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From Kanak Biscuitwala <kana...@hotmail.com>
Subject RE: Questions about custom helix rebalancer/controller/agent
Date Fri, 01 Aug 2014 17:18:02 GMT
a) By default, there is one znode per resource, which as you know is a grouping of partitions.
The biggest limitation is that ZK has a 1MB limit on znode sizing. To get around this, Helix
has the concept of bucketizing, where in your ideal state, you can set a bucket size, which
will effectively create that many znodes to fully represent all your partitions. I believe
that you can have ~2k partitions before you start needing to bucketize.
300k may cause you separate issues, and you may want to consider doing things like enabling
batch message mode in your ideal state so that each message we send to an instance contains
transitions for all partitions hosted on that instance, rather than creating a znode per partition
state change. However, in theory (we've never played with this many in practice), Helix should
be able to function correctly with that many partitions.
b) Yes, if you have a hard limit of 1 master per partition, Helix will transition the first
node to OFFLINE before sending the MASTER transition to the new master.

Date: Fri, 1 Aug 2014 10:09:24 -0700
Subject: Re: Questions about custom helix rebalancer/controller/agent
From: varun@pinterest.com
To: user@helix.apache.org

Sounds fine to me. I can work without the FINALIZE notification for now, but I hope its going
to come out soon. A few more questions:
a) How well does Helix scale with partitions - is each partition a separate znode inside helix
? If I have 300K partitions in Helix would that be an issue ?
b) If a partition which was assigned as a master on node1 is now assigned as a master on node2,
will node1 get a callback execution for transition from MASTER-->OFFLINE

On Thu, Jul 31, 2014 at 11:18 PM, Kanak Biscuitwala <kanak.b@hotmail.com> wrote:

s/run/start/g -- sorry about that, fixed in javadocs for future releases

You may need to register for a notification type; I believe HelixCustomCodeRunner complains
if you don't. However, you can simply ignore that notification type, and just check for INIT
and FINALIZE notification types in your callback to to track whether or not you're the leader.
On INIT, you start your 30 minute timer, and on FINALIZE you stop it. You may need to wait
for us to make a 0.6.4 release (we will likely do this soon) to get the FINALIZE notification.

Here is an example of a custom code runner usage:Registration: https://github.com/kishoreg/fullmatix/blob/master/mysql-cluster/src/main/java/org/apache/fullmatix/mysql/MySQLAgent.java
Callback: https://github.com/kishoreg/fullmatix/blob/master/mysql-cluster/src/main/java/org/apache/fullmatix/mysql/MasterSlaveRebalancer.java

Regarding setting up the Helix controller, you actually don't need to instantiate a GenericHelixController.
If you create a HelixManager with InstanceType.CONTROLLER, then ZKHelixManager automatically
creates a GenericHelixController and sets it up with leader election. We really should update
the documentation to clarify that.

Date: Thu, 31 Jul 2014 23:00:13 -0700
Subject: Re: Questions about custom helix rebalancer/controller/agent
From: varun@pinterest.com

To: user@helix.apache.org

Thanks for the suggestions..
Seems like the HelixCustomCodeRunner could do it. However, it seems like the CustomCodeRunner
only provides hooks for plugging into notifications. The documentation example in the above
link suggests a run() method, which does not seem to exist.

However, this maybe sufficient for my case. I essentially hook in an empty CustomCodeRunner
into my helix manager. Then I can instantiate my own thread which would run above snippet
and keep writing ideal states every 30 minutes. I guess I would still need to attach the GenericHelixController
with the following code snippet to take action whenever the ideal state changes ??

GenericHelixController controller = new GenericHelixController();

On Thu, Jul 31, 2014 at 6:01 PM, kishore g <g.kishore@gmail.com> wrote:

List resourceList = helixAdmin.getResourceList();
for each resource:   Compute target ideal state   helixAdmin.setIdealState(resource, targetIdealState);

This can work right. This code can be as part of CustomCodeRunner. http://helix.apache.org/javadocs/0.6.3/reference/org/apache/helix/participant/HelixCustomCodeRunner.html.
You can say you are interested in notifications but can ignore that.

Kishore G

On Thu, Jul 31, 2014 at 5:45 PM, Kanak Biscuitwala <kanak.b@hotmail.com> wrote:

i.e. helixAdmin.enableCluster(clusterName, false);

From: kanak.b@hotmail.com

To: user@helix.apache.org
Subject: RE: Questions about custom helix rebalancer/controller/agent
Date: Thu, 31 Jul 2014 17:44:40 -0700

Unfortunately HelixAdmin#rebalance is a misnomer, and it is a function of all the configured
instances and not the live instances. The closest you can get to that is to use the third
option I listed related to CUSTOMIZED mode, where you write the mappings yourself based on
what is live.

Another thing you could do is pause the cluster controller and unpause it for a period every
30 minutes. That will essentially enforce that the controller will not send transitions (or
do anything else, really) during the time it is paused. This sounds a little like a hack to
me, but it may do what you want.


Date: Thu, 31 Jul 2014 17:39:40 -0700
Subject: Re: Questions about custom helix rebalancer/controller/agent
From: varun@pinterest.com

To: user@helix.apache.org

Thanks Kanak, for your detailed response and this is really very helpful. I was wondering
if its possible for me do something like the following:

List resourceList = helixAdmin.getResourceList();
for each resource:   Compute target ideal state   helixAdmin.rebalance(resource);
So, the above happens inside a while loop thread and this is the only place where we do the
rebalancing ?


On Thu, Jul 31, 2014 at 5:25 PM, Kanak Biscuitwala <kanak.b@hotmail.com> wrote:

Hi Varun,
Sorry for the delay.

1 and 3) There are a number of ways to do this, with various tradeoffs.
- You can write a user-defined rebalancer. In helix 0.6.x, it involves implementing the following


Essentially what it does is given an existing ideal state, compute a new ideal state. For
0.6.x, this will read the preference lists in the output ideal state and compute a state mapping
based on them. If you need more control, you can also implement:


which will allow you to create a mapping from partition to map of participant and state. In
0.7.x, we consolidated these into a single method.
Here is a tutorial on the user-defined rebalancer: http://helix.apache.org/0.6.3-docs/tutorial_user_def_rebalancer.html

Now, running this every 30 minutes is tricky because by default the controller responds to
all cluster events (and really it needs to because it aggregates all participant current states
into the external view -- unless you don't care about that).

- Combined with the user-defined rebalancer (or not), you can have a GenericHelixController
that doesn't listen on any events, but calls startRebalancingTimer(), into which you can pass
30 minutes. The problem with this is that the instructions at http://helix.apache.org/0.6.3-docs/tutorial_controller.html
won't work as described because of a known issue. The workaround is to connect HelixManager

However, if you connect as ADMINISTRATOR, you have to set up leader election yourself (assuming
you want a fault-tolerant controller). See https://github.com/apache/helix/blob/helix-0.6.x/helix-core/src/main/java/org/apache/helix/manager/zk/DistributedLeaderElection.java
for a controller change listener that can do leader election, but your version will have to
be different, as you actually don't want to add listeners, but rather set up a timer.

This also gives you the benefit of plugging in your own logic into the controller pipeline.
See https://github.com/apache/helix/blob/helix-0.6.x/helix-core/src/main/java/org/apache/helix/controller/GenericHelixController.java
createDefaultRegistry() for how to create an appropriate PipelineRegistry.

- You can take a completely different approach and put your ideal state in CUSTOMIZED rebalance
mode. Then you can have a meta-resource where one participant is a leader and the others are
followers (you can create an ideal state in SEMI_AUTO mode, where the replica count and the
replica count and preference list of resourceName_0 is "ANY_LIVEINSTANCE". When one participant
is told to become leader, you can set a timer for 30 minutes and update and write the map
fields of the ideal state accordingly.

2) I'm not sure I understand the question. If you're in the JVM, you simply need to connect
as a PARTICIPANT for your callbacks, but that can just be something you do at the beginning
of your node startup. The rest of your code is more or less governed by your transitions,
but if there are things you need to do on the side, there is nothing in Helix preventing you
from doing so. See http://helix.apache.org/0.6.3-docs/tutorial_participant.html for participant

4) The current state is per-instance and is literally called CurrentState. For a given participant,
you can query a current state by doing something like:
HelixDataAccessor accessor = helixManager.getHelixDataAccessor();

CurrentState currentState = accessor.getProperty(accessor.keyBuilder().currentState(instanceName,
sessionId, resourceName);
If you implement a user-defined rebalancer as above, we automatically aggregate all these
current states into a CurrentStateOutput object.

5) You can use a Helix spectator:

This basically gives you a live-updating routing table for the mappings of the Helix-managed
resource. However, it requires the external view to be up to date, going back to my other
point of perhaps separating the concept of changing mappings every 30 minutes from the frequency
at which the controller runs.

Hopefully this helps.
Date: Thu, 31 Jul 2014 12:13:27 -0700
Subject: Questions about custom helix rebalancer/controller/agent
From: varun@pinterest.com

To: user@helix.apache.org


I am trying to write a customized rebalancing algorithm. I would like to run the rebalancer
every 30 minutes inside a single thread. I would also like to completely disable Helix triggering
the rebalancer.

I have a few questions:1) What's the best way to run the custom controller ? Can I simply
instantiate a ZKHelixAdmin object and then keep running my rebalancer inside a thread or do
I need to do something more.

Apart from rebalancing, I want to do other things inside the the controller, so it would be
nice if I could simply fire up the controller through code. I could not find this in the documentation.

2) Same question for the Helix agent. My Helix Agent is a JVM process which does other things
apart from exposing the callbacks for state transitions. Is there a code sample for the same

3) How do I disable Helix triggered rebalancing once I am able to run the custom controller

4) During my custom rebalance run, how I can get the current cluster state - is it through
ClusterDataCache.getIdealState() ?

5) For clients talking to the cluster, does helix provide an easy abstraction to find the
partition distribution for a helix resource ?




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