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From Kanak Biscuitwala <kana...@hotmail.com>
Subject RE: helix rebalancing for multiple resources
Date Thu, 02 Jan 2014 04:20:04 GMT
Hi Vu,

Your understanding is basically correct. The controller will rebalance each resource in sequence,
at most one controller pipeline execution is going on at any one time, and there is no parallelism
within the controller pipeline (other than batch reading and writing the cluster at the beginning
and end).

Here are some things that may be of use to know:

1. You can plug in your own code to help decide how to rebalance your cluster in one of two
ways:   - Using the CustomCodeRunner on the participant side so that you can update the IdealState
whenever the cluster changes: https://github.com/apache/incubator-helix/blob/helix-0.6.2-release/helix-core/src/main/java/org/apache/helix/participant/HelixCustomCodeRunner.java?source=c
  - Implementing a Rebalancer with USER_DEFINED rebalance mode: https://github.com/apache/incubator-helix/blob/helix-0.6.2-release/helix-core/src/main/java/org/apache/helix/controller/rebalancer/Rebalancer.java?source=c
In either case, Helix will still fire transitions according to constraints and react to node
2. Helix supports adding tags to nodes (via InstanceConfig), and specifying tags in each resource
IdealState. Then, a tagged resource will only be assigned to nodes with the corresponding
tag present.
3. You can specify max partitions per resource per node in the IdealState of the resource
(this should be 1 in your case)
4. You can combine any of the above 3 if that makes sense (e.g. change node tags whenever
a cluster change happens, thus constraining how Helix will assign everything)
Is that helpful?
> Date: Wed, 1 Jan 2014 19:44:58 -0800 
> Subject: helix rebalancing for multiple resources 
> From: vusilly@gmail.com 
> To: user@helix.incubator.apache.org 
> Hi, 
> We're looking into creating something like a distributed task 
> processing cluster. We already have existing code for the processing 
> task on a single host. So that results in stronger restrictions on 
> what we're doing: 
> - partitioned task A: single partition needs to be assigned to a single 
> node and a node may have only a single partitioned task 
> - another set of non-partitioned tasks (e.g. B, C, D) also needs to be 
> assigned nodes, but it would be most efficient of those tasks are 
> assigned to separate nodes so any single node has at most 1 task 
> (either partitioned A, B, C, D, etc.) 
> This seems to require a global view of a tasks. However, from the 
> examples and the Rebalancer code, it appears that the resource 
> mappings/assignments are independent of each another. Is that correct? 
> If so, is Apache Helix the right framework for us, given the 
> requirements above? 
> I saw that it might be possible to find the current resource assignment 
> for other resources during the rebalancing calculation methods, but I 
> was then concerned about concurrency issues--if the rebalance for task 
> A and rebalance for B was computed at the same time. 
> Thanks for any and all feedback. 
> Vu Nguyen 
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