Java App Engine - ranked counter

I understand the sharded counter, here: http://code.google.com/appengine/articles/sharding_counters.html The problem is that a simple counter will not work in my application. I am sorting my entities by a particular variable so I am returned not so much a count, but more of a rank. My current method is:

SELECT COUNT(this) FROM Entity.class WHERE value <= ?

Result + 1 is then the rank of the parameter in relation to the value variable in the persistent Entity objects. The limitation of this is the highe开发者_开发技巧st rank being returned is 1001 because count() can give a maximum of 1000. The reason I cannot store the rank on the Entity object is that the ranks are updated very often, and re-setting this rank variable would be much too costly.

Any ideas on the best way to accomplish this?

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You might want to consider adapting something like the google-appengine-ranklist project to Java. It builds a tree of 'ranker' nodes, such that the nth ranked item can be found in O(log n) time, and updates are likewise O(log n).

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I would create a class with a static variable to hold the current count value. E.g., you could create something like the following:

public class Counter {
    private static int counter = -1;

    public static synchronized int getNextValue() {
    if (counter == -1) {                
            counter = // Insert code that retrieves the highest value from Google datastore
    }
    counter++;
    return counter;
    }
}