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From Josh Wills <>
Subject Re: Multiple output channels from Crunch DoFn
Date Wed, 21 Aug 2013 02:40:27 GMT
...with the assumption that we would exclude null values in the Pair<A, B>?

On Tue, Aug 20, 2013 at 7:40 PM, Josh Wills <> wrote:

> That does sound pretty clean...
> On Tue, Aug 20, 2013 at 7:34 PM, Chao Shi <> wrote:
>> Is it possible to provide a utility that transforms PCollection<Pair<A,
>> B>> to Pair<PCollection<A>, PCollection<B>>? So one can simply
emit Pairs
>> and then write them to two Targets. This could be generalized to Tuples.
>> 2013/8/21 Josh Wills <>
>>> On Tue, Aug 20, 2013 at 12:23 PM, Inman,Brandon <
>>>> wrote:
>>>>  I like the flexibility of this approach, although would the idea of
>>>> having some official constants defined for a small set of standard channels
>>>> be reasonable (the concepts of "out" and "error" are pretty common, others
>>>> may be warranted as well)?
>>> So I think the way I would handle this would be having a main output
>>> directory and an error output directory that was underneath it. Cascading
>>> does this trick within their existing flows where you can throw exceptions
>>> to "traps," which is essentially the same idea, though I'm not wild about
>>> control flow that relies on throwing exceptions.
>>>>  Is this something that you would see being added to core Crunch APIs
>>>> (for example, directly to Pipeline), or implemented on top of Crunch with
>>>> filtering approach similar to my original post?  If it's implemented on
>>>> top, shouldn't materialization work as-is?
>>> Yes, your model would be simpler. I think that mine would require a
>>> special kind of Target implementation, a custom implementation of the
>>> Emitter interface that would be used for routing the outputs of the DoFn,
>>> and possibly some post-processing code to move the data to a sensible
>>> place. I don't know if that work is strictly necessary, and your impl is
>>> certainly much more straightforward than mine. :)
>>>>  If the type was PTable<String, T>, could Union<S,U> be a choice
for T
>>>> as appropriate? In our case, we would likely be looking at a PTable<String,
>>>> T extends SpecificRecordBase> and not necessarily need Union with this
>>>> approach.
>>> Yeah, I think it would be fine, but we'd have to be cognizant of it when
>>> we were implementing the union type, and it would be up to the client to
>>> ensure that the right data type ended up in the right file, which is maybe
>>> less good?
>>>>   From: Josh Wills <>
>>>> Reply-To: "" <>
>>>> Date: Tuesday, August 20, 2013 1:00 PM
>>>> To: "" <>
>>>> Subject: Re: Multiple output channels from Crunch DoFn
>>>>   A related idea that has come up a few times has been the idea of
>>>> having a way of writing values to different files based on a key: some kind
>>>> of generalization of Target that would itself write multiple outputs under
>>>> the covers, with the name of the output file indicated by some function of
>>>> the key of the PTable.
>>>> For this situation, we would have a PTable that was like PTable<String,
>>>> Union<S, T>>, or just PTable<String, T> if the output types
were all the
>>>> same, and the String would specify the name of an output directory (that
>>>> suppose would live underneath some base output directory for the Target)
>>>> that the record would be written to.
>>>>  There are a couple of limitations to this approach, I think: we
>>>> couldn't consider this kind of PTable "materialized" w/o doing an overhaul
>>>> of the materialization logic-- it would act sort of like an HTableTarget
>>>> that it would be write-only in flows. There are probably some others I
>>>> can't think of off the top of my head. What do you guys think?
>>>>  J
>>>> On Tue, Aug 20, 2013 at 9:49 AM, Brush,Ryan <> wrote:
>>>>> I happen to have some context around this, so I wanted to expand on
>>>>> Brandon's question a bit.  The use case here is we're dealing with a
>>>>> volume of third-party input and expect a certain percentage of bogus
>>>>> malformed data. Rather than simply logging instances of bad records,
>>>>> want to treat it as a signal we can learn from, both for improving our
>>>>> processing logic and for creating structured reports we can use to
>>>>> troubleshoot data sources.
>>>>> This leads to the "standard out" and "standard error" metaphors
>>>>> Brandon mentions: in most cases, our Crunch DoFns would emit a processed
>>>>> structure useful downstream. But we'd also like to be able to emit a
>>>>> structured error -- probably as an Avro object in our case -- and persist
>>>>> that as a byproduct of our main processing pipeline.
>>>>> Would it make sense for such DoFn's to emit something some form of
>>>>> "Option" object? We could then attach two consuming functions to it:
>>>>> that handles the "success" case, sending the resulting Avro object
>>>>> downstream. Another DoFn attached to the "Option" object would be a no-op
>>>>> unless the Option contained an "error" structure, at which point we persist
>>>>> it to some well-known location for later analysis.
>>>>> I think this is entirely achievable using existing mechanisms...but it
>>>>> seems like common enough use case (at least for us) to establish some
>>>>> idioms for dealing it.
>>>>> On Aug 20, 2013, at 11:13 AM, Inman,Brandon wrote:
>>>>> >
>>>>> > We've been looking at ways to do multiple outputs in Crunch jobs,
>>>>> > specifically writing out some kind of Status or Error Avro object,
>>>>> based
>>>>> > on failures that occur processing individual records in various
>>>>> jobs. It
>>>>> > had been suggested that, rather than logging these errors to
>>>>> traditional
>>>>> > loggers, to consider them an output of the Crunch job.  After some
>>>>> > internal discussion, it was suggested to run the ideas past the
>>>>> Crunch
>>>>> > community.
>>>>> >
>>>>> >
>>>>> > A major goal we have is to end with all the error output in a
>>>>> location
>>>>> > that makes it easy to run Hive queries or perform other
>>>>> MapReduce-style
>>>>> > analysis to quickly view all errors across the larger system without
>>>>> the
>>>>> > need go to multiple facilities.  This means standardizing on the
>>>>> > object, but it also necessitates decoupling the storage of the
>>>>> object from
>>>>> > the "standard output" of the job.
>>>>> >
>>>>> >
>>>>> > As Crunch DoFns support a single Emitter per invocation of
>>>>> process(), the
>>>>> > solution that gathered the most support would be to emit an object
>>>>> similar
>>>>> > to Pair<>, where first would be the "standard out" and second
>>>>> be the
>>>>> > "standard error".  A DoFn would generally only populate one (nothing
>>>>> > preventing it from populating both if appropriate, but not really
>>>>> intended
>>>>> > as a part of general use), and separate DoFns would filter out the
>>>>> two
>>>>> > components of the pair and write the values to the appropriate
>>>>> targets.
>>>>> >
>>>>> > As far as the emitted pairing object; the concept of a tagged union
>>>>> was
>>>>> > suggested although there currently isn't support in Java or Avro
>>>>> the
>>>>> > concept; it was noted that
>>>>> ><>might
be a close
>>>>> > candidate. Pair<> would meet the requirements, although it
>>>>> suggested
>>>>> > that a simple object dedicated to the task could make a cleaner
>>>>> approach.
>>>>> >
>>>>> > Any general thoughts on this approach? Are there any other patterns
>>>>> that
>>>>> > might serve us better, or anything on the Crunch roadmap that might
>>>>> be
>>>>> > more appropriate?
>>>>> >
>>>>> >
>>>>> > Brandon Inman
>>>>> > Software Architect
>>>>> >
>>>>> >
>>>>> >
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>>>>  --
>>>> Director of Data Science
>>>> Cloudera<>
>>>> Twitter: @josh_wills<>
> --
> Director of Data Science
> Cloudera <>
> Twitter: @josh_wills <>

Director of Data Science
Cloudera <>
Twitter: @josh_wills <>

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