org.dasein.persist
Class DaseinSequencer

java.lang.Object
  org.dasein.persist.Sequencer
      org.dasein.persist.DaseinSequencer

public class DaseinSequencer
extends Sequencer

Provides a database-independent sequence generation scheme. This class goes to the database once every MAX_KEYS requests to get a new seed for the numbers it generates. This class is thread-safe, meaning multiple threads can be safely requesting unique numbers from it. It is also multi-process safe. In other words, multiple machines can simultaneously be generating unique values and those values will be guaranteed to be unique across all applications. The only caveat is that they all must be using the same algorithm for generating the numbers and getting seeds from the same database.

In order to access the database, this class reads the configuration properties file dasein-persistence.properties to look up the DSN for the sequencer database. This value should be set with the name of the DSN that provides connections to the database with the sequencer table. That table should have the following CREATE:

 CREATE TABLE sequencer (
     name        VARCHAR(20)     NOT NULL,
     spacing    BIGINT UNSIGNED NOT NULL,
     next_key     BIGINT UNSIGNED NOT NULL,
     last_update  BIGINT UNSIGNED NOT NULL,
     PRIMARY KEY ( name, lastUpdate )
 );
 

Last modified $Date: 2009/02/07 18:11:51 $

Version:

$Revision: 1.4 $

Author:

George Reese

Field Summary

Fields inherited from class org.dasein.persist.Sequencer

PROPERTIES

Constructor Summary

DaseinSequencer()

Method Summary

long	next() Generates a new unique number.

Methods inherited from class org.dasein.persist.Sequencer

getInstance, getInstance, getName, setName

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

DaseinSequencer

public DaseinSequencer()

Method Detail

next

public long next()
          throws PersistenceException

Generates a new unique number. The unique number is based on the following algorithm:
unique number = seed multiple by maximum keys per seed added to seed sequence
The method then increments the seed sequence for the next ID to be generated. If the ID to be generated would exhaust the seed, then a new seed is retrieved from the database.

Specified by:

next in class Sequencer

Returns:

a unique number

Throws:

PersistenceException - a data store error occurred while generating the number