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From Varun Sharma <va...@pinterest.com>
Subject Re: Questions about custom helix rebalancer/controller/agent
Date Fri, 01 Aug 2014 00:39:40 GMT
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


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>

>  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
> 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
> 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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