-Re: leader election length
Botond Hejj 2011-12-14, 14:44
Thanks Flavio for looking into this.
If the loadData can be removed from lead() that could be improve
leaderElection a lot on slow disk with high amount of data.
On Tue, Dec 13, 2011 at 14:02, Flavio Junqueira <[EMAIL PROTECTED]> wrote:
> Hi Botond, I'm under the impression that the leader only loads the database
> twice when it is bootstrapping. In QuorumPeer.start(), we call
> loadDataBase(). The second time corresponds to the call to zk.loadData in
> Leader.lead(). There could be a third time if zkDb.isInitialized() is false
> thought the call path lookForLeader()-> getInitLastLoggedZxid() ->
> getLastLoggedZxid(). I don't see how it could be false, though.
> After we get into the main QuorumPeer loop (in run()), we only call
> loadDataBase() through lookForLeader() upon calling getInitLastLoggedZxid()
> -> getLastLoggedZxid(). loadDataBase() is only executed if
> zkDb.isInitialized() is false, and I actually don't see a case in which it
> would be true once for the call coming from lookForLeader().
> The part I can't remember is why we need to call loadData() in Leader.lead()
> in the case that the data tree has been initialized already. If we are
> somehow skipping it when it has been initialized already, then I missed it.
> I'll keep looking into it...
> On Dec 13, 2011, at 11:55 AM, Botond Hejj wrote:
>> Thanks Camille,
>> Sorry, my assumption was wrong. I've made now a test and indeed the
>> session doesn't expire in this case.
>> This means than the only problem is that the service is down for an
>> increasing amount of time if snapshot is increasing. We use SAN to
>> store the snapshot and reading it back twice during leader election
>> can be a little slow (~30 secs). It is not bad at all but still have
>> anybody tried making the leader election faster in this case?
>> On Mon, Dec 12, 2011 at 18:35, Camille Fournier <[EMAIL PROTECTED]>
>>> Existing sessions will not expire from the server side during election.
>>> Your client code may choose to close them on its end if you sit in a
>>> DISCONNECTED state for too long, but nothing should be expiring the
>>> sessions while quorum is not available.
>>> On Mon, Dec 12, 2011 at 12:06 PM, Botond Hejj
>>> <[EMAIL PROTECTED]
>>>> Hi ZooKeeper users,
>>>> I am playing currently with zookeeper and testing what happens if the
>>>> leader of an ensemble goes down.
>>>> I know that during the leader election zookeeper server won't reply to
>>>> any requests and if leader election takes a long time than existing
>>>> sessions might expire.
>>>> What I see in my tests that each server reads the last snapshot file
>>>> to get last zxid for leader election and when the leader is elected
>>>> than the leader reads the snapshot again before it syncs the
>>>> This means that the more data we store in zookeeper the longer it
>>>> takes to elect a new leader. This is also means as load of the
>>>> ensemble increases clients need bigger session timeout to "survive"
>>>> the loss of the leader.
>>>> Is it possible to do anything about this and have a fast leader
>>>> election even if the snapshot is big?
>>>> Botond Hejj
>> Botond Hejj
>> Morgan Stanley | Technology
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