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Transaction接口是基于flume的稳定性考虑的。所有主要的组件（sources、sinks、channels)都必须使用Flume Transaction。我们也可以理解Transaction接口就是flume的事务，sources和sinks的发送数据与接受数据都是在一个Transaction里完成的。
从上图中可以看出，一个Transaction在Channel实现内实现。每一个连接到channel的source和sink都要获取一个Transaction对象。这Sources实际上使用了一个ChannelSelector接口来封装Transaction。存放事件到channel和从channel中提取事件的操作是在一个活跃的Transaction内执行的。
下面是官网例子
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Channel ch = new MemoryChannel();
Transaction txn = ch.getTransaction();
txn.begin();
try {
// This try clause includes whatever Channel operations you want to do
Event eventToStage = EventBuilder.withBody("Hello Flume!",
Charset.forName("UTF-8"));
ch.put(eventToStage);
// Event takenEvent = ch.take();
// ...
txn.commit();
} catch (Throwable t) {
txn.rollback();
// Log exception, handle individual exceptions as needed
// re-throw all Errors
if (t instanceof Error) {
throw (Error)t;
}
} finally {
txn.close();
}
上面的代码是一个很简单的Transaction示例，在自定义Source与自定义Sink中都要使用。
Sink提取event数据从channel中，然后直接将数据发送到下一个flume agent中或者存储到外部库中。
Sink和channel的关联关系可以在配置文件中配置。有一个SinkRunner实例与每一个已配置的Sink关联，当Flume框架调用SinkRunner.start()方法时候，将创建一个新的线程来驱动这Sink。
这个线程将管理这个Sink的生命周期。Sink需要实现LifecycleAware接口的start()和stop()方法。start()方法用于初始化数据；stop()用于释放资源；process()是从channel中提取event数据和转发数据的核心方法。
这Sink需要实现Configurable接口以便操作配置文件。
下面是官网例子：
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public class MySink extends AbstractSink implements Configurable {
private String myProp;
@Override
public void configure(Context context) {
String myProp = context.getString("myProp", "defaultValue");
// Process the myProp value (e.g. validation)
// Store myProp for later retrieval by process() method
this.myProp = myProp;
}
@Override
public void start() {
// Initialize the connection to the external repository (e.g. HDFS) that
// this Sink will forward Events to ..
}
@Override
public void stop () {
// Disconnect from the external respository and do any
// additional cleanup (e.g. releasing resources or nulling-out
// field values) ..
}
@Override
public Status process() throws EventDeliveryException {
Status status = null;
// Start transaction
Channel ch = getChannel();
Transaction txn = ch.getTransaction();
txn.begin();
try {
// This try clause includes whatever Channel operations you want to do
Event event = ch.take();
// Send the Event to the external repository.
// storeSomeData(e);
txn.commit();
status = Status.READY;
} catch (Throwable t) {
txn.rollback();
// Log exception, handle individual exceptions as needed
status = Status.BACKOFF;
// re-throw all Errors
if (t instanceof Error) {
throw (Error)t;
}
} finally {
txn.close();
}
return status;
}
}
下面是测试例子：
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import org.apache.flume.Channel;
import org.apache.flume.Context;
import org.apache.flume.Event;
import org.apache.flume.EventDeliveryException;
import org.apache.flume.Transaction;
import org.apache.flume.conf.Configurable;
import org.apache.flume.sink.AbstractSink;
public class Custom_Sink extends AbstractSink implements Configurable {
private String myProp;
@Override
public void configure(Context context) {
String myProp = context.getString("myProp", "defaultValue");
// Process the myProp value (e.g. validation)
// Store myProp for later retrieval by process() method
this.myProp = myProp;
}
@Override
public void start() {
// Initialize the connection to the external repository (e.g. HDFS) that
// this Sink will forward Events to ..
}
@Override
public void stop () {
// Disconnect from the external respository and do any
// additional cleanup (e.g. releasing resources or nulling-out
// field values) ..
}
@Override
public Status process() throws EventDeliveryException {
Status status = null;
// Start transaction
Channel ch = getChannel();
Transaction txn = ch.getTransaction();
txn.begin();
try {
// This try clause includes whatever Channel operations you want to do
Event event = ch.take();
String out = new String(event.getBody());
// Send the Event to the external repository.
// storeSomeData(e);
System.out.println(out);
txn.commit();
status = Status.READY;
} catch (Throwable t) {
txn.rollback();
// Log exception, handle individual exceptions as needed
status = Status.BACKOFF;
// re-throw all Errors
if (t instanceof Error) {
throw (Error)t;
}
} finally {
txn.close();
}
return status;
}
}
上面的测试例子只输出事件的BODY信息，这里说明下直接用代码event.getBody().tostring() 输出是乱码。因为所有sink都是在Transaction里完成的，因此自定义开发sink是需要加上Transaction相关设置。
然后是测试配置，这里是自定义的jar 包是flumedev.Custom_Sink。注意，打包之后请放在目录$FLUME_HOME/lib下
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#配置文件：custom_sink_case23.conf
# Name the components on this agent
a1.sources = r1
a1.sinks = k1
a1.channels = c1
# Describe/configure the source
a1.sources.r1.type = syslogtcp
a1.sources.r1.port = 50000
a1.sources.r1.bind = 192.168.233.128
a1.sources.r1.channels = c1
# Describe the sink
a1.sinks.k1.channel = c1
a1.sinks.k1.type = flumedev.Custom_Sink
#a1.sinks.k1.type =logger
# Use a channel which buffers events in memory
a1.channels.c1.type = memory
a1.channels.c1.capacity = 1000
a1.channels.c1.transactionCapacity = 100
#敲命令
flume-ng agent -cconf -f conf/custom_sink_case23.conf -n a1 -Dflume.root.logger=INFO,console
启动成功后
打开另一个终端输入，往侦听端口送数据
echo "testcustom_sink" | nc 192.168.233.128 50000
#在启动的终端查看console输出
可以看到数据正常输出。
Source从外面接收数据并把数据存入Channel中。很少有人用。
下面是官网的例子
[java] view plain copy
public class MySource extends AbstractSource implements Configurable, PollableSource {
private String myProp;
@Override
public void configure(Context context) {
String myProp = context.getString("myProp", "defaultValue");
// Process the myProp value (e.g. validation, convert to another type, ...)
// Store myProp for later retrieval by process() method
this.myProp = myProp;
}
@Override
public void start() {
// Initialize the connection to the external client
}
@Override
public void stop () {
// Disconnect from external client and do any additional cleanup
// (e.g. releasing resources or nulling-out field values) ..
}
@Override
public Status process() throws EventDeliveryException {
Status status = null;
// Start transaction
Channel ch = getChannel();
Transaction txn = ch.getTransaction();
txn.begin();
try {
// This try clause includes whatever Channel operations you want to do
// Receive new data
Event e = getSomeData();
// Store the Event into this Source‘s associated Channel(s)
getChannelProcessor().processEvent(e)
txn.commit();
status = Status.READY;
} catch (Throwable t) {
txn.rollback();
// Log exception, handle individual exceptions as needed
status = Status.BACKOFF;
// re-throw all Errors
if (t instanceof Error) {
throw (Error)t;
}
} finally {
txn.close();
}
return status;
}
}
测试的话，主要针对Event e 这里进行传输数据，这里就不测试了。
官网说待定。
下面是美团网的自定义Channel 开发，下面是链接
http://tech.meituan.com/mt-log-system-optimization.html
……
Flume本身提供了MemoryChannel和FileChannel。MemoryChannel处理速度快，但缓存大小有限，且没有持久化；FileChannel则刚好相反。我们希望利用两者的优势，在Sink处理速度够快，Channel没有缓存过多日志的时候，就使用MemoryChannel，当Sink处理速度跟不上，又需要Channel能够缓存下应用端发送过来的日志时，就使用FileChannel，由此我们开发了DualChannel，能够智能的在两个Channel之间切换。
其具体的逻辑如下：
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/***
* putToMemChannel indicate put event to memChannel or fileChannel
* takeFromMemChannel indicate take event from memChannel or fileChannel
* */
private AtomicBoolean putToMemChannel = new AtomicBoolean(true);
private AtomicBoolean takeFromMemChannel = new AtomicBoolean(true);
void doPut(Event event) {
if (switchon && putToMemChannel.get()) {
//往memChannel中写数据
memTransaction.put(event);
if ( memChannel.isFull() || fileChannel.getQueueSize() > 100) {
putToMemChannel.set(false);
}
} else {
//往fileChannel中写数据
fileTransaction.put(event);
}
}
Event doTake() {
Event event = null;
if ( takeFromMemChannel.get() ) {
//从memChannel中取数据
event = memTransaction.take();
if (event == null) {
takeFromMemChannel.set(false);
}
} else {
//从fileChannel中取数据
event = fileTransaction.take();
if (event == null) {
takeFromMemChannel.set(true);
putToMemChannel.set(true);
}
}
return event;
}
这里要说明下，官网是建议使用file channel，虽然它的效率比较低，但是它能保证数据完整性，而memory channel效率高，但是只能对数据丢失和重复不太敏感的业务使用
Flume NG 学习笔记（十） Transaction、Sink、Source和Channel开发
原文地址：http://jackwxh.blog.51cto.com/2850597/1906903
