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Flavio Paiva Junqueira commented on ZOOKEEPER-30:
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Andrew, It would be really good for us to see a preliminary patch, just to have
an idea of how you have implemented a few things.
Although perhaps not exactly appropriate, Ben and I had a discussion offline
about having or not propose returning a zxid. Currently, only the leader
proposes, since broadcast messages are requests transformed into idempotent
transactions. If we follow this model, then returning a zxid is not a problem
because the leader is the one generating a zxid and a propose call does not
block in this case.
I understand, however, that you are interested in having Zab in such a way that
any process in the ensemble can propose. If any process can propose, then it
might not make sense to have a call to propose returning a zxid. The problem of
not returning a zxid is that currently the broadcast layer and the application
layer are tightly coupled and separating them creates a potential problem for
generating snapshots that we use to recover or bring a new follower up to
speed. In a little more detail, if we separate the state from the atomic
broadcast and return no zxid, then the service layer on top of zab will have no
such a notion of transaction identifiers. We currently use such identifiers to
generate and guarantee a consistent state transfer.
> Hooks for atomic broadcast protocol
> -----------------------------------
>
> Key: ZOOKEEPER-30
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-30
> Project: Zookeeper
> Issue Type: New Feature
> Components: quorum
> Reporter: Patrick Hunt
> Assignee: Mahadev konar
>
> Moved from SourceForge to Apache.
> http://sourceforge.net/tracker/index.php?func=detail&aid=1938788&group_id=209147&atid=1008547
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