Isn't the allowing a nulls a factor of the serialization of the payload? As an example if someone had a Writable PType, they could emit nulls so long as how they implemented the Writables readl/write methods supported it. It looks like most of Avro however we don't support nulls and for same for some of the simpler Writables.
Might need to document the discouragement somewhere. On Tue, Aug 20, 2013 at 10:45 PM, Josh Wills <[email protected]> wrote: > I think that we try to disallow it in general, so it's probably okay. > > > On Tue, Aug 20, 2013 at 8:43 PM, Chao Shi <[email protected]> wrote: > >> Yes, I think so. Do we generally allow nulls in crunch APIs? I'm a afraid >> that it would be confusing if some excludes null values while others don't. >> >> >> 2013/8/21 Josh Wills <[email protected]> >> >>> ...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 <[email protected]> wrote: >>> >>>> 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> >>>> >>> >>> >>> >>> -- >>> Director of Data Science >>> Cloudera <http://www.cloudera.com> >>> Twitter: @josh_wills <http://twitter.com/josh_wills> >>> >> >> > > > -- > Director of Data Science > Cloudera <http://www.cloudera.com> > Twitter: @josh_wills <http://twitter.com/josh_wills> >
