NLineInputFormat 在 Spark 中不起作用 [英] NLineInputFormat not working in Spark

问题描述

我想要的基本上是让数据的每个元素由 10 行组成.但是，对于下面的代码，每个元素仍然是一行.我在这里做错了什么?

What I want is basically to have each element of data consist of 10 lines. However, with the following code, each element is still one line. What mistake am I doing here?

val conf = new SparkConf().setAppName("MyApp")
conf.set("spark.serializer", "org.apache.spark.serializer.KryoSerializer")
conf.registerKryoClasses(Array[Class[_]](classOf[NLineInputFormat], classOf[LongWritable], 
 classOf[Text]))
val sc = new SparkContext(conf)

val c = new Configuration(sc.hadoopConfiguration)
c.set("lineinputformat.linespermap", 10);
val data = sc.newAPIHadoopFile(fname, classOf[NLineInputFormat], classOf[LongWritable], 
 classOf[Text], c)

推荐答案

NLineInputFormat by design just 不执行您期望的操作:

NLineInputFormat by design just doesn't perform operation you expect it to:

NLineInputFormat 将 N 行输入拆分为一个拆分.(...) 分割输入文件，这样默认情况下，一行作为值提供给一个 map 任务.

NLineInputFormat which splits N lines of input as one split. (...) that splits the input file such that by default, one line is fed as a value to one map task.

如您所见，它修改了拆分(Spark 命名法中的分区)的计算方式，而不是记录的确定方式.

As you can see it modifies how splits (partitions in the Spark nomenclature) are computed, not how records are determined.

如果描述不清楚，我们可以用下面的例子来说明:

If description is not clear we can illustrate that with a following example:

def nline(n: Int, path: String) = {
  val sc = SparkContext.getOrCreate
  val conf = new Configuration(sc.hadoopConfiguration)
  conf.setInt("mapreduce.input.lineinputformat.linespermap", n);

  sc.newAPIHadoopFile(path,
    classOf[NLineInputFormat], classOf[LongWritable], classOf[Text], conf
  )
}

require(nline(1, "README.md").glom.map(_.size).first == 1)
require(nline(2, "README.md").glom.map(_.size).first == 2)
require(nline(3, "README.md").glom.map(_.size).first == 3)

如上图所示，每个分区(可能不包括最后一个)正好包含 n 行.

As show above each partition (possibly excluding the last one) contains exactly n lines.

虽然您可以尝试对其进行改造以适应您的情况，但不建议将 linespermap 参数用于较小的值.

While you can try to retrofit this to fit your case it won't be recommended for small values of the linespermap parameter.

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