[sheepdog] [PATCH v4] sheep: remove master node

[Liu Yuan]

On Thu, Jul 25, 2013 at 01:14:11PM +0900, MORITA Kazutaka wrote:
> From: MORITA Kazutaka <morita.kazutaka at lab.ntt.co.jp>
> 
> The current procedure to handle sheep join is as follows.
> 
>  1. The joining node sends a join request.
>  2. The master node accepts the request.
>  3. All the nodes update cluster members.
> 
> This procedure has some problems:
> 
>  - The master election is too complex to maintain.
>    It is very difficult to make sure that the implementation is
>    correct.
> 
>  - The master node can fail while it is accepting the joining node.
>    The newly elected master has to take over the process, but it's
>    usually difficult to implement because we have to know what the
>    previous master did and what it did not before its failure.
> 
> This patch changes the sheep join procedure to the following.
> 
>  1. The joining node sends a join request.
>  2. Some of the existing nodes accept the request.
>  3. All the nodes update cluster members.
> 
> It is allowed for the multiple nodes to call sd_join_handler() against
> the same join request, but at least one node must have to do it.  With
> this change, we can eliminate a master, and node failure while
> accepting node join is also allowed.
> 
> Removing a master from zookeeper is not easy since it doesn't expect
> that multiple nodes send EVENT_ACCEPT.  I'll leave this for another
> day.

Applied, thanks

Yuan