如果还没看过Flume-ng源码解析之启动流程，可以点击 Flume-ng源码解析之启动流程 查看
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.Stable
public 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.Stable
public 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.Stable
public 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.Stable
public abstract class BasicChannelSemantics extends AbstractChannel {
private ThreadLocal<BasicTransactionSemantics> currentTransaction
= new ThreadLocal<BasicTransactionSemantics>();
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<Event> takeList;
private LinkedBlockingDeque<Event> putList;
private final ChannelCounter channelCounter;
private int putByteCounter = 0;
private int takeByteCounter = 0;
public MemoryTransaction(int transCapacity, ChannelCounter counter) {
putList = new LinkedBlockingDeque<Event>(transCapacity);
takeList = new LinkedBlockingDeque<Event>(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<Event> 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<Channel> var1);
List<Channel> getRequiredChannels(Event var1);
List<Channel> getOptionalChannels(Event var1);
List<Channel> 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<Channel> channels,
Map<String, String> config) {
...
}
public static ChannelSelector create(List<Channel> 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<? extends ChannelSelector> selectorClass =
(Class<? extends ChannelSelector>) 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-1
agent_foo.sources.avro-AppSrv-source1.selector.mapping.AZ = file-channel-2
agent_foo.sources.avro-AppSrv-source1.selector.mapping.NY = mem-channel-1 file-channel-2
agent_foo.sources.avro-AppSrv-source1.selector.optional.CA = mem-channel-1 file-channel-2
agent_foo.sources.avro-AppSrv-source1.selector.mapping.AZ = file-channel-2
agent_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<String, List<Channel>> channelMapping;
private Map<String, List<Channel>> optionalChannels;
private List<Channel> 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
- 如果是replicating的话，那么如果没有指定optional，那么全部channel都有，如果某个channel指定为option的话，那么就要从requiredChannel移除，只发送给optionalchannel
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。