Dynamic servers addition and persistent storage.

[Maxime Caron <ma...@gmail.com>]

I was investigating scalaris (http://code.google.com/p/scalaris/) but found
it does not support a persistent storage.
In their faq they say it cant be done because they assume that a majority of
the replicas of an item is always available.
If this precondition is violated, a majority of the nodes with replicas of
an item x is not available, the item cannot be changed. It is lost.
Persistent storage cannot help directly.
So i would like to understand if zookeeper work the same way or their is a
recovery model for  when the majority of the goes down and then back up.


Unlike zookeeper with scalaris servers can be added or removed on the fly
without any service downtime.
>From what i can understand in zookeeper you need to have a fixed server list
that every body share.
Would it be possible to add on the fly server addition and removal to
zookeeper?

Re: Dynamic servers addition and persistent storage.

[Ted Dunning <te...@gmail.com>]

In the meantime, the one-at-a-time process is remarkably usable.  We use it
routinely for exactly the purpose proposed by Gustavo.

Scaling by magic will be even better, but in the meantime people shouldn't
be afraid to modify their clusters.

On Wed, Jul 1, 2009 at 10:34 AM, Gustavo Niemeyer <gu...@niemeyer.net>wrote:

> ... I guess the same basic idea that everyone else has for it: bringing
> additional systems up and down dynamically to control scalability and
> reliability vs. cost of having many machines running at once.  I
> understand it is possible to restart the servers one at a time to
> change the server list without a full stop, but if we end up bundling
> this in some open source framework for people to use in the wild, the
> least manual interaction and procedural maintenance the best.  Having
> ZooKeeper clients learning about the server list dynamically will help
> a lot in this scenario too.
>
>

Re: Dynamic servers addition and persistent storage.

[Gustavo Niemeyer <gu...@niemeyer.net>]

Hi again Henry,

> I hope to have a patch for both fairly soon. I should at least get ZK-368 to
> a workable position this week, and ZK-107 will hopefully not be an enormous
> amount of work on top of that. However, there doubtless be some slack time
> for picking up bugs etc. before it gets committed as it will be a reasonably
> sized patch.

That's great news.  I've added myself as a watcher on both tickets.

> Out of interest, what's your application for this?

I guess the same basic idea that everyone else has for it: bringing
additional systems up and down dynamically to control scalability and
reliability vs. cost of having many machines running at once.  I
understand it is possible to restart the servers one at a time to
change the server list without a full stop, but if we end up bundling
this in some open source framework for people to use in the wild, the
least manual interaction and procedural maintenance the best.  Having
ZooKeeper clients learning about the server list dynamically will help
a lot in this scenario too.

-- 
Gustavo Niemeyer
http://niemeyer.net

Re: Dynamic servers addition and persistent storage.

[Henry Robinson <he...@cloudera.com>]

Hi Gustavo -

I hope to have a patch for both fairly soon. I should at least get ZK-368 to
a workable position this week, and ZK-107 will hopefully not be an enormous
amount of work on top of that. However, there doubtless be some slack time
for picking up bugs etc. before it gets committed as it will be a reasonably
sized patch.

Out of interest, what's your application for this?

Henry

On Wed, Jul 1, 2009 at 4:01 PM, Gustavo Niemeyer <gu...@niemeyer.net>wrote:

> Hey Henry,
>
> > We (and myself in particular) are working on dynamic cluster membership,
> see
> > https://issues.apache.org/jira/browse/ZOOKEEPER-107 and the related
> > https://issues.apache.org/jira/browse/ZOOKEEPER-368.
>
> That's fantastic news!  How do you feel this is going so far?  We
> might have an application for this pretty soon.
>
> --
> Gustavo Niemeyer
> http://niemeyer.net
>

Re: Dynamic servers addition and persistent storage.

[Gustavo Niemeyer <gu...@niemeyer.net>]

Hey Henry,

> We (and myself in particular) are working on dynamic cluster membership, see
> https://issues.apache.org/jira/browse/ZOOKEEPER-107 and the related
> https://issues.apache.org/jira/browse/ZOOKEEPER-368.

That's fantastic news!  How do you feel this is going so far?  We
might have an application for this pretty soon.

-- 
Gustavo Niemeyer
http://niemeyer.net

Re: Dynamic servers addition and persistent storage.

[Henry Robinson <he...@cloudera.com>]

Hi Maxime -

When a quorum of ZooKeeper servers have failed, the service stops being
available - you cannot write or read to any item. Once a quorum returns to
operation, the ensemble recovers automatically and continues where it left
off. There is the same requirement that a quorum of servers must see every
write.

We (and myself in particular) are working on dynamic cluster membership, see
https://issues.apache.org/jira/browse/ZOOKEEPER-107 and the related
https://issues.apache.org/jira/browse/ZOOKEEPER-368.

Henry

On Wed, Jul 1, 2009 at 3:07 PM, Maxime Caron <ma...@gmail.com> wrote:

> I was investigating scalaris (http://code.google.com/p/scalaris/) but
> found
> it does not support a persistent storage.
> In their faq they say it cant be done because they assume that a majority
> of
> the replicas of an item is always available.
> If this precondition is violated, a majority of the nodes with replicas of
> an item x is not available, the item cannot be changed. It is lost.
> Persistent storage cannot help directly.
> So i would like to understand if zookeeper work the same way or their is a
> recovery model for  when the majority of the goes down and then back up.
>
>
> Unlike zookeeper with scalaris servers can be added or removed on the fly
> without any service downtime.
> From what i can understand in zookeeper you need to have a fixed server
> list
> that every body share.
> Would it be possible to add on the fly server addition and removal to
> zookeeper?
>