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Mapping entities
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Defining the indexing strategy
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We ve pretty much described how Hibernate Search stores the object structure in the index, but we haven t considered how Lucene should index the data. Lucene offers various strategies on how to index data. Each influences the kind of query that will be available to you. It s important to have a global understanding of these strategies and, of course, to be able to choose among them. We ll discuss those strategies while walking through the Hibernate Search mapping metadata. Even before thinking about the fine-grained indexing strategies, the first question you should ask is, Should I index this data Or more accurately, Should I be able to search by this data At a coarse-grained level, systematically indexing all your entities makes little sense; only those useful for retrieving information searched by your users should be marked for indexing. This is true as well at the fine-grained level. Not all properties deserve to be indexed. While they could be indexed, and while a query could easily filter them out, you should be aware that excluding them will speed up the following: Indexing time Index time is directly correlated to the amount of data to index. Search time While the index size doesn t linearly affect the search query time, it has an influence. Some clustering strategies The smaller the index is, the faster replication will be. Fortunately, Hibernate Search makes it intuitive during the mapping to select the relevant part to index. Enough theory and concepts for now. Since we know why a mapping is needed, let s discover how to map the data structure. The next section will describe how to map the coarse-grained level: an entity.
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Let s describe the steps required to transform a boring Hibernate entity into a colorful indexed entity. To help us stay concrete, we ll come back to the online store example we started in chapter 2. We want to sell DVDs online; the DVD data structure is represented by a Dvd class. We ll first explore what makes an entity indexed and what is happening behind the scenes.
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Marking an entity as indexed
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All Hibernate Search mapping metadata are described through annotations.
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WHY USE ANNOTATIONS Hibernate Search metadata is very code-centric and shares a lot of information with the class structure: Annotations are a natural fit and avoid much redundancy compared to other metadata models such as XML. It would be quite simple to add XML deployment descriptor support for Hibernate Search. Nobody has found the time and interest to write this layer (so far), which seems to indicate that the Hibernate Search community doesn t strongly desire this feature.
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Mapping simple data structures
All Hibernate Search annotations are contained in the org.hibernate.search.annotations package.
To mark an entity as indexed by Hibernate Search, place the @Indexed annotation on the class, as shown in listing 3.1.
Listing 3.1 An entity is indexed by Hibernate Search when marked @Indexed
@Entity @Indexed public class Dvd { ... }
Mark the entity as @Indexed
Much information is inferred, and a lot of work is triggered from this single annotation. When the Hibernate SessionFactory bootstraps, Hibernate Search looks for all mapped entities marked as @Indexed and processes them. We don t have to explicitly list the indexed entities in a configuration file. This reduces work and limits the risk of mistakes. The Lucene directory name is also inferred from this annotation. Because we haven t explicitly defined an index name, the default naming convention applies. The index name is the fully qualified class name of the entity, in our example com.manning.hsia.dvdstore.model.Dvd. You can override this name by using the name attribute of @Indexed (see Listing 3.2).
Listing 3.2 Overriding an indexed name to refine the targeted Lucene Directory.
@Entity @Indexed(name="Item") public class Dvd { ... }
Index names can be customized
The underlying mapping between a Hibernate Search index name and a physical Lucene Directory depends entirely on the directory provider (see section 5.1). Let s explore the two most common scenarios: the in-memory directory provider (RAMDirectoryProvider) and the filesystem directory provider (FSDirectoryProvider). Indexes using the RAMDirectoryProvider are uniquely identified by their index name for a given SessionFactory (EntityManagerFactory if you use Java Persistence). Hibernate Search keeps one instance of RAMDirectoryProvider per index name and per SessionFactory (or EntityManagerFactory). When using FSDirectoryProvider, the index name represents the path to the physical filesystem directory. Relative paths are prefixed with the indexBase property. It s perfectly safe to share the same physical Lucene directory among several entities; Hibernate Search partitions the information. If you want to share the same physical Lucene directory across several entities, they need to share the same @Indexed.name value (as well as the same DirectoryProvider type), like Dvd and Drink in listing 3.3.
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