I built a social android application in which users can see other users around them by gps location. at the beginning thing went well as i had low number of users, But now that I have increasing number of users (about 1500 +100 every day) I revealed a major problem in my design.

In my Google App Engine servlet I have static HashMap that holding all the users profiles objects, currenty 1500 and this number will increase as more users register.

Why I'm doing it

Every user that requesting for the users around him compares his gps with other users and check if they are in his 10km radius, this happens every 5 min on average. That is why I can't get the users from db every time because GAE read/write operation quota will tare me apart.

The problem with this desgin is

As the number of users increased the Hashmap turns to null every 4-6 hours, I thing that this time is getting shorten but I'm not sure. I'm fixing this by reloading the users from the db every time I detect that it became null, But this causes DOS to my users for 30 sec, So I'm looking for better solution.
I'm guessing that it happens because the size of the hashmap, Am I right?

I have been advised to use spatial database, but that mean that I can't work with GAE any more and that mean that I need to build my big server all over again and lose my existing DB.

Is there something I can do with the existing tools?

Thanks.