I'm +1 for the PCollection<Pair<T, U>> -> Pair<PCollection<T>, PCollection<U>> approach outlined by Brandon and Chao. I think the only question is whether or not we want to add in the Union<T, U> (or Either<T, U>?) feature as part of doing that change.
J On Wed, Aug 21, 2013 at 9:19 AM, Inman,Brandon <[email protected]>wrote: > This is close to how I had imagined the implementation to look. Very > roughly- > > public static class FirstEmittingDoFn<T extends Pair<U, ?>, U> extends > DoFn<Pair<U, ?>, U> { > > @Override > public void process(Pair<U, ?> input, Emitter<U> emitter) { > final U first = input.first(); > if (first != null) { > emitter.emit(first); > } > } > } > } > > There would be a very similar DoFn for second() that I'll omit for > brevity. I originally envisioned the utility method calling the DoFn that > generated the pair, but I like the idea of a smaller utility. The utility > method should be as simple as... > > public static <T, U> Pair<PCollection<T>,PCollection<U>> > filterChannels(final PCollection<Pair<T,U>> pCollection, final PType<T> > firstPType, final PType<U> secondPType) { > > final PCollection<T> stdout = collection.parallelDo(new > FirstEmittingDoFn<T>, firstPType); > final PCollection<U> stderr = collection.parallelDo(new > SecondEmittingDoFn<U>, secondPType); > > > return Pair.of(stdout,stderr); > } > > > Disclaimer; I didn't try to compile (all) this code, so treat as > pseudocode. > > From: Josh Wills <[email protected]> > Reply-To: "[email protected]" <[email protected]> > Date: Tuesday, August 20, 2013 9:40 PM > To: "[email protected]" <[email protected]> > Subject: Re: Multiple output channels from Crunch DoFn > > > That does sound pretty clean... > > > On Tue, Aug 20, 2013 at 7:34 PM, Chao Shi > <[email protected]> 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 <[email protected]> > > > On Tue, Aug 20, 2013 at 12:23 PM, Inman,Brandon <[email protected]> > 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 <[email protected]> > Reply-To: "[email protected]" <[email protected]> > Date: Tuesday, August 20, 2013 1:00 PM > To: "[email protected]" <[email protected]> > 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 > <[email protected]> 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 > > > https://issues.apache.org/jira/browse/CRUNCH-239 > < > https://urldefense.proofpoint.com/v1/url?u=https://issues.apache.org/jira/ > browse/CRUNCH-239&k=PmKqfXspAHNo6iYJ48Q45A%3D%3D%0A&r=RiPWMqlVaSiSs74U1fVjr > SpZO%2FvyTEWUW1RhCH7Ftlg%3D%0A&m=ZOuvUFJf2XiQL4mXsKMy9ArJwoDf7VP6eNKgaIHMaf > c%3D%0A&s=dceef88f8fadf4d34b61b47e1728bc63dda36ad51151ccfceb5c84ea45be0e82 > > > 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 > > www.cerner.com <http://www.cerner.com> > > > > > > CONFIDENTIALITY NOTICE This message and any included attachments are > >from Cerner Corporation and are intended only for the addressee. The > >information contained in this message is confidential and may constitute > >inside or non-public information under international, > federal, or state securities laws. Unauthorized forwarding, printing, > copying, distribution, or use of such information is strictly prohibited > and may be unlawful. If you are not the addressee, please promptly delete > this message and notify the sender of the > delivery error by e-mail or you may call Cerner's corporate offices in > Kansas City, Missouri, U.S.A at > > (+1) (816)221-1024 <tel:%28%2B1%29%20%28816%29221-1024>. > > > > > > > > > > > -- > Director of Data Science > Cloudera > < > https://urldefense.proofpoint.com/v1/url?u=http://www.cloudera.com&k=PmKqf > XspAHNo6iYJ48Q45A%3D%3D%0A&r=RiPWMqlVaSiSs74U1fVjrSpZO%2FvyTEWUW1RhCH7Ftlg% > 3D%0A&m=ZOuvUFJf2XiQL4mXsKMy9ArJwoDf7VP6eNKgaIHMafc%3D%0A&s=508adfd2097ef3f > 7c9738fe9f729f47d95ae1d6568dabe09697317fd6d53f9d1> > Twitter: > @josh_wills > < > https://urldefense.proofpoint.com/v1/url?u=http://twitter.com/josh_wills&k > =PmKqfXspAHNo6iYJ48Q45A%3D%3D%0A&r=RiPWMqlVaSiSs74U1fVjrSpZO%2FvyTEWUW1RhCH > 7Ftlg%3D%0A&m=ZOuvUFJf2XiQL4mXsKMy9ArJwoDf7VP6eNKgaIHMafc%3D%0A&s=585b666e2 > 90f5104a6f13a0fcbc52f4fc6cd93365dc1d44d3e49ed09c2fe1996> > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > Director of Data Science > Cloudera > < > https://urldefense.proofpoint.com/v1/url?u=http://www.cloudera.com&k=PmKqf > XspAHNo6iYJ48Q45A%3D%3D%0A&r=RiPWMqlVaSiSs74U1fVjrSpZO%2FvyTEWUW1RhCH7Ftlg% > 3D%0A&m=JjOKxAMa8Miu4X1FpLdnSvt5WCGlwK4xE7i92OmAex0%3D%0A&s=1095ecaa17ab1e4 > 31966b19fec39773cae0b9319fc310155b4ab636cabd4799a> > Twitter: > @josh_wills > < > https://urldefense.proofpoint.com/v1/url?u=http://twitter.com/josh_wills&k > =PmKqfXspAHNo6iYJ48Q45A%3D%3D%0A&r=RiPWMqlVaSiSs74U1fVjrSpZO%2FvyTEWUW1RhCH > 7Ftlg%3D%0A&m=JjOKxAMa8Miu4X1FpLdnSvt5WCGlwK4xE7i92OmAex0%3D%0A&s=8b9feeae6 > 0caabb4edd6caff1fd188790717924e130b1d3533089bee9a85e9a6> > > -- Director of Data Science Cloudera <http://www.cloudera.com> Twitter: @josh_wills <http://twitter.com/josh_wills>
