simple php mysql script

This is for connection slot assignment for dozens of concurrent connections. there is a column called conSlot which never exceeds 6. i need the script to search all available conSlots which are less than 4 and order them in descending order. the topmost conSlot will be assigned in a first in first out basis. I assume this will need to be transactioanlly locked. Concurrency is key here and i have read that locking with FOR UPDATE is not concurrent effecient. once assigned conSlot will be incrememented this has to be mysql cluster friendly. Im currently using the following start transaction Select * from [login to view URL] where conSlot < 6 Order by conSlot DESC limit 1 FOR UPDATE *pseudo - update conslot=conslot+1 commit the probplem with this is that if one of the conslots is 3 and the rest are zero, slots will be assigned randomly to the empty slots and not 3, increment 3. The job is to write a script which completes the task above in the most reliable manner with over 50 concurrent sessions in mind and mysql cluster friendly.