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From Kanak Biscuitwala <kbiscuitw...@linkedin.com>
Subject Re: Getting auto_rebalance right
Date Tue, 15 Oct 2013 17:39:00 GMT
Hi Matthieu,

Please change line 39 in ClusterConfigInit to:


Basically, the leader counts as a replica, so if you want a replica in addition to the leader,
you need to specify 2 for the replica count.

There is a bug where when there are 3 nodes, I see partition 0 has 2 in REPLICA state even
though one of them should be dropped. I'll keep investigating that.


From: Matthieu Morel <mmorel@apache.org<mailto:mmorel@apache.org>>
Reply-To: "user@helix.incubator.apache.org<mailto:user@helix.incubator.apache.org>"
Date: Tuesday, October 15, 2013 8:06 AM
To: "user@helix.incubator.apache.org<mailto:user@helix.incubator.apache.org>" <user@helix.incubator.apache.org<mailto:user@helix.incubator.apache.org>>
Subject: Re: Getting auto_rebalance right

Hi Kishore,

On Oct 15, 2013, at 16:48 , kishore g <g.kishore@gmail.com<mailto:g.kishore@gmail.com>>

Hi Matthieu,

I think the code avoids placing more than one replica of a partition on the same node. So
If you have only 1 node, it will not create only LEADERS. We can make add a configuration
and allow this to happen.

Actually, preventing leader and replica for a partition to be on the same node makes sense
: such a placement defeats the purpose of the replica.

But I do see something weird when you add a node, only 1 additional replica gets created,
that does not make sense. I will take a look at that.

Yes, with 3 nodes and 3 partitions we should expect 3 leaders and 3 replicas. An additional
requirement, related to the above comment, would be that leaders and replica are never colocated.
Should I open a jira for that?

Let me know if you need more feedback.



Kishore G

On Tue, Oct 15, 2013 at 1:31 AM, Matthieu Morel <mmorel@apache.org<mailto:mmorel@apache.org>>
Thanks for your prompt answers!

I used the latest version from the master branch and applied the code changes suggested by

The good news are that:
- the update was trivial - at least for the small code example I provided.
- I always get 3 leaders states for the 3 partitions

The bad news are that:
- I either don't get enough replica (I want 1 replica for each partition, and initially I
only have replica for 2 partitions)
- or simply I get no replica at all (after removing 1 node from the cluster, I have 3 leaders,
0 replica)

I updated my simple example https://github.com/matthieumorel/helix-balancing so you can reproduce
that behavior.

// with only 1 node, I have 3 leaders, 0 replica :

Starting instance Node:myhost:10000
Assigning MY_RESOURCE_1 to Node:myhost:10000
Assigning MY_RESOURCE_0 to Node:myhost:10000
Assigning MY_RESOURCE_2 to Node:myhost:10000
OFFLINE -> REPLICA (Node:myhost:10000, MY_RESOURCE_2)
OFFLINE -> REPLICA (Node:myhost:10000, MY_RESOURCE_1)
OFFLINE -> REPLICA (Node:myhost:10000, MY_RESOURCE_0)
REPLICA -> LEADER (Node:myhost:10000, MY_RESOURCE_0)
REPLICA -> LEADER (Node:myhost:10000, MY_RESOURCE_1)
REPLICA -> LEADER (Node:myhost:10000, MY_RESOURCE_2)

// adding 1 node adds a replica:

Starting instance Node:myhost:10001
Assigning MY_RESOURCE_1 to Node:myhost:10001
OFFLINE -> REPLICA (Node:myhost:10001, MY_RESOURCE_1)

// adding another node adds a new replica:

Starting instance Node:myhost:10002
Assigning MY_RESOURCE_0 to Node:myhost:10002
OFFLINE -> REPLICA (Node:myhost:10002, MY_RESOURCE_0)

// removing a node rebalances things but we end up with 3 leaders, 0 replica

Stopping instance Node:myhost:10000
Assigning MY_RESOURCE_2 to Node:myhost:10002
REPLICA -> LEADER (Node:myhost:10002, MY_RESOURCE_0)
OFFLINE -> REPLICA (Node:myhost:10002, MY_RESOURCE_2)
REPLICA -> LEADER (Node:myhost:10002, MY_RESOURCE_2)
REPLICA -> LEADER (Node:myhost:10001, MY_RESOURCE_1)

I would like to get 1 leader and 1 replica for each partition, regardless of the number of
nodes. Is that possible?



On Oct 15, 2013, at 02:30 , Kanak Biscuitwala <kanak.b@hotmail.com<mailto:kanak.b@hotmail.com>>

Hi Matthieu,

I have just pushed a patch to the master branch (i.e. trunk) that should fix the issue. Please
let me know if the problem persists.


From: zzhang@linkedin.com<mailto:zzhang@linkedin.com>
To: user@helix.incubator.apache.org<mailto:user@helix.incubator.apache.org>
Subject: Re: Getting auto_rebalance right
Date: Mon, 14 Oct 2013 21:32:41 +0000

Hi Matthieu, this is a known bug in 0.6.1 release. We have fixed it in
trunk. If you are building from trunk, change ClusterConfigInit#init()





It should work. We are planing to make 0.6.2 release with a few fixes including this one.



From: Matthieu Morel <mmorel@apache.org<mailto:mmorel@apache.org><mailto:mmorel@apache.org>>
Date: Monday, October 14, 2013 12:09 PM
Subject: Getting auto_rebalance right


I'm trying to use the auto-rebalance mode in Helix.

The use case is the following (standard leader-standby scenario, a bit
like the rsync example in the helix codebase):
- the dataspace is partitioned
- for a given partition, we have
- a leader that is responsible for writing and serving data, logging
operations into a journal
- a replica that fetches updates from a journal and applies them
locally but it does not serve data
Upon failure, the replica becomes leader, applies pending updates and
can write and serve data. Ideally we also get a new replica assigned.

We'd like to use the auto_rebalance mode in Helix so that partitions
are automatically assigned and re-assigned, and so that leaders are
automatically elected.

Unfortunately, I can't really get the balancing right. I might be doing
something wrong, so I uploaded an example here
: https://github.com/matthieumorel/helix-balancing

In this application I would like to get exactly 1 leader and 1 replica
for each of the partitions

In this example we don't reach that result, and when removing a node,
we even get to a situation where there is no leader for a given

Do I have wrong expectations? Is there something wrong with the code,
is it something with helix?



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