org.equanda.persistence
Class UoidGenerator

java.lang.Object
  org.equanda.persistence.UoidGenerator

public final class UoidGenerator
extends Object

generator for Uoid values for the newly created records We need a universal object if which is fast to generate, has a minimal (no) configuration overhead and cross platform. In accordance with Scott Ambler's article about this ("Enterprise Ready Object IDs", @link http://www.sdmagazine.com/print/documentID=11250) we use a HIGH-LOW scheme. Values are unique accross tables and normally also accross databases. This is very important when a database is remotely added to (eg on a portable) and new data has to be merged later.

I am concerned about mistakes caused by improper configuration and therefore do not want to use a configurable HIGH value. A PC's IP address is not guaranteed unique when PC's are connecting and disconnecting from the network. I do not know how to obtain a network card's MAC address in a cross platform method, so I have to search for a different mechanism. A popular fix to this is using a database table to store generated values. I don't like that because lack of transactions can make this very dangerous on some databases.

The idea is to use the time of initialisation of this class as base for the HIGH address. This should prove to be quite random and chances are very, very slim that a second system which needs the same database will also use this timestamp as base for HIGH value. However, to decrease the likelyhood even further, the HIGH value also contains a random number which does not use the time at it's seed.

For LOW values we just use a counter and we force a new HIGH value when the counter runs out. Contrary to normal practice, the LOW counter does not start at zero, but at a random number. This increases the distribution of Uoid values (which should improve the quality of the indexes). Also, in the unlikely event that HIGH values would clash, this reduces the chance of duplicate Uoid values.

The Uoid consists of a 36bit time, and 30bit random number, giving a 66bit high value, and a 18bit LOW value, giving a 84 bit key. This is converted to a 14 digit String using base64 encoding. However, the digits are encoded back to front to make the Uoid strings discriminate from the start.

There are some things which can be chosen in this scheme. The random part of the HIGH value could be regenerated for each Uoid (giving a HIGH/RAND/LOW scheme), or just once (as in true HIGH/LOW scheme). When going for a once generated random, the question is whether you change just the time part or both time and random parts when you run out of LOW values and have to generate a new HIGH as a result of that.

Author:

Joachim Van der Auwera

Method Summary

static Uoid

get() get a new Uoid

Methods inherited from class java.lang.Object

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

Method Detail

get

public static Uoid get()

get a new Uoid

Returns:

Uoid (14 character String)