I read it one by one as I need to maintain the order, but it doesn't mean that I process them one by one later. Input lines refer to different entities I update, so once I read them in order, I group them by the id of the entity I want to update, sort the updates on per-entity basis and process them further in parallel (including writing data to C* and Kafka at the very end). That's what I use Spark for - the first step I ask about is just a requirement related to the input format I get and need to support. Everything what happens after that is just a normal data processing job that you want to distribute.
Kind regards, Michał Michalski, michal.michal...@boxever.com On 24 April 2015 at 16:10, Ganelin, Ilya <ilya.gane...@capitalone.com> wrote: > If you're reading a file one by line then you should simply use Java's > Hadoop FileSystem class to read the file with a BuffereInputStream. I don't > think you need an RDD here. > > > > Sent with Good (www.good.com) > > > -----Original Message----- > *From: *Michal Michalski [michal.michal...@boxever.com] > *Sent: *Friday, April 24, 2015 11:04 AM Eastern Standard Time > *To: *Ganelin, Ilya > *Cc: *Spico Florin; user > *Subject: *Re: Does HadoopRDD.zipWithIndex method preserve the order of > the input data from Hadoop? > > The problem I'm facing is that I need to process lines from input file in > the order they're stored in the file, as they define the order of updates I > need to apply on some data and these updates are not commutative so that > order matters. Unfortunately the input is purely order-based, theres no > timestamp per line etc. in the file and I'd prefer to avoid preparing the > file in advance by adding ordinals before / after each line. From the > approaches you suggested first two won't work as there's nothing I could > sort by. I'm not sure about the third one - I'm just not sure what you > meant there to be honest :-) > > Kind regards, > Michał Michalski, > michal.michal...@boxever.com > > On 24 April 2015 at 15:48, Ganelin, Ilya <ilya.gane...@capitalone.com> > wrote: > >> Michael - you need to sort your RDD. Check out the shuffle documentation >> on the Spark Programming Guide. It talks about this specifically. You can >> resolve this in a couple of ways - either by collecting your RDD and >> sorting it, using sortBy, or not worrying about the internal ordering. You >> can still extract elements in order by using a filter with the zip if e.g >> RDD.filter(s => s._2 < 50).sortBy(_._1) >> >> >> >> Sent with Good (www.good.com) >> >> >> >> -----Original Message----- >> *From: *Michal Michalski [michal.michal...@boxever.com] >> *Sent: *Friday, April 24, 2015 10:41 AM Eastern Standard Time >> *To: *Spico Florin >> *Cc: *user >> *Subject: *Re: Does HadoopRDD.zipWithIndex method preserve the order of >> the input data from Hadoop? >> >> Of course after you do it, you probably want to call >> repartition(somevalue) on your RDD to "get your paralellism back". >> >> Kind regards, >> Michał Michalski, >> michal.michal...@boxever.com >> >> On 24 April 2015 at 15:28, Michal Michalski <michal.michal...@boxever.com >> > wrote: >> >>> I did a quick test as I was curious about it too. I created a file with >>> numbers from 0 to 999, in order, line by line. Then I did: >>> >>> scala> val numbers = sc.textFile("./numbers.txt") >>> scala> val zipped = numbers.zipWithUniqueId >>> scala> zipped.foreach(i => println(i)) >>> >>> Expected result if the order was preserved would be something like: (0, >>> 0), (1, 1) etc. >>> Unfortunately, the output looks like this: >>> >>> (126,1) >>> (223,2) >>> (320,3) >>> (1,0) >>> (127,11) >>> (2,10) >>> (...) >>> >>> The workaround I found that works for me for my specific use case >>> (relatively small input files) is setting explicitly the number of >>> partitions to 1 when reading a single *text* file: >>> >>> scala> val numbers_sp = sc.textFile("./numbers.txt", 1) >>> >>> Than the output is exactly as I would expect. >>> >>> I didn't dive into the code too much, but I took a very quick look at it >>> and figured out - correct me if I missed something, it's Friday afternoon! >>> ;-) - that this workaround will work fine for all the input formats >>> inheriting from org.apache.hadoop.mapred.FileInputFormat including >>> TextInputFormat, of course - see the implementation of getSplits() method >>> there ( >>> http://grepcode.com/file/repo1.maven.org/maven2/org.jvnet.hudson.hadoop/hadoop-core/0.19.1-hudson-2/org/apache/hadoop/mapred/FileInputFormat.java#FileInputFormat.getSplits%28org.apache.hadoop.mapred.JobConf%2Cint%29 >>> ). >>> The numSplits variable passed there is exactly the same value as you >>> provide as a second argument to textFile, which is minPartitions. However, >>> while *min* suggests that we can only define a minimal number of >>> partitions, while we have no control over the max, from what I can see in >>> the code, that value specifies the *exact* number of partitions per the >>> FileInputFormat.getSplits implementation. Of course it can differ for other >>> input formats, but in this case it should work just fine. >>> >>> >>> Kind regards, >>> Michał Michalski, >>> michal.michal...@boxever.com >>> >>> On 24 April 2015 at 14:05, Spico Florin <spicoflo...@gmail.com> wrote: >>> >>>> Hello! >>>> I know that HadoopRDD partitions are built based on the number of >>>> splits in HDFS. I'm wondering if these partitions preserve the initial >>>> order of data in file. >>>> As an example, if I have an HDFS (myTextFile) file that has these >>>> splits: >>>> >>>> split 0-> line 1, ..., line k >>>> split 1->line k+1,..., line k+n >>>> splt 2->line k+n, line k+n+m >>>> >>>> and the code >>>> val lines=sc.textFile("hdfs://mytextFile") >>>> lines.zipWithIndex() >>>> >>>> will the order of lines preserved? >>>> (line 1, zipIndex 1) , .. (line k, zipIndex k), and so one. >>>> >>>> I found this question on stackoverflow ( >>>> http://stackoverflow.com/questions/26046410/how-can-i-obtain-an-element-position-in-sparks-rdd) >>>> whose answer intrigued me: >>>> "Essentially, RDD's zipWithIndex() method seems to do this, but it >>>> won't preserve the original ordering of the data the RDD was created from" >>>> >>>> Can you please confirm that is this the correct answer? >>>> >>>> Thanks. >>>> Florin >>>> >>>> >>>> >>>> >>>> >>>> >>> >> >> ------------------------------ >> >> The information contained in this e-mail is confidential and/or >> proprietary to Capital One and/or its affiliates. The information >> transmitted herewith is intended only for use by the individual or entity >> to which it is addressed. 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