聊聊storm tuple的序列化

序

本文主要研究一下storm tuple的序列化

ExecutorTransfer.tryTransfer

storm-2.0.0/storm-client/src/jvm/org/apache/storm/executor/ExecutorTransfer.java

// Every executor has an instance of this class
public class ExecutorTransfer {
    private static final Logger LOG = LoggerFactory.getLogger(ExecutorTransfer.class);

    private final WorkerState workerData;
    private final KryoTupleSerializer serializer;
    private final boolean isDebug;
    private int indexingBase = 0;
    private ArrayList<JCQueue> localReceiveQueues; // [taskId-indexingBase] => queue : List of all recvQs local to this worker
    private AtomicReferenceArray<JCQueue> queuesToFlush;
        // [taskId-indexingBase] => queue, some entries can be null. : outbound Qs for this executor instance


    public ExecutorTransfer(WorkerState workerData, Map<String, Object> topoConf) {
        this.workerData = workerData;
        this.serializer = new KryoTupleSerializer(topoConf, workerData.getWorkerTopologyContext());
        this.isDebug = ObjectReader.getBoolean(topoConf.get(Config.TOPOLOGY_DEBUG), false);
    }

    //......

    // adds addressedTuple to destination Q if it is not full. else adds to pendingEmits (if its not null)
    public boolean tryTransfer(AddressedTuple addressedTuple, Queue<AddressedTuple> pendingEmits) {
        if (isDebug) {
            LOG.info("TRANSFERRING tuple {}", addressedTuple);
        }

        JCQueue localQueue = getLocalQueue(addressedTuple);
        if (localQueue != null) {
            return tryTransferLocal(addressedTuple, localQueue, pendingEmits);
        }
        return workerData.tryTransferRemote(addressedTuple, pendingEmits, serializer);
    }

    //......
}

• ExecutorTransfer在构造器里头创建了KryoTupleSerializer
• 这里先判断目标地址是否是在localQueue中，如果是则进行local transfer，否则进行remote transfer
• remote transfer的时候调用了workerData.tryTransferRemote，并传递了serializer

WorkerState.tryTransferRemote

storm-2.0.0/storm-client/src/jvm/org/apache/storm/daemon/worker/WorkerState.java

    /* Not a Blocking call. If cannot emit, will add 'tuple' to pendingEmits and return 'false'. 'pendingEmits' can be null */
    public boolean tryTransferRemote(AddressedTuple tuple, Queue<AddressedTuple> pendingEmits, ITupleSerializer serializer) {
        return workerTransfer.tryTransferRemote(tuple, pendingEmits, serializer);
    }

• WorkerState.tryTransferRemote实际上使用的是workerTransfer.tryTransferRemote

workerTransfer.tryTransferRemote

storm-2.0.0/storm-client/src/jvm/org/apache/storm/daemon/worker/WorkerTransfer.java

    /* Not a Blocking call. If cannot emit, will add 'tuple' to 'pendingEmits' and return 'false'. 'pendingEmits' can be null */
    public boolean tryTransferRemote(AddressedTuple addressedTuple, Queue<AddressedTuple> pendingEmits, ITupleSerializer serializer) {
        if (pendingEmits != null && !pendingEmits.isEmpty()) {
            pendingEmits.add(addressedTuple);
            return false;
        }

        if (!remoteBackPressureStatus[addressedTuple.dest].get()) {
            TaskMessage tm = new TaskMessage(addressedTuple.getDest(), serializer.serialize(addressedTuple.getTuple()));
            if (transferQueue.tryPublish(tm)) {
                return true;
            }
        } else {
            LOG.debug("Noticed Back Pressure in remote task {}", addressedTuple.dest);
        }
        if (pendingEmits != null) {
            pendingEmits.add(addressedTuple);
        }
        return false;
    }

• 这里可以看到创建TaskMessage的时候，使用serializer.serialize(addressedTuple.getTuple())对tuple进行了序列化；该serializer为ITupleSerializer类型，它的实现类为KryoTupleSerializer

KryoTupleSerializer

storm-2.0.0/storm-client/src/jvm/org/apache/storm/serialization/KryoTupleSerializer.java

public class KryoTupleSerializer implements ITupleSerializer {
    KryoValuesSerializer _kryo;
    SerializationFactory.IdDictionary _ids;
    Output _kryoOut;

    public KryoTupleSerializer(final Map<String, Object> conf, final GeneralTopologyContext context) {
        _kryo = new KryoValuesSerializer(conf);
        _kryoOut = new Output(2000, 2000000000);
        _ids = new SerializationFactory.IdDictionary(context.getRawTopology());
    }

    public byte[] serialize(Tuple tuple) {
        try {

            _kryoOut.clear();
            _kryoOut.writeInt(tuple.getSourceTask(), true);
            _kryoOut.writeInt(_ids.getStreamId(tuple.getSourceComponent(), tuple.getSourceStreamId()), true);
            tuple.getMessageId().serialize(_kryoOut);
            _kryo.serializeInto(tuple.getValues(), _kryoOut);
            return _kryoOut.toBytes();
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    //    public long crc32(Tuple tuple) {
    //        try {
    //            CRC32OutputStream hasher = new CRC32OutputStream();
    //            _kryo.serializeInto(tuple.getValues(), hasher);
    //            return hasher.getValue();
    //        } catch (IOException e) {
    //            throw new RuntimeException(e);
    //        }
    //    }
}

• KryoTupleSerializer创建了KryoValuesSerializer，在serialize tuple的时候调用了_kryo.serializeInto(tuple.getValues(), _kryoOut)

KryoValuesSerializer

storm-2.0.0/storm-client/src/jvm/org/apache/storm/serialization/KryoValuesSerializer.java

public class KryoValuesSerializer {
    Kryo _kryo;
    ListDelegate _delegate;
    Output _kryoOut;

    public KryoValuesSerializer(Map<String, Object> conf) {
        _kryo = SerializationFactory.getKryo(conf);
        _delegate = new ListDelegate();
        _kryoOut = new Output(2000, 2000000000);
    }

    public void serializeInto(List<Object> values, Output out) {
        // this ensures that list of values is always written the same way, regardless
        // of whether it's a java collection or one of clojure's persistent collections 
        // (which have different serializers)
        // Doing this lets us deserialize as ArrayList and avoid writing the class here
        _delegate.setDelegate(values);
        _kryo.writeObject(out, _delegate);
    }

    public byte[] serialize(List<Object> values) {
        _kryoOut.clear();
        serializeInto(values, _kryoOut);
        return _kryoOut.toBytes();
    }

    public byte[] serializeObject(Object obj) {
        _kryoOut.clear();
        _kryo.writeClassAndObject(_kryoOut, obj);
        return _kryoOut.toBytes();
    }
}

• KryoValuesSerializer在构造器里头调用SerializationFactory.getKryo(conf)方法创建_kryo
• 这里的_delegate使用的是ListDelegate(即用它来包装一下List<Object> values)，_kryoOut为new Output(2000, 2000000000)
• serialize方法调用的是serializeInto方法，该方法最后调用的是原生的_kryo.writeObject方法进行序列化

SerializationFactory.getKryo

storm-2.0.0/storm-client/src/jvm/org/apache/storm/serialization/SerializationFactory.java

    public static Kryo getKryo(Map<String, Object> conf) {
        IKryoFactory kryoFactory = (IKryoFactory) ReflectionUtils.newInstance((String) conf.get(Config.TOPOLOGY_KRYO_FACTORY));
        Kryo k = kryoFactory.getKryo(conf);
        k.register(byte[].class);

        /* tuple payload serializer is specified via configuration */
        String payloadSerializerName = (String) conf.get(Config.TOPOLOGY_TUPLE_SERIALIZER);
        try {
            Class serializerClass = Class.forName(payloadSerializerName);
            Serializer serializer = resolveSerializerInstance(k, ListDelegate.class, serializerClass, conf);
            k.register(ListDelegate.class, serializer);
        } catch (ClassNotFoundException ex) {
            throw new RuntimeException(ex);
        }

        k.register(ArrayList.class, new ArrayListSerializer());
        k.register(HashMap.class, new HashMapSerializer());
        k.register(HashSet.class, new HashSetSerializer());
        k.register(BigInteger.class, new BigIntegerSerializer());
        k.register(TransactionAttempt.class);
        k.register(Values.class);
        k.register(org.apache.storm.metric.api.IMetricsConsumer.DataPoint.class);
        k.register(org.apache.storm.metric.api.IMetricsConsumer.TaskInfo.class);
        k.register(ConsList.class);
        k.register(BackPressureStatus.class);

        synchronized (loader) {
            for (SerializationRegister sr : loader) {
                try {
                    sr.register(k);
                } catch (Exception e) {
                    throw new RuntimeException(e);
                }
            }
        }

        kryoFactory.preRegister(k, conf);

        boolean skipMissing = (Boolean) conf.get(Config.TOPOLOGY_SKIP_MISSING_KRYO_REGISTRATIONS);

        register(k, conf.get(Config.TOPOLOGY_KRYO_REGISTER), conf, skipMissing);

        kryoFactory.postRegister(k, conf);

        if (conf.get(Config.TOPOLOGY_KRYO_DECORATORS) != null) {
            for (String klassName : (List<String>) conf.get(Config.TOPOLOGY_KRYO_DECORATORS)) {
                try {
                    Class klass = Class.forName(klassName);
                    IKryoDecorator decorator = (IKryoDecorator) klass.newInstance();
                    decorator.decorate(k);
                } catch (ClassNotFoundException e) {
                    if (skipMissing) {
                        LOG.info("Could not find kryo decorator named " + klassName + ". Skipping registration...");
                    } else {
                        throw new RuntimeException(e);
                    }
                } catch (InstantiationException e) {
                    throw new RuntimeException(e);
                } catch (IllegalAccessException e) {
                    throw new RuntimeException(e);
                }
            }
        }

        kryoFactory.postDecorate(k, conf);

        return k;
    }

    public static void register(Kryo k, Object kryoRegistrations, Map<String, Object> conf, boolean skipMissing) {
        Map<String, String> registrations = normalizeKryoRegister(kryoRegistrations);
        for (Map.Entry<String, String> entry : registrations.entrySet()) {
            String serializerClassName = entry.getValue();
            try {
                Class klass = Class.forName(entry.getKey());
                Class serializerClass = null;
                if (serializerClassName != null) {
                    serializerClass = Class.forName(serializerClassName);
                }
                if (serializerClass == null) {
                    k.register(klass);
                } else {
                    k.register(klass, resolveSerializerInstance(k, klass, serializerClass, conf));
                }
            } catch (ClassNotFoundException e) {
                if (skipMissing) {
                    LOG.info("Could not find serialization or class for " + serializerClassName + ". Skipping registration...");
                } else {
                    throw new RuntimeException(e);
                }
            }
        }
    }

• SerializationFactory.getKryo静态方法首先根据Config.TOPOLOGY_KRYO_FACTORY创建IKryoFactory，默认是org.apache.storm.serialization.DefaultKryoFactory
• 之后通过IKryoFactory.getKryo创建Kryo，之后就是对Kryo进行一系列配置，这里注册了byte[].class、ListDelegate.class、ArrayList.class、HashMap.class、HashSet.class、BigInteger.class、TransactionAttempt.class、Values.class、org.apache.storm.metric.api.IMetricsConsumer.DataPoint.class、org.apache.storm.metric.api.IMetricsConsumer.TaskInfo.class、ConsList.class、BackPressureStatus.class
• ListDelegate.class为payload的容器，采用Config.TOPOLOGY_TUPLE_SERIALIZER(topology.tuple.serializer，默认是org.apache.storm.serialization.types.ListDelegateSerializer)配置的类进行序列化
• Config.TOPOLOGY_SKIP_MISSING_KRYO_REGISTRATIONS(topology.skip.missing.kryo.registrations，默认为false)，当kryo找不到配置的要序列化的class对应serializers的时候，是抛出异常还是直接跳过注册；
• 最后通过Config.TOPOLOGY_KRYO_DECORATORS(topology.kryo.decorators)加载自定义的serialization

DefaultKryoFactory

storm-2.0.0/storm-client/src/jvm/org/apache/storm/serialization/DefaultKryoFactory.java

public class DefaultKryoFactory implements IKryoFactory {

    @Override
    public Kryo getKryo(Map<String, Object> conf) {
        KryoSerializableDefault k = new KryoSerializableDefault();
        k.setRegistrationRequired(!((Boolean) conf.get(Config.TOPOLOGY_FALL_BACK_ON_JAVA_SERIALIZATION)));
        k.setReferences(false);
        return k;
    }

    @Override
    public void preRegister(Kryo k, Map<String, Object> conf) {
    }

    public void postRegister(Kryo k, Map<String, Object> conf) {
        ((KryoSerializableDefault) k).overrideDefault(true);
    }

    @Override
    public void postDecorate(Kryo k, Map<String, Object> conf) {
    }

    public static class KryoSerializableDefault extends Kryo {
        boolean _override = false;

        public void overrideDefault(boolean value) {
            _override = value;
        }

        @Override
        public Serializer getDefaultSerializer(Class type) {
            if (_override) {
                return new SerializableSerializer();
            } else {
                return super.getDefaultSerializer(type);
            }
        }
    }
}

• 这里从配置读取Config.TOPOLOGY_FALL_BACK_ON_JAVA_SERIALIZATION(topology.fall.back.on.java.serialization)，默认该值为true，则registrationRequired这里设置为false，即序列化的时候不要求该class必须在已注册的列表中

Kryo

kryo-4.0.2-sources.jar!/com/esotericsoftware/kryo/Kryo.java

    /** If the class is not registered and {@link Kryo#setRegistrationRequired(boolean)} is false, it is automatically registered
     * using the {@link Kryo#addDefaultSerializer(Class, Class) default serializer}.
     * @throws IllegalArgumentException if the class is not registered and {@link Kryo#setRegistrationRequired(boolean)} is true.
     * @see ClassResolver#getRegistration(Class) */
    public Registration getRegistration (Class type) {
        if (type == null) throw new IllegalArgumentException("type cannot be null.");

        Registration registration = classResolver.getRegistration(type);
        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) && !Enum.class.equals(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);
            } else if (isClosure(type)) {
                registration = classResolver.getRegistration(ClosureSerializer.Closure.class);
            }
            if (registration == null) {
                if (registrationRequired) {
                    throw new IllegalArgumentException(unregisteredClassMessage(type));
                }
                if (warnUnregisteredClasses) {
                    warn(unregisteredClassMessage(type));
                }
                registration = classResolver.registerImplicit(type);
            }
        }
        return registration;
    }

    /** Registers the class using the lowest, next available integer ID and the {@link Kryo#getDefaultSerializer(Class) default
     * serializer}. If the class is already registered, no change will be made and the existing registration will be returned.
     * Registering a primitive also affects the corresponding primitive wrapper.
     * <p>
     * Because the ID assigned is affected by the IDs registered before it, the order classes are registered is important when
     * using this method. The order must be the same at deserialization as it was for serialization. */
    public Registration register (Class type) {
        Registration registration = classResolver.getRegistration(type);
        if (registration != null) return registration;
        return register(type, getDefaultSerializer(type));
    }

    /** Returns the best matching serializer for a class. This method can be overridden to implement custom logic to choose a
     * serializer. */
    public Serializer getDefaultSerializer (Class type) {
        if (type == null) throw new IllegalArgumentException("type cannot be null.");

        final Serializer serializerForAnnotation = getDefaultSerializerForAnnotatedType(type);
        if (serializerForAnnotation != null) return serializerForAnnotation;

        for (int i = 0, n = defaultSerializers.size(); i < n; i++) {
            DefaultSerializerEntry entry = defaultSerializers.get(i);
            if (entry.type.isAssignableFrom(type)) {
                Serializer defaultSerializer = entry.serializerFactory.makeSerializer(this, type);
                return defaultSerializer;
            }
        }

        return newDefaultSerializer(type);
    }

    /** Called by {@link #getDefaultSerializer(Class)} when no default serializers matched the type. Subclasses can override this
     * method to customize behavior. The default implementation calls {@link SerializerFactory#makeSerializer(Kryo, Class)} using
     * the {@link #setDefaultSerializer(Class) default serializer}. */
    protected Serializer newDefaultSerializer (Class type) {
        return defaultSerializer.makeSerializer(this, type);
    }

    /** Registers the class using the lowest, next available integer ID and the specified serializer. If the class is already
     * registered, the existing entry is updated with the new serializer. Registering a primitive also affects the corresponding
     * primitive wrapper.
     * <p>
     * Because the ID assigned is affected by the IDs registered before it, the order classes are registered is important when
     * using this method. The order must be the same at deserialization as it was for serialization. */
    public Registration register (Class type, Serializer serializer) {
        Registration registration = classResolver.getRegistration(type);
        if (registration != null) {
            registration.setSerializer(serializer);
            return registration;
        }
        return classResolver.register(new Registration(type, serializer, getNextRegistrationId()));
    }

    /** Returns the lowest, next available integer ID. */
    public int getNextRegistrationId () {
        while (nextRegisterID != -2) {
            if (classResolver.getRegistration(nextRegisterID) == null) return nextRegisterID;
            nextRegisterID++;
        }
        throw new KryoException("No registration IDs are available.");
    }

• Kryo的getRegistration方法，当遇到class没有注册时会判断registrationRequired，如果为true，则抛出IllegalArgumentException；如果为false，则调用classResolver.registerImplicit进行隐式注册，同时如果warnUnregisteredClasses为true则会打印warning信息
• Kryo的register方法如果没有指定Serializer时，会通过getDefaultSerializer获取最匹配的Serializer，如果从已经注册的defaultSerializers没匹配到，则调用newDefaultSerializer创建一个，这里可能存在无法创建的异常，会抛出IllegalArgumentException
• register(Class type, Serializer serializer)方法最后是调用ClassResolver.register(Registration registration)方法，对于没有Registration的，这里new了一个，同时通过getNextRegistrationId，给Registration分配一个id

DefaultClassResolver.register

kryo-4.0.2-sources.jar!/com/esotericsoftware/kryo/util/DefaultClassResolver.java

    static public final byte NAME = -1;

    protected final IntMap<Registration> idToRegistration = new IntMap();
    protected final ObjectMap<Class, Registration> classToRegistration = new ObjectMap();
    protected IdentityObjectIntMap<Class> classToNameId;

    public Registration registerImplicit (Class type) {
        return register(new Registration(type, kryo.getDefaultSerializer(type), NAME));
    }

    public Registration register (Registration registration) {
        if (registration == null) throw new IllegalArgumentException("registration cannot be null.");
        if (registration.getId() != NAME) {
            if (TRACE) {
                trace("kryo", "Register class ID " + registration.getId() + ": " + className(registration.getType()) + " ("
                    + registration.getSerializer().getClass().getName() + ")");
            }
            idToRegistration.put(registration.getId(), registration);
        } else if (TRACE) {
            trace("kryo", "Register class name: " + className(registration.getType()) + " ("
                + registration.getSerializer().getClass().getName() + ")");
        }
        classToRegistration.put(registration.getType(), registration);
        if (registration.getType().isPrimitive()) classToRegistration.put(getWrapperClass(registration.getType()), registration);
        return registration;
    }

    public Registration writeClass (Output output, Class type) {
        if (type == null) {
            if (TRACE || (DEBUG && kryo.getDepth() == 1)) log("Write", null);
            output.writeVarInt(Kryo.NULL, true);
            return null;
        }
        Registration registration = kryo.getRegistration(type);
        if (registration.getId() == NAME)
            writeName(output, type, registration);
        else {
            if (TRACE) trace("kryo", "Write class " + registration.getId() + ": " + className(type));
            output.writeVarInt(registration.getId() + 2, true);
        }
        return registration;
    }

    protected void writeName (Output output, Class type, Registration registration) {
        output.writeVarInt(NAME + 2, true);
        if (classToNameId != null) {
            int nameId = classToNameId.get(type, -1);
            if (nameId != -1) {
                if (TRACE) trace("kryo", "Write class name reference " + nameId + ": " + className(type));
                output.writeVarInt(nameId, true);
                return;
            }
        }
        // Only write the class name the first time encountered in object graph.
        if (TRACE) trace("kryo", "Write class name: " + className(type));
        int nameId = nextNameId++;
        if (classToNameId == null) classToNameId = new IdentityObjectIntMap();
        classToNameId.put(type, nameId);
        output.writeVarInt(nameId, true);
        output.writeString(type.getName());
    }

    public void reset () {
        if (!kryo.isRegistrationRequired()) {
            if (classToNameId != null) classToNameId.clear(2048);
            if (nameIdToClass != null) nameIdToClass.clear();
            nextNameId = 0;
        }
    }

• DefaultClassResolver.register(Registration registration)方法里头针对registration的id进行了判断，如果是NAME(这里用-1表示)则注册到ObjectMap<Class, Registration> classToRegistration，如果有id不是NAME的，则注册到IntMap<Registration> idToRegistration
• 前面提到如果registrationRequired是false，则调用classResolver.registerImplicit进行隐式注册，这里可以看到registerImplicit注册的registration的id是NAME
• registration的id是NAME与否具体在writeClass中有体现(如果要序列化的类的字段中不仅仅有基本类型，还有未注册的类，会调用这里的writeClass方法)，从代码可以看到如果是NAME，则使用的是writeName；不是NAME的则直接使用output.writeVarInt(registration.getId() + 2, true)，写入int；writeName方法第一次遇到NAME的class时会给它生成一个nameId，然后放入到IdentityObjectIntMap<Class> classToNameId中，然后写入int，再写入class.getName，第二次遇到该class的时候，由于classToNameId中已经存在nameId，因而直接写入int；但是DefaultClassResolver的reset方法在registrationRequired是false这种情况下会调用classToNameId.clear(2048)，进行清空或者resize，这个时候一旦这个方法被调用，那么下次可能无法利用classToNameId用id替代className来序列化。

Kryo.writeObject

kryo-4.0.2-sources.jar!/com/esotericsoftware/kryo/Kryo.java

    /** Writes an object using the registered serializer. */
    public void writeObject (Output output, Object object) {
        if (output == null) throw new IllegalArgumentException("output cannot be null.");
        if (object == null) throw new IllegalArgumentException("object cannot be null.");
        beginObject();
        try {
            if (references && writeReferenceOrNull(output, object, false)) {
                getRegistration(object.getClass()).getSerializer().setGenerics(this, null);
                return;
            }
            if (TRACE || (DEBUG && depth == 1)) log("Write", object);
            getRegistration(object.getClass()).getSerializer().write(this, output, object);
        } finally {
            if (--depth == 0 && autoReset) reset();
        }
    }

    /** Resets unregistered class names, references to previously serialized or deserialized objects, and the
     * {@link #getGraphContext() graph context}. If {@link #setAutoReset(boolean) auto reset} is true, this method is called
     * automatically when an object graph has been completely serialized or deserialized. If overridden, the super method must be
     * called. */
    public void reset () {
        depth = 0;
        if (graphContext != null) graphContext.clear();
        classResolver.reset();
        if (references) {
            referenceResolver.reset();
            readObject = null;
        }

        copyDepth = 0;
        if (originalToCopy != null) originalToCopy.clear(2048);

        if (TRACE) trace("kryo", "Object graph complete.");
    }

• 这里要注意一下，writeObject方法在finally的时候判断如果depth为0且autoReset为true，会调用reset方法；而reset方法会调用classResolver.reset()，清空nameIdToClass以及classToNameId(classToNameId.clear(2048))

小结

• storm默认是用kryo来进行tuple的序列化，storm额外注册了byte[].class、ListDelegate.class、ArrayList.class、HashMap.class、HashSet.class、BigInteger.class、TransactionAttempt.class、Values.class、org.apache.storm.metric.api.IMetricsConsumer.DataPoint.class、org.apache.storm.metric.api.IMetricsConsumer.TaskInfo.class、ConsList.class、BackPressureStatus.class等类型
• Config.TOPOLOGY_FALL_BACK_ON_JAVA_SERIALIZATION(topology.fall.back.on.java.serialization)如果为true，则kryo.setRegistrationRequired(false)，也就是如果一个class没有在kryo进行注册，不会抛异常；这个命名可能存在歧义(不是使用java自身的序列化机制来进行fallback)，它实际上要表达的是对于遇到没有注册的class要不要fallback，如果不fallback则直接抛异常，如果fallback，则会进行隐式注册，在classToNameId不会被reset的前提下，第一次使用className来序列化，同时分配一个id写入classToNameId，第二次则直接使用classToNameId中获取到的id，也就相当于手工注册的效果
• Config.TOPOLOGY_TUPLE_SERIALIZER(topology.tuple.serializer，默认是org.apache.storm.serialization.types.ListDelegateSerializer)用于配置tuple的payload的序列化类
• Config.TOPOLOGY_KRYO_DECORATORS(topology.kryo.decorators)用于加载自定义的serialization，可以直接通过Config.registerDecorator注册一个IKryoDecorator，在decorate方法中对Kyro注册要序列化的class
• Config.TOPOLOGY_SKIP_MISSING_KRYO_REGISTRATIONS(topology.skip.missing.kryo.registrations，默认为false)这个属性容易跟Config.TOPOLOGY_FALL_BACK_ON_JAVA_SERIALIZATION(topology.fall.back.on.java.serialization)混淆起来，前者是storm自身的属性而后者storm包装的kryo的属性(registrationRequired)；Config.TOPOLOGY_SKIP_MISSING_KRYO_REGISTRATIONS配置的是在有自定义Config.TOPOLOGY_KRYO_DECORATORS的场景下，如果storm加载不到用户自定义的IKryoDecorator类时是skip还是抛异常
• Kryo的registrationRequired为false的话，则会自动对未注册的class进行隐式注册(注册到classToNameId)，只在第一次序列化的时候使用className，之后都用id替代，来节省空间；不过要注意的是如果Kryo的autoReset为true的话，那么classToNameId会被reset，因而隐式注册在非第一次遇到未注册的class的时候并不能一直走使用id代替className来序列化

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• Serialization
• Spark调优之Data Serialization
• Spark 2.0.2, double[], 使用Kyro序列化加速，和手动注册类名