spark 变量使用 broadcast、accumulator

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.  

函数原型:

def broadcast[T](value: T): Broadcast[T] 

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。

源码分析:

def broadcast[T: ClassTag](value: T): Broadcast[T] = { assertNotStopped() if (classOf[RDD[_]].isAssignableFrom(classTag[T].runtimeClass)) { // This is a warning instead of an exception in order to avoid breaking user programs that // might have created RDD broadcast variables but not used them: logWarning("Can not directly broadcast RDDs; instead, call collect() and " + "broadcast the result (see SPARK-5063)") } val bc = env.broadcastManager.newBroadcast[T](value, isLocal) val callSite = getCallSite logInfo("Created broadcast " + bc.id + " from " + callSite.shortForm) cleaner.foreach(_.registerBroadcastForCleanup(bc)) bc } 

实例:

 1 List<Integer> data = Arrays.asList(5, 1, 1, 4, 4, 2, 2);
 2 JavaRDD<Integer> javaRDD = javaSparkContext.parallelize(data,5);
 3 final Broadcast<List<Integer>> broadcast = javaSparkContext.broadcast(data);
 4 JavaRDD<Integer> result = javaRDD.map(new Function<Integer, Integer>() {    
 5   List<Integer> iList = broadcast.value();    
 6   @Override    
 7   public Integer call(Integer v1) throws Exception {        
 8     Integer isum = 0;        
 9     for(Integer i : iList)            
10       isum += i;        
11     return v1 + isum;    
12   }
13 });
14 System.out.println(result.collect());

 

 

accumulator

官方文档描述:

 Create an [[org.apache.spark.Accumulator]] variable of a given type, which tasks can "add"
 values to using the `add` method. Only the master can access the accumulator's `value`.

函数原型:

def accumulator[T](initialValue: T, accumulatorParam: AccumulatorParam[T]): Accumulator[T] def accumulator[T](initialValue: T, name: String, accumulatorParam: AccumulatorParam[T]) : Accumulator[T]

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。

源码分析:

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)) acc }

实例:

 1 class VectorAccumulatorParam implements AccumulatorParam<Vector> {    
 2   @Override    
 3   //合并两个累加器的值。
 4   //参数r1是一个累加数据集合
 5   //参数r2是另一个累加数据集合
 6   public Vector addInPlace(Vector r1, Vector r2) {
 7     r1.addAll(r2);
 8     return r1;    
 9   }    
10   @Override 
11   //初始值   
12   public Vector zero(Vector initialValue) {        
13      return initialValue;    
14   }    
15   @Override
16   //添加额外的数据到累加值中
17   //参数t1是当前累加器的值
18   //参数t2是被添加到累加器的值    
19   public Vector addAccumulator(Vector t1, Vector t2) {        
20       t1.addAll(t2);        
21       return t1;    
22   }
23 }
24 List<Integer> data = Arrays.asList(5, 1, 1, 4, 4, 2, 2);
25 JavaRDD<Integer> javaRDD = javaSparkContext.parallelize(data,5);
26 
27 final Accumulator<Integer> accumulator = javaSparkContext.accumulator(0);
28 Vector initialValue = new Vector();
29 for(int i=6;i<9;i++)    
30   initialValue.add(i);
31 //自定义累加器
32 final Accumulator accumulator1 = javaSparkContext.accumulator(initialValue,new VectorAccumulatorParam());
33 JavaRDD<Integer> result = javaRDD.map(new Function<Integer, Integer>() {    
34   @Override    
35   public Integer call(Integer v1) throws Exception {        
36     accumulator.add(1);        
37     Vector term = new Vector();        
38     term.add(v1);        
39     accumulator1.add(term);        
40     return v1;    
41   }
42 });
43 System.out.println(result.collect());
44 System.out.println("~~~~~~~~~~~~~~~~~~~~~" + accumulator.value());
45 System.out.println("~~~~~~~~~~~~~~~~~~~~~" + accumulator1.value());

 

 

原文引自:https://www.jianshu.com/p/082ef79c63c1?utm_campaign=maleskine&utm_content=note&utm_medium=pc_all_hots&utm_source=recommendation

    原文作者:spark
    原文地址: https://www.cnblogs.com/jinggangshan/p/8117155.html
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