判斷是RDD action的操作的一個標誌是
其函式實現裡得有

sc.runJob

RDD 是怎麼觸發job的

以 rdd.count 為例

RDD.scala

/**
   * Return the number of elements in the RDD.
   */
  def count(): Long = sc.runJob(this, Utils.getIteratorSize _).sum

SparkContext.scala

/**
   * Run a job on all partitions in an RDD and return the results in an array.
   *
   * @param rdd target RDD to run tasks on
   * @param func a function to run on each partition of the RDD
   * @return in-memory collection with a result of the job (each collection element will contain
   * a result from one partition)
   */
  def runJob[T, U: ClassTag](rdd: RDD[T], func: Iterator[T] => U): Array[U] = {
    runJob(rdd, func, 0 until rdd.partitions.length)
  }

 

DataFrame 是怎麼觸發job的

以 df.count 為例
比rdd 要囉嗦很多

DataSet.scala

/**
   * Returns the number of rows in the Dataset.
   * @group action
   * @since 1.6.0
   */
  def count(): Long = withAction("count", groupBy().count().queryExecution) { plan =>
    plan.executeCollect().head.getLong(0)
  }

SparkPlan.scala

/**
   * Runs this query returning the result as an array.
   */
  def executeCollect(): Array[InternalRow] = {
    val byteArrayRdd = getByteArrayRdd()

    val results = ArrayBuffer[InternalRow]()
    byteArrayRdd.collect().foreach { countAndBytes =>
      decodeUnsafeRows(countAndBytes._2).foreach(results.+=)
    }
    results.toArray
  }
 

RDD.scala

/**
   * Return an array that contains all of the elements in this RDD.
   *
   * @note This method should only be used if the resulting array is expected to be small, as
   * all the data is loaded into the driver's memory.
   */
  def collect(): Array[T] = withScope {
    val results = sc.runJob(this, (iter: Iterator[T]) => iter.toArray)
    Array.concat(results: _*)
  }

SparkContext.scala

/**
   * Run a job on all partitions in an RDD and return the results in an array.
   *
   * @param rdd target RDD to run tasks on
   * @param func a function to run on each partition of the RDD
   * @return in-memory collection with a result of the job (each collection element will contain
   * a result from one partition)
   */
  def runJob[T, U: ClassTag](rdd: RDD[T], func: Iterator[T] => U): Array[U] = {
    runJob(rdd, func, 0 until rdd.partitions.length)
  }

至此殊途同歸

spark.read.parquet 也能觸發job

val df: DataFrame = spark.read.parquet("/tmp/spark-975aa02f-fa2e-4b03-a3bd-7d57e3927787/" +
      "part-00000-61784fcc-e6cc-4ea8-bb14-7405f680681d.snappy.parquet"

DataFrameReader.scala

def parquet(path: String): DataFrame = {
    // This method ensures that calls that explicit need single argument works, see SPARK-16009
    parquet(Seq(path): _*)
  }

DataFrameReader.scala

def parquet(paths: String*): DataFrame = {
    format("parquet").load(paths: _*)
  }

DataFrameReader.scala

def load(paths: String*): DataFrame = {
    sparkSession.baseRelationToDataFrame(
      DataSource.apply(
        sparkSession,
        paths = paths,
        userSpecifiedSchema = userSpecifiedSchema,
        className = source,
        options = extraOptions.toMap).resolveRelation())
  }

DataSource.scala

def resolveRelation()
{
...
format.inferSchema(
        sparkSession,
        caseInsensitiveOptions,
        tempFileIndex.allFiles())
        ...
        }

ParquetFileFormat.scala

override def inferSchema()
{
...
ParquetFileFormat.mergeSchemasInParallel(filesToTouch, sparkSession)
...
}

ParquetFileFormat.scala

def mergeSchemasInParallel()
{
...
// Issues a Spark job to read Parquet schema in parallel.
    val partiallyMergedSchemas =
      sparkSession
        .sparkContext
        .parallelize(partialFileStatusInfo, numParallelism)
        .mapPartitions { iterator =>
          // Resembles fake `FileStatus`es with serialized path and length information.
          val fakeFileStatuses = iterator.map { case (path, length) =>
            new FileStatus(length, false, 0, 0, 0, 0, null, null, null, new Path(path))
          }.toSeq

          // Reads footers in multi-threaded manner within each task
          val footers =
            ParquetFileFormat.readParquetFootersInParallel(
              serializedConf.value, fakeFileStatuses, ignoreCorruptFiles)

          // Converter used to convert Parquet `MessageType` to Spark SQL `StructType`
          val converter = new ParquetToSparkSchemaConverter(
            assumeBinaryIsString = assumeBinaryIsString,
            assumeInt96IsTimestamp = assumeInt96IsTimestamp)
          if (footers.isEmpty) {
            Iterator.empty
          } else {
            var mergedSchema = ParquetFileFormat.readSchemaFromFooter(footers.head, converter)
            footers.tail.foreach { footer =>
              val schema = ParquetFileFormat.readSchemaFromFooter(footer, converter)
              try {
                mergedSchema = mergedSchema.merge(schema)
              } catch { case cause: SparkException =>
                throw new SparkException(
                  s"Failed merging schema of file ${footer.getFile}:\n${schema.treeString}", cause)
              }
            }
            Iterator.single(mergedSchema)
          }
        }.collect()
        ...
}

RDD.scala

/**
   * Return an array that contains all of the elements in this RDD.
   *
   * @note This method should only be used if the resulting array is expected to be small, as
   * all the data is loaded into the driver's memory.
   */
  def collect(): Array[T] = withScope {
    val results = sc.runJob(this, (iter: Iterator[T]) => iter.toArray)
    Array.concat(results: _*)
  }