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Kiyan Ahmadizadeh commented on CRUNCH-58:
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Discussion of implementing PObjects started on the CRUNCH-57 ticket. Josh gave
this suggestion for an implementation:
--
Kiyan, do you have an opinion on how you want to go about this one? Do you want
to take on defining PObject (which in my mind, could just be a simple wrapper
that materialized a PCollection and then implemented some abstract function
that did a computation on the materialized Iterable) and incorporate it here?
--
Josh, I think PObject should be a wrapper around PCollection, but the
underlying PCollection should contain only one element (or be treated as such).
In other words, it should wrap the result of a distributed computation that
reduced/combined a source PCollection into a target PCollection of 1 element.
Then PObject could have a getValue method that materialized the underlying
PCollection and returned the singleton element found within. I'm not sure if
we want to strongly enforce that the underlying PCollection for a PObject
contains one element by throwing an exception, or if we simply ignore any
element but the first in the underlying PCollection.
Your suggestion for "some abstract function that did a computation on the
materialized Iterable" doesn't make sense to me, since in my mind a PObject
should only care about the first element in its underlying PCollection. Could
you clarify?
> Implement PObject in Crunch/Scrunch
> -----------------------------------
>
> Key: CRUNCH-58
> URL: https://issues.apache.org/jira/browse/CRUNCH-58
> Project: Crunch
> Issue Type: New Feature
> Affects Versions: 0.3.0
> Reporter: Kiyan Ahmadizadeh
> Assignee: Kiyan Ahmadizadeh
>
> FlumeJava has the concept of a PObject<T>, a container for a singleton of
> type T. It is meant represent the result of a distributed computation that
> yields a singleton value (for example max, min, and length methods on
> PCollection<T>). Generally speaking, the result of any computation that
> combines/reduces a PCollection into a singleton value could be represented by
> a PObject.
> Like PCollection, a PObject defers distributed computation until its value is
> actually used.
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