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=RiPWMqlVaSiSs74U1fVjrSpZO%2FvyTEWUW1RhCH7Ftlg%3D%0A&m=ZOuvUFJf2XiQL4mXsKMy9ArJwoDf7VP6eNKgaIHMafc%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 >>> > >>> > >>> > 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. >>> >>> >>> >> >> >> -- >> Director of Data Science >> Cloudera<https://urldefense.proofpoint.com/v1/url?u=http://www.cloudera.com&k=PmKqfXspAHNo6iYJ48Q45A%3D%3D%0A&r=RiPWMqlVaSiSs74U1fVjrSpZO%2FvyTEWUW1RhCH7Ftlg%3D%0A&m=ZOuvUFJf2XiQL4mXsKMy9ArJwoDf7VP6eNKgaIHMafc%3D%0A&s=508adfd2097ef3f7c9738fe9f729f47d95ae1d6568dabe09697317fd6d53f9d1> >> Twitter: >> @josh_wills<https://urldefense.proofpoint.com/v1/url?u=http://twitter.com/josh_wills&k=PmKqfXspAHNo6iYJ48Q45A%3D%3D%0A&r=RiPWMqlVaSiSs74U1fVjrSpZO%2FvyTEWUW1RhCH7Ftlg%3D%0A&m=ZOuvUFJf2XiQL4mXsKMy9ArJwoDf7VP6eNKgaIHMafc%3D%0A&s=585b666e290f5104a6f13a0fcbc52f4fc6cd93365dc1d44d3e49ed09c2fe1996> >> > >
