Hello

I'm considering situation when I have 2 data centers, In 1s DC I'm having
leader + followers, in 2nd only observers.
In case when my 1st DC goes down, can I somehow reconfigure observers in
2nd DC to leader+followers?

>From what I found out the observers doesn't get transaction logs from the
leader, is there any option I could set to change this behavior?

Thanks.

Hi,

You can probably do this manually now, but I'll let someone else comment on
this.

In 3.5.0 release (with ZK-107 in) you will be able to turn observers into
leader + followers and vise versa by running a simple zookeeper command.
But unfortunately not in the scenario you described.

In your scenario, there are two possible problems that make it unsafe to
reconfigure:
- it possible that the connection between the datacenters is down and not
DC1, and therefore if DC2 independently decides
to start leading we may have "split brain" - both quorums will be
independently processing and committing operations.
- it is very possible that DC1 is down but managed to commit things before
it went
down which are unknown to DC2, and therefore if we activate DC2 we'll loose
committed state.

You will only be able to reconfigure if a quorum (majority) of the old
configuration and a quorum of the new configuration are up and connected to
the current leader.

Alex

On Thu, Feb 7, 2013 at 4:24 AM, Kuba Lekstan <[EMAIL PROTECTED]> wrote:

> Hello
>
> I'm considering situation when I have 2 data centers, In 1s DC I'm having
> leader + followers, in 2nd only observers.
> In case when my 1st DC goes down, can I somehow reconfigure observers in
> 2nd DC to leader+followers?
>
> From what I found out the observers doesn't get transaction logs from the
> leader, is there any option I could set to change this behavior?
>
> Thanks.
>

Yea I can reconfigure cluster manually but because I have only observers in
DC2 it's not possible because I don't have transaction logs in DC2, or
maybe it is?

2013/2/7 Alexander Shraer <[EMAIL PROTECTED]>

> Hi,
>
> You can probably do this manually now, but I'll let someone else comment on
> this.
>
> In 3.5.0 release (with ZK-107 in) you will be able to turn observers into
> leader + followers and vise versa by running a simple zookeeper command.
> But unfortunately not in the scenario you described.
>
> In your scenario, there are two possible problems that make it unsafe to
> reconfigure:
> - it possible that the connection between the datacenters is down and not
> DC1, and therefore if DC2 independently decides
> to start leading we may have "split brain" - both quorums will be
> independently processing and committing operations.
> - it is very possible that DC1 is down but managed to commit things before
> it went
> down which are unknown to DC2, and therefore if we activate DC2 we'll loose
> committed state.
>
> You will only be able to reconfigure if a quorum (majority) of the old
> configuration and a quorum of the new configuration are up and connected to
> the current leader.
>
> Alex
>
> On Thu, Feb 7, 2013 at 4:24 AM, Kuba Lekstan <[EMAIL PROTECTED]> wrote:
>
> > Hello
> >
> > I'm considering situation when I have 2 data centers, In 1s DC I'm having
> > leader + followers, in 2nd only observers.
> > In case when my 1st DC goes down, can I somehow reconfigure observers in
> > 2nd DC to leader+followers?
> >
> > From what I found out the observers doesn't get transaction logs from the
> > leader, is there any option I could set to change this behavior?
> >
> > Thanks.
> >
>