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
>
>
>
>
>
>
