This tutorial explains the benefits of using an automated schema migration tool such as FlywayDB, as well as the limitations of the Hibernate schema generation utility (hbm2ddl). The article assumes almost no knowledge of Hibernate, and it can be read by both junior and senior developers alike.
Introduction
In a relational database, data is stored in table rows, and relations are expressed through a foreign key. While the database is responsible for storing and retrieving data, the business logic is usually embedded in the application layer. When using an object-oriented programming language, the object graph becomes the most natural representation of data. The discrepancy between graph-like structures and database tuples is typically known as the object-relational impedance mismatch.
The object-relational mapping (ORM) pattern aims to address this problem, and Hibernate ORM is undoubtedly one of the most successful implementations of this pattern. However, Hibernate is more than an ORM framework that conforms to the Java Persistence API specification.
Hibernate is a data access framework implementing many patterns that have been popularized by Martin Fowler in his book Patterns of Enterprise Application Architecture. For instance, aside from the typical object-relational mapping feature, Hibernate also implements the following data access patterns:
- Unit of work
- Identity map
- Lazy load
- Optimistic and pessimistic locking
- Application-level conversations that span over multiple web requests
Data Model Duality
While the ORM tool allows us to translate the object graph state changes into SQL statements, we are still managing two distinct data models. On one side, the database schema defines the rules for how data is ought to be stored, whereas on the application side, the domain model is an object-graph mirror of the underlying database schema.
So, who is in charge of driving the model representation? Is it the database or the domain model?
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by Vlad Mihalcea via SitePoint
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