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JPA identifier generators

JPA defines the following identifier strategies:

Strategy Description AUTO The persistence provider picks the most appropriate identifier strategy supported by the underlying database IDENTITY Identifiers are assigned by a database IDENTITY column SEQUENCE The persistence provider uses a database sequence for generating identifiers TABLE The persistence provider uses a separate database table to emulate a sequence object

In my previous post I exampled the pros and cons of all these surrogate identifier strategies.

Identifier optimizers

While there’s not much application-side IDENTITY generator optimization (other than configuring database identity preallocation), the sequence identifiers offer much more flexibility in this regard. One of the most common optimization strategy is based on the hi/lo allocation algorithm.

For this Hibernate offers:

Generator Description SequenceHiLoGenerator It uses a database sequence to generate the hi value, while the low value is incremented according to the hi/lo algorithm TableHiLoGenerator A database table is used for generating the hi values. This generator is deprecated in favour of the MultipleHiLoPerTableGenerator, the enhanced TableGenerator or the SequenceStyleGenerator. MultipleHiLo

PerTableGenerator It’s a hi/lo table generator capable of using a single database table even for multiple identifier sequences. SequenceStyleGenerator It’s an enhanced version of the previous sequence generator. It uses a sequence if the underlying database supports them. If the current database doesn’t support sequences it switches to using a table for generating sequence values. While the previous generators were having a predefined optimization algorithm, the enhanced generators can be configured with an optimizer strategy: none : there is no optimizing strategy applied, so every identifier is fetched from the database

: there is no optimizing strategy applied, so every identifier is fetched from the database hi/lo : it uses the original hi/lo algorithm. This strategy makes it difficult for other systems to share the same identifier sequence, requiring other systems to implement the same identifier generation logic.

: it uses the original hi/lo algorithm. This strategy makes it difficult for other systems to share the same identifier sequence, requiring other systems to implement the same identifier generation logic. pooled: This optimizer uses a hi/lo optimization strategy, but instead of saving the current hi value it stores the current range upper boundary (or lower boundary – hibernate.id.optimizer.pooled.prefer_lo). Pooled is the default optimizer strategy. TableGenerator Like MultipleHiLoPerTableGenerator it may use one single table for multiple identifier generators, while offering configurable optimizer strategies. Pooled is the default optimizer strategy.

JPA to Hibernate identifier mapping

Having such an abundant generator offer, we cannot help asking which of those is being used as the default JPA generators.

While the JPA specification doesn’t imply any particular optimization, Hibernate will prefer an optimized generator over one that always hit the database for every new identifier.

The JPA SequenceGenerator

We’ll define one entity configured with the SEQUENCE JPA identifier generator. A unit test is going to persist five such entities.

@Entity(name = "sequenceIdentifier") public class SequenceIdentifier { @Id @GeneratedValue(generator = "sequence", strategy=GenerationType.SEQUENCE) @SequenceGenerator(name = "sequence", allocationSize = 10) private Long id; } @Test public void testSequenceIdentifierGenerator() { LOGGER.debug("testSequenceIdentifierGenerator"); doInTransaction(new TransactionCallable<Void>() { @Override public Void execute(Session session) { for (int i = 0; i < 5; i++) { session.persist(new SequenceIdentifier()); } session.flush(); return null; } }); }

Running this test we’ll give us the following output

Query:{[call next value for hibernate_sequence][]} Generated identifier: 10, using strategy: org.hibernate.id.SequenceHiLoGenerator Generated identifier: 11, using strategy: org.hibernate.id.SequenceHiLoGenerator Generated identifier: 12, using strategy: org.hibernate.id.SequenceHiLoGenerator Generated identifier: 13, using strategy: org.hibernate.id.SequenceHiLoGenerator Generated identifier: 14, using strategy: org.hibernate.id.SequenceHiLoGenerator Query:{[insert into sequenceIdentifier (id) values (?)][10]} Query:{[insert into sequenceIdentifier (id) values (?)][11]} Query:{[insert into sequenceIdentifier (id) values (?)][12]} Query:{[insert into sequenceIdentifier (id) values (?)][13]} Query:{[insert into sequenceIdentifier (id) values (?)][14]}

Hibernate chooses to use the legacy SequenceHiLoGenerator for backward compatibility with all those applications that were developed prior to releasing the enhanced generators. Migrating a legacy application to the new generators is not an easy process, so the enhanced generators are a better alternative for new applications instead.

Hibernate prefers using the “seqhilo” generator by default, which is not an intuitive assumption since many might expect the raw “sequence” generator (always calling the database sequence for every new identifier value).

To enable the enhanced generators we need to set the following Hibernate property:

properties.put("hibernate.id.new_generator_mappings", "true");

Giving us the following output:

Query:{[call next value for hibernate_sequence][]} Query:{[call next value for hibernate_sequence][]} Generated identifier: 1, using strategy: org.hibernate.id.enhanced.SequenceStyleGenerator Generated identifier: 2, using strategy: org.hibernate.id.enhanced.SequenceStyleGenerator Generated identifier: 3, using strategy: org.hibernate.id.enhanced.SequenceStyleGenerator Generated identifier: 4, using strategy: org.hibernate.id.enhanced.SequenceStyleGenerator Generated identifier: 5, using strategy: org.hibernate.id.enhanced.SequenceStyleGenerator Query:{[insert into sequenceIdentifier (id) values (?)][1]} Query:{[insert into sequenceIdentifier (id) values (?)][2]} Query:{[insert into sequenceIdentifier (id) values (?)][3]} Query:{[insert into sequenceIdentifier (id) values (?)][4]} Query:{[insert into sequenceIdentifier (id) values (?)][5]}

The new SequenceStyleGenerator generates other identifier values than the legacy SequenceHiLoGenerator. The reason why the update statements differ between the old and the new generators is because the new generators default optimizer strategy is “pooled” while the old generators can only use the “hi/lo” strategy.

The JPA TableGenerator

@Entity(name = "tableIdentifier") public class TableSequenceIdentifier { @Id @GeneratedValue(generator = "table", strategy=GenerationType.TABLE) @TableGenerator(name = "table", allocationSize = 10) private Long id; }

Running the following test:

@Test public void testTableSequenceIdentifierGenerator() { LOGGER.debug("testTableSequenceIdentifierGenerator"); doInTransaction(new TransactionCallable<Void>() { @Override public Void execute(Session session) { for (int i = 0; i < 5; i++) { session.persist(new TableSequenceIdentifier()); } session.flush(); return null; } }); }

Generates the following SQL statement output:

Query:{[select sequence_next_hi_value from hibernate_sequences where sequence_name = 'tableIdentifier' for update][]} Query:{[insert into hibernate_sequences(sequence_name, sequence_next_hi_value) values('tableIdentifier', ?)][0]} Query:{[update hibernate_sequences set sequence_next_hi_value = ? where sequence_next_hi_value = ? and sequence_name = 'tableIdentifier'][1,0]} Generated identifier: 1, using strategy: org.hibernate.id.MultipleHiLoPerTableGenerator Generated identifier: 2, using strategy: org.hibernate.id.MultipleHiLoPerTableGenerator Generated identifier: 3, using strategy: org.hibernate.id.MultipleHiLoPerTableGenerator Generated identifier: 4, using strategy: org.hibernate.id.MultipleHiLoPerTableGenerator Generated identifier: 5, using strategy: org.hibernate.id.MultipleHiLoPerTableGenerator Query:{[insert into tableIdentifier (id) values (?)][1]} Query:{[insert into tableIdentifier (id) values (?)][2]} Query:{[insert into tableIdentifier (id) values (?)][3]} Query:{[insert into tableIdentifier (id) values (?)][4]} Query:{[insert into tableIdentifier (id) values (?)][5]}

As with the previous SEQUENCE example, Hibernate uses the MultipleHiLoPerTableGenerator to maintain the backward compatibility.

Switching to the enhanced id generators:

properties.put("hibernate.id.new_generator_mappings", "true");

Give us the following output:

Query:{[select tbl.next_val from hibernate_sequences tbl where tbl.sequence_name=? for update][tableIdentifier]} Query:{[insert into hibernate_sequences (sequence_name, next_val) values (?,?)][tableIdentifier,1]} Query:{[update hibernate_sequences set next_val=? where next_val=? and sequence_name=?][11,1,tableIdentifier]} Query:{[select tbl.next_val from hibernate_sequences tbl where tbl.sequence_name=? for update][tableIdentifier]} Query:{[update hibernate_sequences set next_val=? where next_val=? and sequence_name=?][21,11,tableIdentifier]} Generated identifier: 1, using strategy: org.hibernate.id.enhanced.TableGenerator Generated identifier: 2, using strategy: org.hibernate.id.enhanced.TableGenerator Generated identifier: 3, using strategy: org.hibernate.id.enhanced.TableGenerator Generated identifier: 4, using strategy: org.hibernate.id.enhanced.TableGenerator Generated identifier: 5, using strategy: org.hibernate.id.enhanced.TableGenerator Query:{[insert into tableIdentifier (id) values (?)][1]} Query:{[insert into tableIdentifier (id) values (?)][2]} Query:{[insert into tableIdentifier (id) values (?)][3]} Query:{[insert into tableIdentifier (id) values (?)][4]} Query:{[insert into tableIdentifier (id) values (?)][5]}

You can see that the new enhanced TableGenerator was used this time.

Conclusion

The extended identifier generators are very useful as they allow us to reduce the number of database roundtrips when persisting entities using sequence-based identifiers.

Code available on GitHub.

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