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From Puneet Zaroo <puneetza...@gmail.com>
Subject A state transition requirement.
Date Tue, 26 Feb 2013 20:12:25 GMT

I wanted to know how to implement a specific state machine requirement in Helix.
Lets say a partition is in the state S2.

1. On an instance hosting it going down, the partition moves to state
S3 (but stays on the same instance).
2. If the instance comes back up before a timeout expires, the
partition moves to state S1 (stays on the same instance).
3. If the instance does not come back up before the timeout expiry,
the partition moves to state S0 (the initial state, on a different
instance picked up by the controller).

I have a few questions.

1. I believe in order to implement Requirement 1, I have to use the
CUSTOM rebalancing feature (as otherwise the partitions will get
assigned to a new node).
The wiki page says the following about the CUSTOM mode.

"Applications will have to implement an interface that Helix will
invoke when the cluster state changes. Within this callback, the
application can recompute the partition assignment mapping"

Which interface does one have to implement ?  I am assuming the
callbacks are triggered inside the controller.

 2. The transition from S2 -> S3 should not issue a callback on the
participant (instance) holding that partition. This is because the
participant is unavailable and so cannot execute the callback. Is this
doable ?

3. One way the time-out (Requirement 3) can be implemented is to
occasionally trigger IdealState calculation after a time-out and not
only on liveness changes. Does that sound doable ?

- Puneet

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