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From Varun Sharma <va...@pinterest.com>
Subject Re: Questions about custom helix rebalancer/controller/agent
Date Fri, 01 Aug 2014 18:10:01 GMT
Thanks a lot. Most of my questions are answered, except I have one follow
up question.

Lets say I have a situation with 3 masters per partition. For partition X,
these are on node1, node2 and node3. Upon dropping the resource, would the
partition X be offlined on all three nodes and then dropped or can that be
independent as in, node1 offlines and drops, followed by node2 and so on.
Just want to check if we first wait for all the masters to offline and then
initiate the offline->drop or the other way round.

Thanks !
Varun


On Fri, Aug 1, 2014 at 10:33 AM, Kanak Biscuitwala <kanak.b@hotmail.com>
wrote:

>
> Dropping a resource will cause the controller to first send MASTER -->
> OFFLINE for all partitions, and then OFFLINE --> DROPPED.
>
> Kanak
> ------------------------------
> Date: Fri, 1 Aug 2014 10:30:54 -0700
>
> Subject: Re: Questions about custom helix rebalancer/controller/agent
> From: varun@pinterest.com
> To: user@helix.apache.org
>
> In my case, I will have many resources - like say upto a 100 resources.
> Each of them will have partitions in the range of 100-5K.  So I guess, I do
> require the bucket size. 300K partitions is the sum of partitions across
> all resources, rather than the # of partitions within a single resource.
>
> Another question, I had was regarding removing a resource in Helix. When a
> removeResource is called from HelixAdmin, would it trigger the
> MASTER->OFFLINE the respective partitions before the resource is removing ?
> To concretize my use case, we have many resources with a few thousand
> partitions being loaded every day. New versions of the resources keep
> getting loaded as brand new resources into Helix and the older versions are
> decommissioned/garbage collected. So we would be issuing upto a 100 or so
> resource additions per day and upto a 100 or so resource deletions every
> day. Just want to check that deleting a resource would also trigger the
> appropriate MASTER->OFFLINE transitions.
>
> Thanks
> Varun
>
>
> On Fri, Aug 1, 2014 at 10:18 AM, Kanak Biscuitwala <kanak.b@hotmail.com>
> wrote:
>
> 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.
>
> Kanak
>
> ------------------------------
> 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
>
> Thanks
> Varun
>
>
> 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();
>      manager.addConfigChangeListener(controller);
>      manager.addLiveInstanceChangeListener(controller);
>      manager.addIdealStateChangeListener(controller);
>      manager.addExternalViewChangeListener(controller);
>      manager.addControllerListener(controller);
>
>
>
>
>
> 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);
>
> Thread.sleep(30minutes);
>
> 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.
>
> thanks,
> 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.
>
> 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 ?
>
> 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
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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