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From matt hoffman <matthoff...@acm.org>
Subject Modeling a pipeline in Helix
Date Sun, 02 Nov 2014 00:39:31 GMT
I'm doing a quick POC to write an event processing pipeline using Helix,
and I had a couple questions about the best way to model it.

For simplicity, these pipelines are made up of a linear set of stages, and
an item flows through each stage in order.

I'd like to be able to scale a particular pipeline up and down (3 instances
of Pipeline A on the cluster, for example).  That seems straightforward to
do in Helix -- it's just changing the number of replicas of a resource.

Sometimes particular stages have to run on a particular set of machines
(say, a stage that requires a GPU that is only on some machines in the
cluster, for example).  It looks like I would do that in Helix using a
SEMI_AUTO rebalancing mode.

For efficiency, I'd like Helix to try to group as many stages of the same
pipeline onto the same machine as possible.  I don't want it to spread the
stages across the cluster so that a single item has to hop from machine to
machine any more than necessary.  I'm not sure how best to model this in
Helix.
In Helix's world, does it make sense to model stages as "partitions" within
a pipeline "resource"?  Or should the stages be resources themselves?   And
if they are resources, can I define a constraint or rebalancing algorithm
that attempts to colocate them?   I see that the task execution recipe is
pretty similar to what I want, and it models the individual stages as
resources... so I'm guessing that's the best way to model it, but I don't
know enough about Helix yet to know the pros and cons.

I'm also assuming that the new "task" abstraction in Helix 0.7 probably
isn't what I want; it seems to be modeling something with a discrete
execution like a MapReduce mapper, as opposed to a stage that items flow
through.  Am I correct?

Thanks for any advice you can give!

matt

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