Hi Radim,

I've implemented something similar in my application.  It's actually pretty easy and very performant if you use the distributed executor framework.  Here are some rough guidelines:

• You assuredly will either need a lock/increment/release pattern to get counter values.  However, if you execute a task on the data owner, you're acquiring local locks.
• You will need to look at the cache configuration options for eager locks.
• You may want to consider allocating "chunks" of possible counter values for your clients to use locally.  This will reduce contention on the cache, while guaranteeing uniqueness. 
• (Don't forget about overflow conditions)