Generate Primary key without using Database

I came across a question recently that was for "Generating primary key in a clustered environment of 5 App-Servers - [OAS Version 10] without using database".

Usually we generate PK by a DB sequence, or storing the values in a database table and then using a SP to generate the new PK value...However current requirement is to generate primary key for my application without referencing the database using JDK 1.4.

Need expert's help to arrive on better ways to handle this.

Thanks,