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From Kanak Biscuitwala <kana...@hotmail.com>
Subject RE: Questions about custom helix rebalancer/controller/agent
Date Fri, 01 Aug 2014 00:45:58 GMT
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.
Kanak

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);
Thread.sleep(30minutes);
So, the above happens inside a while loop thread and this is the only place where we do the
rebalancing ?

ThanksVarun

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
interface:

https://github.com/apache/helix/blob/helix-0.6.x/helix-core/src/main/java/org/apache/helix/controller/rebalancer/Rebalancer.java

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:

https://github.com/apache/helix/blob/helix-0.6.x/helix-core/src/main/java/org/apache/helix/controller/rebalancer/internal/MappingCalculator.java

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
as role ADMINISTRATOR instead of CONTROLLER.

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
logic.

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:
http://helix.apache.org/0.6.3-docs/tutorial_spectator.html

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.
Kanak
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

Hi,


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 ?


Thanks
 		 	   		  

 		 	   		   		 	   		  
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