Hi, (1) Zab is an atomic broadcast protocol, it satisfies strong ordering properties. Check out the paper: http://www.stanford.edu/class/cs347/reading/zab.pdf
Its used to propagate updates in the same order to all the ZooKeeper servers. The provided consistency in ZooKeeper is basically sequential consistency + real-time-order on writes. This is strong, although not linearizable (its possible that you read from a server that has stale data). If you use the sync call before a read, ZooKeeper provides ilnearizability for sync+read and write operations (this is true with certain timing assumption made in ZooKeeper for efficiency). (2) No, only a suffix of uncommitted operations can disappear. Operations acked by a quorum never dissappear. (3) For read operations the server responds immediately using local state. For writes it responds after a quorum acked the update. Alex On Mon, Oct 21, 2013 at 1:47 AM, 聂安 <[email protected]> wrote: > hi all, > > About zk, I got some questions bothered me for one week, which can be > described as follows: > > (1) Does zab guarantee a strong consistency? > > (2) Could those committed proposals on new-elected-followers be deleted when > initially synced? > > (3) When does the server in zk respond to the client? After commiting the > proposal? > > thank you！ > > > > > Regards, > > An.Nie
