I am not sure how to handle this in a BigTable datastore. 

Imagine the following example (just to explain the concept. The example does not match my actual data model):

* I have a Counter entity that keeps track of the number of Transactions in my dataStore. Let's say the current 'count' is 100.
* Now two web requests read this value at the same time.
* Both web requests add a new Transaction
* And finally both update the counter (to 101).

The counter value is now inaccurate. It should be 102. 

Any suggestions on how to handle this situation? Can I 'lock' the counter to ensure that the second web request doesn't even **read** it until the first web request completes?