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1 接口介绍
组件的分析顺序是按照上一篇中启动顺序来分析的，首先是Channel，然后是Sink，最后是Source，在开始看组件源码之前我们先来看一下两个重要的接口，一个是LifecycleAware ，另一个是NamedComponent
1.1 LifecycleAware
@InterfaceAudience.Public@InterfaceStability.Stablepublic interface LifecycleAware { public void start(); public void stop(); public LifecycleState getLifecycleState();
}非常简单就是三个方法，start()、stop()和getLifecycleState，这个接口是flume好多类都要实现的接口，包括Flume-ng源码解析之启动流程
所中提到PollingPropertiesFileConfigurationProvider()，只要涉及到生命周期的都会实现该接口，当然组件们也是要实现的！
1.2 NamedComponent
@InterfaceAudience.Public@InterfaceStability.Stablepublic interface NamedComponent { public void setName(String name); public String getName();
}这个没什么好讲的，就是用来设置名字的。
2 Channel
作为Flume三大核心组件之一的Channel，我们有必要来看看它的构成：
@InterfaceAudience.Public@InterfaceStability.Stablepublic interface Channel extends LifecycleAware, NamedComponent { public void put(Event event) throws ChannelException; public Event take() throws ChannelException; public Transaction getTransaction();
}那么从上面的接口中我们可以看到Channel的主要功能就是put()和take()，那么我们就来看一下它的具体实现。这里我们选择MemoryChannel作为例子，但是MemoryChannel太长了，我们就截取一小段来看看
public class MemoryChannel extends BasicChannelSemantics { private static Logger LOGGER = LoggerFactory.getLogger(MemoryChannel.class); private static final Integer defaultCapacity = Integer.valueOf(100); private static final Integer defaultTransCapacity = Integer.valueOf(100);
public MemoryChannel() {
}
...
}我们又看到它继承了BasicChannelSemantics ，从名字我们可以看出它是一个基础的Channel，我们继续看看看它的实现
@InterfaceAudience.Public@InterfaceStability.Stablepublic abstract class BasicChannelSemantics extends AbstractChannel { private ThreadLocal currentTransaction
= new ThreadLocal(); private boolean initialized = false; protected void initialize() {} protected abstract BasicTransactionSemantics createTransaction(); @Override
public void put(Event event) throws ChannelException {
BasicTransactionSemantics transaction = currentTransaction.get();
Preconditions.checkState(transaction != null, "No transaction exists for this thread");
transaction.put(event);
} @Override
public Event take() throws ChannelException {
BasicTransactionSemantics transaction = currentTransaction.get();
Preconditions.checkState(transaction != null, "No transaction exists for this thread"); return transaction.take();
} @Override
public Transaction getTransaction() { if (!initialized) { synchronized (this) { if (!initialized) {
initialize();
initialized = true;
}
}
}
BasicTransactionSemantics transaction = currentTransaction.get(); if (transaction == null || transaction.getState().equals(
BasicTransactionSemantics.State.CLOSED)) {
transaction = createTransaction();
currentTransaction.set(transaction);
} return transaction;
}
}找了许久，终于发现了put()和take()，但是仔细一看，它们内部调用的是BasicTransactionSemantics 的put()和take()，有点失望，继续来看看BasicTransactionSemantics
public abstract class BasicTransactionSemantics implements Transaction { private State state; private long initialThreadId; protected void doBegin() throws InterruptedException {} protected abstract void doPut(Event event) throws InterruptedException; protected abstract Event doTake() throws InterruptedException; protected abstract void doCommit() throws InterruptedException; protected abstract void doRollback() throws InterruptedException; protected void doClose() {} protected BasicTransactionSemantics() {
state = State.NEW;
initialThreadId = Thread.currentThread().getId();
} protected void put(Event event) {
Preconditions.checkState(Thread.currentThread().getId() == initialThreadId, "put() called from different thread than getTransaction()!");
Preconditions.checkState(state.equals(State.OPEN), "put() called when transaction is %s!", state);
Preconditions.checkArgument(event != null, "put() called with null event!"); try {
doPut(event);
} catch (InterruptedException e) {
Thread.currentThread().interrupt(); throw new ChannelException(e.toString(), e);
}
} protected Event take() {
Preconditions.checkState(Thread.currentThread().getId() == initialThreadId, "take() called from different thread than getTransaction()!");
Preconditions.checkState(state.equals(State.OPEN), "take() called when transaction is %s!", state); try { return doTake();
} catch (InterruptedException e) {
Thread.currentThread().interrupt(); return null;
}
} protected State getState() { return state;
}
...//我们这里只是讨论put和take，所以一些暂时不涉及的方法就被我干掉，有兴趣恩典朋友可以自行阅读
protected static enum State {
NEW, OPEN, COMPLETED, CLOSED
}
}又是一个抽象类，put()和take()内部调用的还是抽象方法doPut()和doTake()，看到这里，我相信没有耐心的同学已经崩溃了，但是就差最后一步了，既然是抽象类，那么最终Channel所使用的肯定是它的一个实现类，这时候我们可以回到一开始使用的MemoryChannel，到里面找找有没有线索，一看，MemoryChannel中就藏着个内部类
private class MemoryTransaction extends BasicTransactionSemantics { private LinkedBlockingDeque takeList; private LinkedBlockingDeque putList; private final ChannelCounter channelCounter; private int putByteCounter = 0; private int takeByteCounter = 0; public MemoryTransaction(int transCapacity, ChannelCounter counter) {
putList = new LinkedBlockingDeque(transCapacity);
takeList = new LinkedBlockingDeque(transCapacity);
channelCounter = counter;
}
@Override protected void doPut(Event event) throws InterruptedException {
channelCounter.incrementEventPutAttemptCount(); int eventByteSize = (int) Math.ceil(estimateEventSize(event) / byteCapacitySlotSize); if (!putList.offer(event)) { throw new ChannelException( "Put queue for MemoryTransaction of capacity " +
putList.size() + " full, consider committing more frequently, " + "increasing capacity or increasing thread count");
}
putByteCounter += eventByteSize;
}
@Override protected Event doTake() throws InterruptedException {
channelCounter.incrementEventTakeAttemptCount(); if (takeList.remainingCapacity() == 0) { throw new ChannelException("Take list for MemoryTransaction, capacity " +
takeList.size() + " full, consider committing more frequently, " + "increasing capacity, or increasing thread count");
} if (!queueStored.tryAcquire(keepAlive, TimeUnit.SECONDS)) { return null;
}
Event event;
synchronized (queueLock) { event = queue.poll();
}
Preconditions.checkNotNull(event, "Queue.poll returned NULL despite semaphore " + "signalling existence of entry");
takeList.put(event); int eventByteSize = (int) Math.ceil(estimateEventSize(event) / byteCapacitySlotSize);
takeByteCounter += eventByteSize; return event;
} //...依然删除暂时不需要的方法
}在这个类中我们可以看到doPut()和doTake()的实现方法，也明白MemoryChannel的put()和take()最终调用的是MemoryTransaction 的doPut()和doTake()。
有朋友看到这里以为这次解析就要结束了，其实好戏还在后头，Channel中还有两个重要的类ChannelProcessor和ChannelSelector，耐心地听我慢慢道来。
3 ChannelProcessor
ChannelProcessor 的作用就是执行put操作，将数据放到channel里面。每个ChannelProcessor实例都会配备一个ChannelSelector来决定event要put到那个channl当中
public class ChannelProcessor implements Configurable { private static final Logger LOG = LoggerFactory.getLogger(ChannelProcessor.class); private final ChannelSelector selector; private final InterceptorChain interceptorChain; public ChannelProcessor(ChannelSelector selector) { this.selector = selector; this.interceptorChain = new InterceptorChain();
} public void initialize() { this.interceptorChain.initialize();
} public void close() { this.interceptorChain.close();
} public void configure(Context context) { this.configureInterceptors(context);
} private void configureInterceptors(Context context) { //配置拦截器
} public ChannelSelector getSelector() { return this.selector;
} public void processEventBatch(List events) {
... while(i$.hasNext()) {
Event optChannel = (Event)i$.next();
List tx = this.selector.getRequiredChannels(optChannel);
...//将event放到Required队列
t1 = this.selector.getOptionalChannels(optChannel);
Object eventQueue;
...//将event放到Optional队列
}
...//event的分配操作
} public void processEvent(Event event) { event = this.interceptorChain.intercept(event); if(event != null) {
List requiredChannels = this.selector.getRequiredChannels(event);
Iterator optionalChannels = requiredChannels.iterator();
...//event的分配操作
List optionalChannels1 = this.selector.getOptionalChannels(event);
Iterator i$1 = optionalChannels1.iterator();
...//event的分配操作
}
}
}为了简化代码，我进行了一些删除，只保留需要讲解的部分，说白了Channel中的两个写入方法，都是需要从作为参数传入的selector中获取对应的channel来执行event的put操作。接下来我们来看看ChannelSelector
4 ChannelSelector
ChannelSelector是一个接口，我们可以通过ChannelSelectorFactory来创建它的子类，Flume提供了两个实现类MultiplexingChannelSelector和ReplicatingChannelSelector。
public interface ChannelSelector extends NamedComponent, Configurable { void setChannels(List var1); List getRequiredChannels(Event var1); List getOptionalChannels(Event var1); List getAllChannels();
}通过ChannelSelectorFactory 的create来创建，create中调用getSelectorForType来获得一个selector，通过配置文件中的type来创建相应的子类
public class ChannelSelectorFactory { private static final Logger LOGGER = LoggerFactory.getLogger(
ChannelSelectorFactory.class); public static ChannelSelector create(List channels,
Map config) {
...
} public static ChannelSelector create(List channels,
ChannelSelectorConfiguration conf) {
String type = ChannelSelectorType.REPLICATING.toString(); if (conf != null) { type = conf.getType();
}
ChannelSelector selector = getSelectorForType(type);
selector.setChannels(channels);
Configurables.configure(selector, conf); return selector;
} private static ChannelSelector getSelectorForType(String type) {
if (type == null || type.trim().length() == 0) { return new ReplicatingChannelSelector();
}
String selectorClassName = type;
ChannelSelectorType selectorType = ChannelSelectorType.OTHER; try {
selectorType = ChannelSelectorType.valueOf(type.toUpperCase(Locale.ENGLISH));
} catch (IllegalArgumentException ex) {
LOGGER.debug("Selector type {} is a custom type", type);
} if (!selectorType.equals(ChannelSelectorType.OTHER)) {
selectorClassName = selectorType.getChannelSelectorClassName();
}
ChannelSelector selector = null; try {
@SuppressWarnings("unchecked")
Class selectorClass =
(Class) Class.forName(selectorClassName);
selector = selectorClass.newInstance();
} catch (Exception ex) {
throw new FlumeException("Unable to load selector type: " + type
+ ", class: " + selectorClassName, ex);
} return selector;
}
}对于这两种Selector简单说一下：
1）MultiplexingChannelSelector
下面是一个channel selector 配置文件
agent_foo.sources.avro-AppSrv-source1.selector.type = multiplexing
agent_foo.sources.avro-AppSrv-source1.selector.header = State
agent_foo.sources.avro-AppSrv-source1.selector.mapping.CA = mem-channel-1agent_foo.sources.avro-AppSrv-source1.selector.mapping.AZ = file-channel-2agent_foo.sources.avro-AppSrv-source1.selector.mapping.NY = mem-channel-1 file-channel-2agent_foo.sources.avro-AppSrv-source1.selector.optional.CA = mem-channel-1 file-channel-2agent_foo.sources.avro-AppSrv-source1.selector.mapping.AZ = file-channel-2agent_foo.sources.avro-AppSrv-source1.selector.default = mem-channel-1
MultiplexingChannelSelector类中定义了三个属性，用于存储不同类型的channel
private Map> channelMapping; private Map> optionalChannels; private List defaultChannels;
那么具体分配原则如下：
如果设置了maping，那么会event肯定会给指定的channel，如果同时设置了optional，也会发送给optionalchannel
如果没有设置maping，设置default，那么event会发送给defaultchannel，如果还同时设置了optional，那么也会发送给optionalchannel
如果maping和default都没指定，如果有指定option，那么会发送给optionalchannel，但是发送给optionalchannel不会进行失败重试
2）ReplicatingChannelSelector
分配原则比较简单
5 总结：
作为一个承上启下的组件，Channel的作用就是将source来的数据通过自己流向sink，那么ChannelProcessor就起到将event put到分配好的channel中，而分配的规则是由selector决定的，flume提供的selector有multiplexing和replicating两种。所以ChannelProcessor一般都是在Source中被调用。那么Channel的take()肯定是在Sink中调用的。
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