Spark 中的序列化

1.序列化常用于网络传输和数据持久化以便于存储和传输，Spark通过两种方式来创建序列化器

val serializer = instantiateClassFromConf[Serializer](
"spark.serializer", "org.apache.spark.serializer.JavaSerializer")
logDebug(s"Using serializer: ${serializer.getClass}")
//暂时在blockManager中没有用到该序列化方式
val closureSerializer = instantiateClassFromConf[Serializer](
"spark.closure.serializer", "org.apache.spark.serializer.JavaSerializer")

2.Spark中两种典型的序列化场景

序列化场景A：执行map等RDD操作时，首先执行cleanF，内部左F解析和F序列化

private def ensureSerializable(func: AnyRef) {
try {
if (SparkEnv.get != null) {
      SparkEnv.get.closureSerializer.newInstance().serialize(func)
    }
valclosureSerializer = instantiateClassFromConf[Serializer](
"spark.closure.serializer", "org.apache.spark.serializer.JavaSerializer")
结论：
spark.closure.serializer配置决定了函数序列化的方式

序列化场景B：blockManager中

val blockManager = new BlockManager(executorId, rpcEnv, blockManagerMaster,
serializer, conf, memoryManager, mapOutputTracker, shuffleManager,
  blockTransferService, securityManager, numUsableCores)
val serializer = instantiateClassFromConf[Serializer](
"spark.serializer", "org.apache.spark.serializer.JavaSerializer")
结论：
spark.serializer决定了BlockManager工作中序列化的方式
3.Spark默认采用Java的序列化器，本人建议采用Kryo序列化提高性能，下面分析下这两种序列化机制的异同

A.首先看看java的序列化机制

override def serialize[T: ClassTag](t: T): ByteBuffer = {
val bos = new ByteArrayOutputStream()
val out = serializeStream(bos)
  out.writeObject(t)
  out.close()
  ByteBuffer.wrap(bos.toByteArray)
}

B.再看看Kyro的序列化机制

//创建两个延迟执行的工作流(Kyro输入输出流)

private lazy val output = ks.newKryoOutput()
private lazy val input = new KryoInput()

override def serialize[T: ClassTag](t: T): ByteBuffer = {
//设置position=0 && total=0  
output.clear()
//获取Kryo对象
val kryo = borrowKryo()
try {
    //开始对t函数进行序列化
    kryo.writeClassAndObject(output, t)
  } catch {
   ....
  } finally {
    //将当前Kryo作为缓存
    releaseKryo(kryo)
  }
  ByteBuffer.wrap(output.toBytes)
}
===================================Kryo分析华丽丽的分界线============================================
A:获取Kryo对象并初始化
KryoSerializer
的过程
首先查看是否有缓存的Kryo对象，如果有就rest清除掉kryo的状态，将缓存设置为null后返回
如果缓存中没有就创建一个新的Kryo,新建Kryo是一个繁琐的过程，包括新建Kryo对象，
public Kryo () {
this(new DefaultClassResolver(), new MapReferenceResolver());
}
public Kryo (ClassResolver classResolver, ReferenceResolver referenceResolver) {
if (classResolver == null) throw new IllegalArgumentException("classResolver cannot be null.");

this.classResolver = classResolver;
classResolver.setKryo(this);

this.referenceResolver = referenceResolver;
if (referenceResolver != null) {
referenceResolver.setKryo(this);
references = true;
   }

   addDefaultSerializer(byte[].class, ByteArraySerializer.class);
   addDefaultSerializer(char[].class, CharArraySerializer.class);
   addDefaultSerializer(short[].class, ShortArraySerializer.class);
   addDefaultSerializer(int[].class, IntArraySerializer.class);
   addDefaultSerializer(long[].class, LongArraySerializer.class);
   addDefaultSerializer(float[].class, FloatArraySerializer.class);
   addDefaultSerializer(double[].class, DoubleArraySerializer.class);
   addDefaultSerializer(boolean[].class, BooleanArraySerializer.class);
   addDefaultSerializer(String[].class, StringArraySerializer.class);
   addDefaultSerializer(Object[].class, ObjectArraySerializer.class);
   addDefaultSerializer(BigInteger.class, BigIntegerSerializer.class);
   addDefaultSerializer(BigDecimal.class, BigDecimalSerializer.class);
   addDefaultSerializer(Class.class, ClassSerializer.class);
   addDefaultSerializer(Date.class, DateSerializer.class);
   addDefaultSerializer(Enum.class, EnumSerializer.class);
   addDefaultSerializer(EnumSet.class, EnumSetSerializer.class);
   addDefaultSerializer(Currency.class, CurrencySerializer.class);
   addDefaultSerializer(StringBuffer.class, StringBufferSerializer.class);
   addDefaultSerializer(StringBuilder.class, StringBuilderSerializer.class);
   addDefaultSerializer(Collections.EMPTY_LIST.getClass(), CollectionsEmptyListSerializer.class);
   addDefaultSerializer(Collections.EMPTY_MAP.getClass(), CollectionsEmptyMapSerializer.class);
   addDefaultSerializer(Collections.EMPTY_SET.getClass(), CollectionsEmptySetSerializer.class);
   addDefaultSerializer(Collections.singletonList(null).getClass(), CollectionsSingletonListSerializer.class);
   addDefaultSerializer(Collections.singletonMap(null, null).getClass(), CollectionsSingletonMapSerializer.class);
   addDefaultSerializer(Collections.singleton(null).getClass(), CollectionsSingletonSetSerializer.class);
   addDefaultSerializer(Collection.class, CollectionSerializer.class);
   addDefaultSerializer(TreeMap.class, TreeMapSerializer.class);
   addDefaultSerializer(Map.class, MapSerializer.class);
   addDefaultSerializer(KryoSerializable.class, KryoSerializableSerializer.class);
   addDefaultSerializer(TimeZone.class, TimeZoneSerializer.class);
   addDefaultSerializer(Calendar.class, CalendarSerializer.class);
lowPriorityDefaultSerializerCount = defaultSerializers.size();

// Primitives and string. Primitive wrappers automatically use the same registration as primitives.
register(int.class, new IntSerializer());
   register(String.class, new StringSerializer());
   register(float.class, new FloatSerializer());
   register(boolean.class, new BooleanSerializer());
   register(byte.class, new ByteSerializer());
   register(char.class, new CharSerializer());
   register(short.class, new ShortSerializer());
   register(long.class, new LongSerializer());
   register(double.class, new DoubleSerializer());
}

注册Spark应用中必须使用的类型，比如
kryo.register(classOf[HttpBroadcast[_]], new KryoJavaSerializer())
kryo.register(classOf[Array[Tuple2[Any, Any]]])
kryo.register(classOf[GenericRecord], new GenericAvroSerializer(avroSchemas))
结论：register方法有很多种重载，能够给同一个class注册不同的序列化器，序列化器决定了执行序列化(读，写)的方法，register过程实际上是从
ObjectMap<Class, Registration>中根据Class获取对应的
Registratio然后更新
Registratio中的
Serializer
，若不存在Value则使用class,
Serializer,创建一个新的
Registratio，然后完成注册给
IntMap<Registration>和
ObjectMap<Class, Registration>
B:序列化函数F的过程
Registration registration = writeClass(output, object.getClass());
//获取到序列化器后，最终使用 Kryo定制化的
Serializer进行序列化
registration.getSerializer().write(this, output, object);
writeClass的过程,实际上调用classResolver的writeClass方法，这个classResolver就是实例化Kryo时所用的
DefaultClassResolver，然后调用
DefaultClassResolver的
writeClass方法
classResolver.writeClass(output, type);
DefaultClassResolver的writeClass方法
Registration registration = kryo.getRegistration(type);
根据不同的类型来获取
Registration,如果原来的
ObjectMap<Class, Registration>中没有
Registration，则会进行类型变幻
if (registration == null) {
if (Proxy.isProxyClass(type)) {
// If a Proxy class, treat it like an InvocationHandler because the concrete class for a proxy is generated.
registration = getRegistration(InvocationHandler.class);
   } else if (!type.isEnum() && Enum.class.isAssignableFrom(type)) {
// This handles an enum value that is an inner class. Eg: enum A {b{}};
registration = getRegistration(type.getEnclosingClass());
   } else if (EnumSet.class.isAssignableFrom(type)) {
      registration = classResolver.getRegistration(EnumSet.class);
   }
if (registration == null) {
if (registrationRequired) {
throw new IllegalArgumentException("Class is not registered: " + className(type)
            + "\nNote: To register this class use: kryo.register(" + className(type) + ".class);");
      }
      registration = classResolver.registerImplicit(type);
   }
}
========================Spark 序列化 精华分界线==================================================
通上面的过程，已经弄清楚了spark序列化机制的来龙去脉，问道解惑，神明自得