> Facebook likes. Do they need to be atomic? Does it matter if a web request gets a stale "like count" and the number of likes for an item is 12,544 > instead

> Facebook likes. Do they need to be atomic? Does it matter if a web request gets a stale "like count" and the number of likes for an item is 12,544 > instead of the 12,545 on someones page view because the database isn't atomic? > No, it doesn't matter. Actually, it does matter. Atomicity isn't the specific issue there, but rather it's read-after-write consistency guarantees that are important. That is, product-data writes need to be immediately consistent (synchronous) in the context of whichever data store is serving subsequent read requests for the same user/session. Read-after-write consistency is essential for social networks. If you take an action and then click a new link -- or even just refresh the current page -- the effect of your action definitely needs to still be visible. Facebook doesn't use Cassandra, fwiw.