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=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> >>> >> >> > -- Director of Data Science Cloudera <http://www.cloudera.com> Twitter: @josh_wills <http://twitter.com/josh_wills>
