Hi hequn, I dig into the source of spark a bit deeper, and I got some ideas,
firstly, immutable is a feather of rdd but not a solid rule, there are ways to
change it, for excample, a rdd with non-idempotent "compute" function, though
it is really a bad design to make that function non-idempotent for
uncontrollable side-effect. I agree with Mark that foreach can modify the
elements of a rdd, but we should avoid this because it will effect all the rdds
generate by this changed rdd , make the whole process inconsistent and unstable.
Some rough opinions on the immutable feature of rdd, full discuss can make it
more clear. Any ideas?
-----原始邮件-----
发件人: "hequn cheng" <chenghe...@gmail.com>
发送时间: 2014/3/25 10:40
收件人: "user@spark.apache.org" <user@spark.apache.org>
主题: Re: 答复: RDD usage
First question:
If you save your modified RDD like this:
points.foreach(p=>p.y = another_value).collect() or
points.foreach(p=>p.y = another_value).saveAsTextFile(...)
the modified RDD will be materialized and this will not use any work's memory.
If you have more transformatins after the map(), the spark will pipelines all
transformations and build a DAG. Very little memory will be used in this stage
and the memory will be free soon.
Only cache() will persist your RDD in memory for a long time.
Second question:
Once RDD be created, it can not be changed due to the immutable feature.You can
only create a new RDD from the existing RDD or from file system.
2014-03-25 9:45 GMT+08:00 林武康 <vboylin1...@gmail.com>:
Hi hequn, a relative question, is that mean the memory usage will doubled? And
further more, if the compute function in a rdd is not idempotent, rdd will
changed during the job running, is that right?
发件人: hequn cheng
发送时间: 2014/3/25 9:35
收件人: user@spark.apache.org
主题: Re: RDD usage
points.foreach(p=>p.y = another_value) will return a new modified RDD.
2014-03-24 18:13 GMT+08:00 Chieh-Yen <r01944...@csie.ntu.edu.tw>:
Dear all,
I have a question about the usage of RDD.
I implemented a class called AppDataPoint, it looks like:
case class AppDataPoint(input_y : Double, input_x : Array[Double]) extends
Serializable {
var y : Double = input_y
var x : Array[Double] = input_x
......
}
Furthermore, I created the RDD by the following function.
def parsePoint(line: String): AppDataPoint = {
/* Some related works for parsing */
......
}
Assume the RDD called "points":
val lines = sc.textFile(inputPath, numPartition)
var points = lines.map(parsePoint _).cache()
The question is that, I tried to modify the value of this RDD, the operation is:
points.foreach(p=>p.y = another_value)
The operation is workable.
There doesn't have any warning or error message showed by the system and the
results are right.
I wonder that if the modification for RDD is a correct and in fact workable
design.
The usage web said that the RDD is immutable, is there any suggestion?
Thanks a lot.
Chieh-Yen Lin
