Yes, I checked and you're right. It gets queued at the leader until all previously proposed requests at the leader are committed. But still if the request is only on its way between server 1 and the leader sync won't immediately help, right ?
On Tue, Aug 4, 2015 at 11:39 AM, Camille Fournier <[email protected]> wrote: > I thought that sync forced a flush of the queued events on a quorum member > before completing/got it in the path of events from the leader, so that it > won't return until all of the pending leader events before it have been > seen by this quorum member. Is that not correct? > > On Tue, Aug 4, 2015 at 2:20 PM, Alexander Shraer <[email protected]> > wrote: > > > It seems that since the delete may be in-flight (between server 1 and > > leader, or still being proposed by the leader) > > when the client connects to server 2, doing a sync right a way may not > help > > since the operation hasn't been committed yet. Perhaps the client should > > wait some multiple of synclimit time (3x ?) before invoking the sync to > > allow the delete to commit or disappear for sure. This is all related to > > https://issues.apache.org/jira/browse/ZOOKEEPER-22, which is still open > > unfortunately... > > > > On Tue, Aug 4, 2015 at 10:15 AM, Camille Fournier <[email protected]> > > wrote: > > > > > True, I'm not sure when the xid increments. If that is the case, you > can > > > force a sync before the read of the path, to prevent reading stale > data. > > So > > > that would be the solve for that edge case although it's an expensive > > > solve. > > > > > > C > > > > > > On Tue, Aug 4, 2015 at 12:52 PM, Alexander Shraer <[email protected]> > > > wrote: > > > > > > > Hi Camille, > > > > > > > > if the client received a response for the delete then sure it > shouldn't > > > be > > > > able to connect > > > > to servers that didn't see it. But if it disconnected before seeing > the > > > > response the example seems possible to me. > > > > I haven't checked the code to see when exactly the transaction number > > is > > > > incremented at > > > > the client, so I may be wrong, but suppose for example that > zkserver-1 > > > > crashes before > > > > sending the delete request to the leader. Then, the request is gone > > > > forever. If you don't let the client > > > > connect to another server that hasn't seen the delete, the client > will > > > > never be able to connect. > > > > So it seems quite possible that it connects, then the request is > > executed > > > > (if zkserver-1 hasn't crashed > > > > after all) and the znode disappears. > > > > > > > > Alex > > > > > > > > > > > > On Tue, Aug 4, 2015 at 8:33 AM, Camille Fournier <[email protected] > > > > > > wrote: > > > > > > > > > ZooKeeper provides a session-coherent single system image > guarantee. > > > Any > > > > > request from the same session will see the results of all of its > > > writes, > > > > > regardless of which server it connects to. See: > > > > > > > > > > > > > > > > > > > > http://zookeeper.apache.org/doc/r3.4.6/zookeeperProgrammers.html#ch_zkGuarantees > > > > > > > > > > So, if your session deletes, and the delete is successfully > processed > > > by > > > > > the quorum, you will not see the path that you have deleted no > matter > > > > what > > > > > server your session connects to. I believe in practice that this > > means > > > > that > > > > > the ZK servers that might be behind your session (say server 2 is > > > lagging > > > > > behind a few commits) will refuse to allow your session to connect > to > > > it, > > > > > so that you will not see stale data. > > > > > > > > > > This means that the example Lokesh gave: > > > > > > > > > > "1. Quorum leader has forwarded request to zkserver-2 for "delete > > > /path". > > > > > 2. If your client connects to "zkserver-2" after step 1 is executed > > > (get > > > > > /path). Then your "/path" will not be available. > > > > > 3. If your client connects to "zkserver-2" before step1 is executed > > > (get > > > > > /path) then your "/path" would be available and after some time > your > > > path > > > > > would not be available (after zkserver-2 is synched with the > leader)" > > > > > > > > > > Cannot happen, so long as you are in the same session. > > > > > > > > > > C > > > > > > > > > > On Tue, Aug 4, 2015 at 6:49 AM, Lokesh Shrivastava < > > > > > [email protected]> wrote: > > > > > > > > > > > I think it depends on whether your request reaches zkserver-1 and > > > > whether > > > > > > it is able to send the request to quorum leader. Considering that > > > > "delete > > > > > > /path" request has reached the quorum leader then following may > > > happen > > > > > > > > > > > > 1. Quorum leader has forwarded request to zkserver-2 for "delete > > > > /path". > > > > > > 2. If your client connects to "zkserver-2" after step 1 is > executed > > > > (get > > > > > > /path). Then your "/path" will not be available. > > > > > > 3. If your client connects to "zkserver-2" before step1 is > executed > > > > (get > > > > > > /path) then your "/path" would be available and after some time > > your > > > > path > > > > > > would not be available (after zkserver-2 is synched with the > > leader) > > > > > > > > > > > > Others can correct me if this is not how it works. > > > > > > > > > > > > Thanks. > > > > > > Lokesh > > > > > > > > > > > > On 4 August 2015 at 12:09, [email protected] < > > > > [email protected]> > > > > > > wrote: > > > > > > > > > > > > > Hi, > > > > > > > I'm thinking about a program desgin with libzookeeper, > here > > is > > > > my > > > > > > > doubts: > > > > > > > > > > > > > > 1) first, I connnect to zkserver-1, and there exists the > path > > > > > > "/path". > > > > > > > 2) I sends "delete /path", the request reaches(may not, i > > don't > > > > > know > > > > > > > about that) zkserver-1 and dont't know whether this effected, > and > > > > then > > > > > > lost > > > > > > > connection before response returns. > > > > > > > 3) reconnect the same session to zkserver-2, and I sends > > "get > > > > > > /path". > > > > > > > > > > > > > > which one will the "get /path" return possibly : > > > > > > > 1, "not exists" > > > > > > > 2, "exists" and "always exists" > > > > > > > 3, "exists" and "not exists" afterwards > > > > > > > > > > > > > > my biggist problem is wether the 3) will occur or not, > > thanks! > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > [email protected] > > > > > > > > > > > > > > > > > > > > > > > > > > > >
