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From Varun Sharma <va...@pinterest.com>
Subject Re: Questions about custom helix rebalancer/controller/agent
Date Fri, 01 Aug 2014 00:41:38 GMT
The reason I want to do it this way, is because for my case, the source of
truth is not Helix but rather another distributed system. That system
dictates the ideal state - so we are essentially getting a snapshot of the
ideal state in accordance with that distributed system and having helix try
to converge its participants to it.

So, we don't want Helix to do any of the rebalancing, rather just follow
whatever ideal state we ask it to achieve every X minutes.

On Thu, Jul 31, 2014 at 5:39 PM, Varun Sharma <varun@pinterest.com> wrote:

> 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 ?
> Thanks
> Varun
> 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
>> 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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