Yes Alex, in my post I mentioned that this (small) optimization can only work with 3-servers cluster.
Who could confirm the optimization can work? Ibrahim -----Original Message----- From: Alexander Shraer [mailto:[email protected]] Sent: Tuesday, September 29, 2015 12:11 ص To: [email protected] Subject: Re: 3-server Zab cluster I'm not 100% sure whether operations that were pending on the leader are sent out during sync when this leader looses quorum and re-elected. If so, then maybe you're right. But in any case, this would not work for 5 or more servers... On Mon, Sep 28, 2015 at 3:51 PM, Ibrahim El-sanosi (PGR) < [email protected]> wrote: > Thank you Alex for replaying. > > When you said " the leader gets re-elected and the operation is > truncated from logs at other servers". I though the new leader will > sync the its logs with other followers (synchronization phase), > resulting in the operation will commit by new quorum. Let me make the > scenarios as steps: > > 1. leader (L) sends a proposal p with zxid =10 to F1 and F2. > 2. F1 logs, sends an ACK, commits, replays to clients and crashes. F2 > crashes before receiving P10. L has not received any ACKs > > Possible solution (1) > The leader will move to LOOKING phase as there is no quorum supporting > its leadership. Now Assume F2 wakes up. F2 forms a quorum with the L > (pervious leader), L becomes new leader again as it has latest zxid (10) in > its log. > L syncs its state with F2, as a result L, F1 (before crashing) and F2 > commit P10. Is that correct? > > Possible solution (2) > The leader will move to LOOKING phase as there is no quorum supporting > its leadership. Now Assume F1 (with Zxid =10 committed) wakes up. I > am not sure who should be a leader (F1 with Zxid =10 committed or L > (pervious > leader) with Zxid = 10 logged), I think F1 become a new leader as it > has Zxid = 10 committed. F1 forms a quorum with the L (pervious > leader), F1 becomes new leader as it has latest zxid (10) . L (new > leader) syncs its state with L (pervious leader now become a > follower), as a result Zxid10 commits by new quorum. Is that correct? > > What do you think? > > Ibrahim > > > > > > -----Original Message----- > From: Alexander Shraer [mailto:[email protected]] > Sent: Monday, September 28, 2015 07:27 م > To: [email protected] > Cc: [email protected] > Subject: Re: 3-server Zab cluster > > Committing locally when sending an ACK at a server would lead to loss > of consistency - it is possible that this is the only server that > acks, e.g., this server is temporarily disconnected from the leader, > the leader gets re-elected and the operation is truncated from logs at > other servers. Its ok to ACK it but its not ok to commit since this > exposes this to users as a committed operation that they can see. > > On Mon, Sep 28, 2015 at 4:19 AM, Ibrahim El-sanosi (PGR) < > [email protected]> wrote: > > > In Zab, assume we have a cluster consists of 3-servers. To deliver a > > write request, it must run 3 communication steps proposal, > > acknowledgement and commit. > > As Zab uses reliable FIFO, it is possible to remove commit round. As > > soon as a follower receives a proposal, it logs, sends an ACK and > > commits locally. Upon receiving ACK from any follower, leader > > commits a proposal locally, no COMMIT message need to be sent to > > followers. In this case, all servers commit a proposal in two > > round-trips, resulting in reducing latency particularly in followers. > > > > Note that this optimization can only work in 3-servers cluster > > (follower reaches a majority as soon as it acks). > > Does anyone see any problems with such (small) optimization? > > Ibrahim > > >
