In this tutorial, we’ll explain the\u00a0Hi\/Lo Algorithm. It is used mostly as a\u00a0database identifier generation strategy<\/strong>.<\/p>\n We’ll start with the algorithm overview. Then, we’ll show a practical example based on the Hibernate framework. Finally, we’ll discuss the algorithm’s use cases, its benefits, and its drawbacks.<\/p>\n The main purpose of the Hi\/Lo algorithm is to create a range of numbers that can be safely used as database identifiers<\/strong>. In order to do that, it uses three number variables commonly called high, low<\/em>, and incrementSize<\/em>.<\/p>\n The incrementSize<\/em> variable holds the maximum number of identifiers that can be generated in one batch. It should be treated as a constant value defined at the beginning of the algorithm. Any runtime modification might cause serious problems in environments where multiple clients use the same Hi\/Lo configuration to persist entries.<\/p>\n The high<\/em> variable is usually assigned from a database sequence. In that case, we’re sure that no one will get the same number twice.<\/p>\n The low<\/em> variable holds the currently assigned number in the range [0, incrementSize<\/em>).<\/p>\n Given these points, the Hi\/Lo algorithm generates values in range [(hi <\/em>– 1) * incrementSize <\/em>+ 1, <\/em>(hi * incrementSize<\/em>)).<\/p>\n Let\u2019s take a look at the steps for generating a new value using the Hi\/Lo algorithm:<\/p>\n Let’s see the Hi\/Lo algorithm in action. To do that, we’ll use the Hibernate framework and its Hi\/Lo implementation.<\/p>\n First, let’s define a database entity to work with:<\/p>\n It’s a simple restaurant order with one\u00a0id<\/em> field. To correctly define the Hi\/Lo algorithm in Hibernate, in the definition of the id<\/em> field, we must choose a sequence<\/em> strategy \u2013 hilo<\/em> optimizer \u2013 and specify the increment_size<\/em> parameter.<\/p>\n To show the Hi\/Lo algorithm in action, we’ll persist nine restaurant orders in a loop:<\/p>\n According to the specified increment size in the entity, we should have only three calls to the database for the next high<\/em> value. Assuming the database sequence starts from 1, the first batch of generated identifiers will be in the range [1,3].<\/p>\n When the Hi\/Lo algorithm returns 3 and Hibernate asks for the next identifier’s value, the value of the low<\/em> variable is equal to the incrementSize<\/em> constant. In that case, the next call to the database for the new high<\/em> value must be made. Having 2 as the new high<\/em> value, the algorithm generates values in the range [4,6].<\/p>\n Finally, the last call to the database for the next high<\/em> value is made, and values in the range [7, 9] are assigned to the entities.<\/p>\n Hibernate logs captured during the execution of the persist()<\/em> method confirm those values:<\/p>\n The main advantage of the Hi\/Lo algorithm is the reduced number of database calls<\/strong> for the next sequence values. Increasing the value of incrementSize<\/em> decreases the number of round-trips to the database. Obviously, that means a performance gain in our application. In addition to that, the Hi\/Lo algorithm is a preferred choice in environments with a weak Internet connection<\/strong>.<\/p>\n On the other hand, the Hi\/Lo algorithm isn’t the best choice in environments where multiple different clients persist data to the same table in a database<\/strong>. Third-party applications might be unaware of the Hi\/Lo strategy we’re using to generate identifiers. As a result, they might use entity ids from the generated range of numbers used currently in our application. In that case, when persisting data, we may encounter errors that are difficult to fix.<\/p>\n In this tutorial, we discussed the Hi\/Lo algorithm.<\/p>\n First, we explained how it works and discussed its pseudocode implementation. Then, we showed a practical example using Hibernate’s algorithm implementation. Finally, we listed Hi\/Lo benefits and drawbacks.<\/p>\n As always, the code shown in this article is available over on GitHub<\/a>.<\/p>\n Learn how to the use Hi\/Lo algorithm as a database identifier generation strategy.<\/p>\n2. Hi\/Lo\u00a0Algorithm Overview<\/h2>\n
2.1 Definition<\/h3>\n
2.2 Pseudocode<\/h3>\n
\n
3.\u00a0Practical Example<\/h2>\n
@Entity\r\npublic class RestaurantOrder {\r\n @Id\r\n @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = \"hilo_sequence_generator\")\r\n @GenericGenerator(\r\n name = \"hilo_sequence_generator\",\r\n strategy = \"sequence\",\r\n parameters = {\r\n @Parameter(name = \"sequence_name\", value = \"hilo_seqeunce\"),\r\n @Parameter(name = \"initial_value\", value = \"1\"),\r\n @Parameter(name = \"increment_size\", value = \"3\"),\r\n @Parameter(name = \"optimizer\", value = \"hilo\")\r\n }\r\n )\r\n private Long id;\r\n}<\/code><\/pre>\npublic void persist() {\r\n Transaction transaction = session.beginTransaction();\r\n for (int i = 0; i < 9; i++) {\r\n session.persist(new RestaurantOrder());\r\n session.flush();\r\n }\r\n transaction.commit();\r\n}<\/code><\/pre>\nHibernate: call next value for hilo_seqeunce\r\norg.hibernate.id.enhanced.SequenceStructure - Sequence value obtained: 1\r\norg.hibernate.event.internal.AbstractSaveEventListener - Generated identifier: 1, using strategy: org.hibernate.id.enhanced.SequenceStyleGenerator\r\norg.hibernate.event.internal.AbstractSaveEventListener - Generated identifier: 2, using strategy: org.hibernate.id.enhanced.SequenceStyleGenerator\r\norg.hibernate.event.internal.AbstractSaveEventListener - Generated identifier: 3, using strategy: org.hibernate.id.enhanced.SequenceStyleGenerator\r\nHibernate: call next value for hilo_seqeunce\r\norg.hibernate.id.enhanced.SequenceStructure - Sequence value obtained: 2\r\norg.hibernate.event.internal.AbstractSaveEventListener - Generated identifier: 4, using strategy: org.hibernate.id.enhanced.SequenceStyleGenerator\r\norg.hibernate.event.internal.AbstractSaveEventListener - Generated identifier: 5, using strategy: org.hibernate.id.enhanced.SequenceStyleGenerator\r\norg.hibernate.event.internal.AbstractSaveEventListener - Generated identifier: 6, using strategy: org.hibernate.id.enhanced.SequenceStyleGenerator\r\nHibernate: call next value for hilo_seqeunce\r\norg.hibernate.id.enhanced.SequenceStructure - Sequence value obtained: 3\r\norg.hibernate.event.internal.AbstractSaveEventListener - Generated identifier: 7, using strategy: org.hibernate.id.enhanced.SequenceStyleGenerator\r\norg.hibernate.event.internal.AbstractSaveEventListener - Generated identifier: 8, using strategy: org.hibernate.id.enhanced.SequenceStyleGenerator\r\norg.hibernate.event.internal.AbstractSaveEventListener - Generated identifier: 9, using strategy: org.hibernate.id.enhanced.SequenceStyleGenerator<\/code><\/pre>\n4. Algorithm Benefits and Drawbacks<\/h2>\n
5. Conclusion<\/h2>\n
<\/p>\n
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