CareerCup

Was it to test that all the servers are in Sync.
ex: delete data from one server n check whether second is updated or not...

There will be many clarification questions to test this.

Assuming that we are testing read and write to a single server first, Using two or more threads make the following checks:
1. Try two threads to obtain locks at the same time and check that lock is granted to just one
2. Once the resource is locked, make sure other threads cannot obtain lock.
3. Try to change the resource when it is locked by other thread, it should throw an error.
4. Once unlocked, other threads should be able to get the lock i.e. the lock is released properly

Then we need to test syncing issues if data is duplicated(the outcome of this depends on what is decided during design phase) and test cases are the same as above except performed on different servers.

Apart from functional testing, non functional testing like stress testing, load balancing testing, security testing etc should be carried out too.

For optimistic concurrency

h t t p : //msdn.microsoft.com/en-us/library/aa0416cz%28v=vs.71%29.aspx

1) what about the private datastore?
how to test it?

even before that, what is to be tested?