Flink异步I/O是一种基于操作系统异步I/O模型的高效IO模式，可以减少I/O操作的阻塞时间，提高程序的性能。本文主要介绍Flink异步I/O的原理及实现代码细节。
一、Flink异步I/O原理
Flink异步I/O的关键在于回调机制和Java NIO库。当程序需要进行I/O操作时，它不会像传统的阻塞I/O那样一直等待I/O操作的完成，而是立刻返回，然后通过注册回调函数的方式，在I/O操作完成后由系统自动调用回调函数来处理结果。Java NIO库则提供了高性能的非阻塞I/O IO模型和高效的Selector机制，可以监听多个通道上的事件，并选择处理已就绪的事件。这样，就可以利用Flink的并发性能进行异步I/O操作，提高程序效率。
二、Flink异步I/O的实现
Flink异步I/O的实现分为三个部分：数据源的开启、数据源的异步读取以及读取数据结果的回调处理。
- 数据源的开启
数据源的开启可以使用Flink提供的RichParallelSourceFunction接口实现。该接口中提供了open方法，可以在并行度中只执行一次，用于初始化一些资源。在open方法中，需要初始化异步读取I/O事件的Selector，注册感兴趣的事件和对应的回调函数。
public class AsyncDataSource extends RichParallelSourceFunction<String> {
private transient AsynchronousFileChannel channel;
private transient ByteBuffer buffer;
private transient Selector selector;
@Override
public void open(Configuration parameters) throws Exception {
super.open(parameters);
channel = AsynchronousFileChannel.open(Paths.get("data.txt"), StandardOpenOption.READ);
buffer = ByteBuffer.allocate(1024);
selector = Selector.open();
channel.read(buffer, 0, null, new CompletionHandler<Integer, Void>() {
@Override
public void completed(Integer result, Void attachment) {
if (result < 0) {
return;
}
buffer.flip();
String line = Charset.forName("UTF-8").decode(buffer).toString();
buffer.clear();
collector.collect(line);
channel.read(buffer, 0, null, this);
}
@Override
public void failed(Throwable exc, Void attachment) {
exc.printStackTrace();
}
});
channel.register(selector, SelectionKey.OP_READ);
}
@Override
public void run(SourceContext<String> ctx) throws Exception {
while (true) {
selector.select();
Set<SelectionKey> selectedKeys = selector.selectedKeys();
Iterator<SelectionKey> iterator = selectedKeys.iterator();
while (iterator.hasNext()) {
SelectionKey key = iterator.next();
if (key.isReadable()) {
channel.read(buffer, 0, null, new CompletionHandler<Integer, Void>() {
@Override
public void completed(Integer result, Void attachment) {
if (result < 0) {
return;
}
buffer.flip();
String line = Charset.forName("UTF-8").decode(buffer).toString();
buffer.clear();
collector.collect(line);
channel.read(buffer, 0, null, this);
}
@Override
public void failed(Throwable exc, Void attachment) {
exc.printStackTrace();
}
});
}
iterator.remove();
}
}
}
@Override
public void cancel() {
try {
channel.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}- 数据源的异步读取
异步读取数据源的方法可以通过Java NIO库提供的AsynchronousFileChannel类来实现，该类提供了异步读取文件的方法，可以通过注册回调函数的方式来处理读取结果。
channel.read(buffer, 0, null, new CompletionHandler<Integer, Void>() {
@Override
public void completed(Integer result, Void attachment) {
if (result < 0) {
return;
}
buffer.flip();
String line = Charset.forName("UTF-8").decode(buffer).toString();
buffer.clear();
collector.collect(line);
channel.read(buffer, 0, null, this);
}
@Override
public void failed(Throwable exc, Void attachment) {
exc.printStackTrace();
}
});- 读取数据结果的回调处理
读取数据结果的回调处理可以在异步读取数据源的方法中设置，利用CompletionHandler接口的completed方法来处理读取结果。在回调函数中，可以将读取的数据行数据转换成字符串，通过Flink的SourceContext将数据发送到下游算子处理。
@Override
public void completed(Integer result, Void attachment) {
if (result < 0) {
return;
}
buffer.flip();
String line = Charset.forName("UTF-8").decode(buffer).toString();
buffer.clear();
collector.collect(line);
channel.read(buffer, 0, null, this);
}完整代码如下：
public class AsyncDataSource extends RichParallelSourceFunction<String> {
private transient AsynchronousFileChannel channel;
private transient ByteBuffer buffer;
private transient Selector selector;
@Override
public void open(Configuration parameters) throws Exception {
super.open(parameters);
channel = AsynchronousFileChannel.open(Paths.get("data.txt"), StandardOpenOption.READ);
buffer = ByteBuffer.allocate(1024);
selector = Selector.open();
channel.read(buffer, 0, null, new CompletionHandler<Integer, Void>() {
@Override
public void completed(Integer result, Void attachment) {
if (result < 0) {
return;
}
buffer.flip();
String line = Charset.forName("UTF-8").decode(buffer).toString();
buffer.clear();
collector.collect(line);
channel.read(buffer, 0, null, this);
}
@Override
public void failed(Throwable exc, Void attachment) {
exc.printStackTrace();
}
});
channel.register(selector, SelectionKey.OP_READ);
}
@Override
public void run(SourceContext<String> ctx) throws Exception {
while (true) {
selector.select();
Set<SelectionKey> selectedKeys = selector.selectedKeys();
Iterator<SelectionKey> iterator = selectedKeys.iterator();
while (iterator.hasNext()) {
SelectionKey key = iterator.next();
if (key.isReadable()) {
channel.read(buffer, 0, null, new CompletionHandler<Integer, Void>() {
@Override
public void completed(Integer result, Void attachment) {
if (result < 0) {
return;
}
buffer.flip();
String line = Charset.forName("UTF-8").decode(buffer).toString();
buffer.clear();
collector.collect(line);
channel.read(buffer, 0, null, this);
}
@Override
public void failed(Throwable exc, Void attachment) {
exc.printStackTrace();
}
});
}
iterator.remove();
}
}
}
@Override
public void cancel() {
try {
channel.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
