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From Josh Wills <>
Subject Re: Multiple output channels from Crunch DoFn
Date Tue, 20 Aug 2013 21:42:25 GMT
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 a
> 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 I
> 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 in 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 large
>> volume of third-party input and expect a certain percentage of bogus or
>> malformed data. Rather than simply logging instances of bad records, we
>> 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: one
>> 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 Avro
>> > 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 would 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 for the
>> > concept; it was noted that
>> ><>might
be a close
>> > candidate. Pair<> would meet the requirements, although it was 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<>

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