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From Alexander Shraer <shra...@gmail.com>
Subject Re: 3-server Zab cluster
Date Mon, 28 Sep 2015 18:26:53 GMT
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) <
i.s.el-sanosi@newcastle.ac.uk> 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

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