database tables, views, and their relationships with other tables.
The DataNode is the lowest layer of abstraction in Cayenne and represents a physical data source. It directly interacts with the physical database and performs the actual database operations. The DataNode converts Cayenne queries into the target database specific queries and executes them against the target database. It then converts the resultant JDBC ResultSet into objects and sends them back to the calling program. It also takes handles target, database-specific features like, sequences, outer joins, and primary key generation.
The DataDomain is the next layer of abstraction that lies above the DataNode. It acts as a controller to multiple DataNodes and routes requests to appropriate DataNodes. DataDomain can abstract multiple databases and database types and transparently manage the connection to these different data sources.
The next higher level of abstraction is the DataContext and is directly exposed to the developer. The DataContext acts as a bridge to multiple data sources through the DataDomain. The DataContext keeps track of changes made to the database records (DataObjects) and synchronizes the changes with the database when the user commits the changes. You can batch multiple database operations without writing any SQLs and then decide to commit all of them to the database at one shot. The DataContext will analyze the changes and order them so that database integrity constraints are respected and data inconsistencies are resolved. You can also use the DataContext to pre-fetch primary data and use lazy loading to load the rest of the related data.
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