broadcast
官方文档描述：
Broadcast a read-only variable to the cluster, returning a [[org.apache.spark.broadcast.Broadcast]] object for reading it in distributed functions. The variable will be sent to each cluster only once.源码分析：
这里使用告警方式代替异常，为了是避免用户进程中断；可能有用户创建了广播变量但是没有使用他们；
/**
* Broadcast a read-only variable to the cluster, returning a
* [[org.apache.spark.broadcast.Broadcast]] object for reading it in distributed functions.
* The variable will be sent to each cluster only once.
*/
def broadcast[T: ClassTag](value: T): Broadcast[T] = {
assertNotStopped()
require(!classOf[RDD[_]].isAssignableFrom(classTag[T].runtimeClass),
"Can not directly broadcast RDDs; instead, call collect() and broadcast the result.")
val bc = env.broadcastManager.newBroadcast[T](value, isLocal)
val callSite = getCallSite
logInfo("Created broadcast " + bc.id + " from " + callSite.shortForm)
cleaner.foreach(_.registerBroadcastForCleanup(bc))
bc
}
广播变量允许程序员将一个只读的变量缓存在每台机器上，而不用在任务之间传递变量。广播变量可被用于有效地给每个节点一个大输入数据集的副本。Spark还尝试使用高效地广播算法来分发变量，进而减少通信的开销。 Spark的动作通过一系列的步骤执行，这些步骤由分布式的洗牌操作分开。Spark自动地广播每个步骤每个任务需要的通用数据。这些广播数据被序列化地缓存，在运行任务之前被反序列化出来。这意味着当我们需要在多个阶段的任务之间使用相同的数据，或者以反序列化形式缓存数据是十分重要的时候，显式地创建广播变量才有用。
实例
List<Integer> data = Arrays.asList(5, 1, 1, 4, 4, 2, 2);
JavaRDD<Integer> javaRDD = javaSparkContext.parallelize(data,5);
final Broadcast<List<Integer>> broadcast = javaSparkContext.broadcast(data);
JavaRDD<Integer> result = javaRDD.map(new Function<Integer, Integer>() {
List<Integer> iList = broadcast.value();
@Override
public Integer call(Integer v1) throws Exception {
Integer isum = 0;
for(Integer i : iList)
isum += i;
return v1 + isum;
}
});
System.out.println(result.collect());
accumulator
源码分析：
// Methods for creating shared variables
/**
* Create an [[org.apache.spark.Accumulator]] variable of a given type, which tasks can "add"
* values to using the `+=` method. Only the driver can access the accumulator's `value`.
*/
@deprecated("use AccumulatorV2", "2.0.0")
def accumulator[T](initialValue: T)(implicit param: AccumulatorParam[T]): Accumulator[T] = {
val acc = new Accumulator(initialValue, param)
cleaner.foreach(_.registerAccumulatorForCleanup(acc.newAcc))
acc
}
/**
* Create an [[org.apache.spark.Accumulator]] variable of a given type, with a name for display
* in the Spark UI. Tasks can "add" values to the accumulator using the `+=` method. Only the
* driver can access the accumulator's `value`.
*/
@deprecated("use AccumulatorV2", "2.0.0")
def accumulator[T](initialValue: T, name: String)(implicit param: AccumulatorParam[T])
: Accumulator[T] = {
val acc = new Accumulator(initialValue, param, Some(name))
cleaner.foreach(_.registerAccumulatorForCleanup(acc.newAcc))
acc
}
累加器是仅仅被相关操作累加的变量，因此可以在并行中被有效地支持。它可以被用来实现计数器和sum。Spark原生地只支持数字类型的累加器，开发者可以添加新类型的支持。如果创建累加器时指定了名字，可以在Spark的UI界面看到。这有利于理解每个执行阶段的进程（对于Python还不支持） 。
累加器通过对一个初始化了的变量v调用SparkContext.accumulator(v)来创建。在集群上运行的任务可以通过add或者”+=”方法在累加器上进行累加操作。但是，它们不能读取它的值。只有驱动程序能够读取它的值，通过累加器的value方法。
class VectorAccumulatorParam implements AccumulatorParam<Vector> {
@Override
//合并两个累加器的值。
//参数r1是一个累加数据集合
//参数r2是另一个累加数据集合
public Vector addInPlace(Vector r1, Vector r2) {
r1.addAll(r2);
return r1;
}
@Override
//初始值
public Vector zero(Vector initialValue) {
return initialValue;
}
@Override
//添加额外的数据到累加值中
//参数t1是当前累加器的值
//参数t2是被添加到累加器的值
public Vector addAccumulator(Vector t1, Vector t2) {
t1.addAll(t2);
return t1;
}
}
List<Integer> data = Arrays.asList(5, 1, 1, 4, 4, 2, 2);
JavaRDD<Integer> javaRDD = javaSparkContext.parallelize(data,5);
final Accumulator<Integer> accumulator = javaSparkContext.accumulator(0);
Vector initialValue = new Vector();
for(int i=6;i<9;i++)
initialValue.add(i);
//自定义累加器
final Accumulator accumulator1 = javaSparkContext.accumulator(initialValue,new VectorAccumulatorParam());
JavaRDD<Integer> result = javaRDD.map(new Function<Integer, Integer>() {
@Override
public Integer call(Integer v1) throws Exception {
accumulator.add(1);
Vector term = new Vector();
term.add(v1);
accumulator1.add(term);
return v1;
}
});
System.out.println(result.collect());
System.out.println("~~~~~~~~~~~~~~~~~~~~~" + accumulator.value());
System.out.println("~~~~~~~~~~~~~~~~~~~~~" + accumulator1.value());