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From Matthieu Morel <mmo...@apache.org>
Subject Getting auto_rebalance right
Date Mon, 14 Oct 2013 19:09:25 GMT

I'm trying to use the auto-rebalance mode in Helix.

The use case is the following (standard leader-standby scenario, a bit like the rsync example
in the helix codebase):
- the dataspace is partitioned
- for a given partition, we have 
	- a leader that is responsible for writing and serving data, logging operations into a journal
	- a replica that fetches updates from a journal and applies them locally but it does not
serve data
Upon failure, the replica becomes leader, applies pending updates and can write and serve
data. Ideally we also get a new replica assigned.

We'd like to use the auto_rebalance mode in Helix so that partitions are automatically assigned
and re-assigned, and so that leaders are automatically elected.

Unfortunately, I can't really get the balancing right. I might be doing something wrong, so
I uploaded an example here : https://github.com/matthieumorel/helix-balancing

In this application I would like to get exactly 1 leader and 1 replica for each of the partitions

In this example we don't reach that result, and when removing a node, we even get to a situation
where there is no leader for a given partition.

Do I have wrong expectations? Is there something wrong with the code, is it something with


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