一、概述
本例展示了如何通过 IPC 在调度于不同线程的两个代理之间传输缓冲区。在继续学习本示例之前，最好先复习一下Simplest Full Example ，因为该示例展示的是 IPC 通信，没有增加代理的复杂性。读者还应熟悉Media Driver
流程构建如下：
- 以默认模式运行的嵌入式Media Driver（发送器、接收器和指挥器的代理（an agent for Sender, Receiver, Conductor））（an embedded media driver running in default mode (an agent for Sender, Receiver, Conductor)）
- 通过publication发送 IPC 数据的代理（SendAgent）（an agent to send the IPC data over a publication (SendAgent)）
- 通过subscription接收 IPC 数据的代理（ReceiveAgent）（an agent to receive the IPC data over a subscription (ReceiveAgent)）
Code Sample overview
代码示例包含在 ipc-core 项目 com.aeroncookbook.ipc.agents 命名空间中的三个文件中。它们是：
StartHere.java- the class responsible for setting up Aeron and scheduling the agents;（负责设置 Aeron 和调度代理的类）SendAgent.java- the class holding the Agent responsible for sending data;（负责发送数据的代理的类）ReceiveAgent.java- the class holding the Agent responsible for receiving data.（负责接收数据的代理的类）
下文将对每个部分进行细分和讨论。
Execution Output
15:13:42.814 [main] starting
15:13:42.964 [receiver] received: 1000000二、StartHere.java
public static void main(String[] args)
{final String channel = "aeron:ipc";final int stream = 10;final int sendCount = 1_000_000;final IdleStrategy idleStrategySend = new BusySpinIdleStrategy();final IdleStrategy idleStrategyReceive = new BusySpinIdleStrategy();final ShutdownSignalBarrier barrier = new ShutdownSignalBarrier();//construct Media Driver, cleaning up media driver folder on start/stopfinal MediaDriver.Context mediaDriverCtx = new MediaDriver.Context().dirDeleteOnStart(true).threadingMode(ThreadingMode.SHARED).sharedIdleStrategy(new BusySpinIdleStrategy()).dirDeleteOnShutdown(true);final MediaDriver mediaDriver = MediaDriver.launchEmbedded(mediaDriverCtx);//construct Aeron, pointing at the media driver's folderfinal Aeron.Context aeronCtx = new Aeron.Context().aeronDirectoryName(mediaDriver.aeronDirectoryName());final Aeron aeron = Aeron.connect(aeronCtx);//construct the subs and pubsfinal Subscription subscription = aeron.addSubscription(channel, stream);final Publication publication = aeron.addPublication(channel, stream);//construct the agentsfinal SendAgent sendAgent = new SendAgent(publication, sendCount);final ReceiveAgent receiveAgent = new ReceiveAgent(subscription, barrier,sendCount);//construct agent runnersfinal AgentRunner sendAgentRunner = new AgentRunner(idleStrategySend,Throwable::printStackTrace, null, sendAgent);final AgentRunner receiveAgentRunner = new AgentRunner(idleStrategyReceive,Throwable::printStackTrace, null, receiveAgent);LOGGER.info("starting");//start the runnersAgentRunner.startOnThread(sendAgentRunner);AgentRunner.startOnThread(receiveAgentRunner);//wait for the final item to be received before closingbarrier.await();//close the resourcesreceiveAgentRunner.close();sendAgentRunner.close();aeron.close();mediaDriver.close();
}Constructing support objects
final String channel = "aeron:ipc";
final int stream = 10;
final int sendCount = 1000;
final IdleStrategy idleStrategySend = new BusySpinIdleStrategy();
final IdleStrategy idleStrategyReceive = new BusySpinIdleStrategy();
final ShutdownSignalBarrier barrier = new ShutdownSignalBarrier();这部分代码构建了一些支持对象。（This section of the code constructs a few support objects.）
- Line 1 holds the channel definition, in this case
aeron:ipc- Line 2 holds the stream ID to use, in this case 10
- Line 3 is the number of integers to send over IPC
- 第 4、5 行构建了代理使用的空闲策略（IdleStrategy）。在这种情况下，只要 doWork 工作周期返回 0，空闲策略就会忙于旋转。（Line 4,5 constructs the
IdleStrategyto be used by the agents. In this case, whenever the doWork duty cycle returns 0, the idle strategy will busy spin.）- 第 6 行是一个屏障，用于协调样本的关闭。一旦 ReceiveAgent 总共接收到一个发送计数整数，它就会向屏障发出信号，触发关闭。(Line 6 is a barrier that will be used to co-ordinate a shutdown of the sample. Once the
ReceiveAgenthas received a total ofsendCountintegers, it will signal the barrier, triggering the shutdown.)
Constructing the Media Driver
//construct Media Driver, cleaning up media driver folder on start/stop
final MediaDriver.Context mediaDriverCtx = new MediaDriver.Context().dirDeleteOnStart(true).threadingMode(ThreadingMode.SHARED).sharedIdleStrategy(new BusySpinIdleStrategy()).dirDeleteOnShutdown(true);
final MediaDriver mediaDriver = MediaDriver.launchEmbedded(mediaDriverCtx);本节代码使用定义的上下文构建Media Driver。上下文是一个对象，其中包含Media Driver的所有可选配置参数。在本例中，有两项配置被重写，以确保Media Driver在启动和关闭时整理Media Driver目录。一旦上下文准备就绪，Media Driver就会作为嵌入式代理启动。
See also: Media Driver
Constructing Aeron, the Publication and the Subscription
//construct Aeron, pointing at the media driver's folder
final Aeron.Context aeronCtx = new Aeron.Context().aeronDirectoryName(mediaDriver.aeronDirectoryName());
final Aeron aeron = Aeron.connect(aeronCtx);这部分代码再次使用 Context 构建 Aeron 对象。有了这个上下文，我们就能让 Aeron 知道Media Driver的 Aeron 目录在哪里。一旦上下文准备就绪，Aeron 对象就会连接到Media Driver。接下来，我们将使用之前定义的通道和流 id 创建 IPC 发布和订阅（IPC publication and subscription）。
//construct the subs and pubs
final Subscription subscription = aeron.addSubscription(channel, stream);
final Publication publication = aeron.addPublication(channel, stream);Constructing and scheduling the agents
//construct the agents
final SendAgent sendAgent = new SendAgent(publication, sendCount);
final ReceiveAgent receiveAgent = new ReceiveAgent(subscription, barrier, sendCount);//construct agent runners
final AgentRunner sendAgentRunner = new AgentRunner(idleStrategySend,Throwable::printStackTrace, null, sendAgent);
final AgentRunner receiveAgentRunner = new AgentRunner(idleStrategyReceive,Throwable::printStackTrace, null, receiveAgent);//start the runners
AgentRunner.startOnThread(sendAgentRunner);
AgentRunner.startOnThread(receiveAgentRunner);
这部分代码构建发送代理（SendAgent）和接收代理（ReceiveAgent），创建代理运行程序来管理它们，然后在特定线程上启动它们。关键行如下：
- 第 6-7 行和第 8-9 行：这两行分别构建了发送和接收的代理运行程序。请注意，每行都给出了空闲策略，用于控制线程在 doWork 工作周期后如何使用资源。
- 第 12 和 13 行：这两行为每个代理创建新线程，并开始工作周期。
Shutting down cleanly
//wait for the final item to be received before closing
barrier.await();//close the resources
receiveAgentRunner.close();
sendAgentRunner.close();
aeron.close();
mediaDriver.close();代码的最后部分负责等待 ReceiveAgent 触发屏障，然后正确清理资源。首先关闭代理，然后关闭 aeron 对象，最后关闭Media Driver。如果不注意关闭过程中的执行顺序，可能会出现核心转储或其他看似严重的故障。
三、SendAgent.java
public class SendAgent implements Agent
{private final Publication publication;private final int sendCount;private final UnsafeBuffer unsafeBuffer;private int currentCountItem = 1;private final Logger logger = LoggerFactory.getLogger(SendAgent.class);public SendAgent(final Publication publication, int sendCount){this.publication = publication;this.sendCount = sendCount;this.unsafeBuffer = new UnsafeBuffer(ByteBuffer.allocate(64));unsafeBuffer.putInt(0, currentCountItem);}@Overridepublic int doWork(){if (currentCountItem > sendCount){return 0;}if (publication.isConnected()){if (publication.offer(unsafeBuffer) > 0){currentCountItem += 1;unsafeBuffer.putInt(0, currentCountItem);}}return 0;}@Overridepublic String roleName(){return "sender";}
}send 对象负责通过提供的 Aeron Publication 发送 sendCount 整数。doWork 方法用于保持代理的工作周期，该方法会被持续调用，直至代理关闭。一旦达到 sendCount 限制，它就会停止向publication发送更多信息，并开始闲置。
这段代码中最有趣的部分是：
- Line 18 to 34: the doWork method holding the duty cycle for this agent
- 第 22 行和第 34 行：这两条返回语句都返回 0，这将导致选定的空闲策略
BusySpinIdleStrategy调用ThreadHints.onSpinWait() - 第 25 行：只有当publication已连接时，才会返回 true。一旦连接，就可以安全地向publication提供信息。
- 第 27 行：这将为publication提供缓冲数据。
- Line 30: this logs the last sent integer, for example
15:13:42.818 [sender] sent: 123 - Line 41: this sets the thread name to
sender, as is visible in the log output.
四、ReceiveAgent.java
public class ReceiveAgent implements Agent
{private final Subscription subscription;private final ShutdownSignalBarrier barrier;private final int sendCount;private final Logger logger = LoggerFactory.getLogger(ReceiveAgent.class);public ReceiveAgent(final Subscription subscription,ShutdownSignalBarrier barrier, int sendCount){this.subscription = subscription;this.barrier = barrier;this.sendCount = sendCount;}@Overridepublic int doWork() throws Exception{subscription.poll(this::handler, 1000);return 0;}private void handler(DirectBuffer buffer, int offset, int length,Header header){final int lastValue = buffer.getInt(offset);if (lastValue >= sendCount){logger.info("received: {}", lastValue);barrier.signal();}}@Overridepublic String roleName(){return "receiver";}
}接收代理负责轮询所提供的订阅并记录接收到的值。一旦达到 sendCount 值，接收代理就会发出屏障信号。该对象中最有趣的部分是：
- 第 17-21 行 - doWork 方法保持着该代理的duty cycle 。duty cycle由两部分组成，一部分是轮询订阅，将事件传递给提供的处理程序，另一部分是返回 0。通过配置的 IdleStrategy，返回 0 将导致线程停顿一微秒。
- Line 26 - this logs the integer value received, for example:
15:13:42.814 [receiver] received: 5 - Lines 29-32 - this signals the barrier, triggering the clean shutdown of the process.
- Line 38 - this sets the role name to
receiver, as visible in log output.
五、Performance
在英特尔笔记本电脑上，本示例每秒可传输约 1 千万条 4 字节信息。如果使用的是 Linux 系统，且有可用的 /dev/shm，代码会自动使用。通过交换
NoOpIdleStrategy，并将media driver线程移至DEDICATED，每秒可传输超过 2000 万条信息。主要更改见下文。请注意，您需要确保硬件上至少有 8 个物理内核。
final IdleStrategy idleStrategySend = new NoOpIdleStrategy();
final IdleStrategy idleStrategyReceive = new NoOpIdleStrategy();
final ShutdownSignalBarrier barrier = new ShutdownSignalBarrier();//construct Media Driver, cleaning up media driver folder on start/stop
final MediaDriver.Context mediaDriverCtx = new MediaDriver.Context().dirDeleteOnStart(true).threadingMode(ThreadingMode.DEDICATED).conductorIdleStrategy(new BusySpinIdleStrategy()).senderIdleStrategy(new NoOpIdleStrategy()).receiverIdleStrategy(new NoOpIdleStrategy()).dirDeleteOnShutdown(true);
final MediaDriver mediaDriver = MediaDriver.launchEmbedded(mediaDriverCtx);//construct Aeron, pointing at the media driver's folder
final Aeron.Context aeronCtx = new Aeron.Context().idleStrategy(new NoOpIdleStrategy()).aeronDirectoryName(mediaDriver.aeronDirectoryName());
final Aeron aeron = Aeron.connect(aeronCtx); 
有一个相关的 Two Agent example of OneToOneRingBuffer 非常相似，只不过它使用了 Agrona 的 OneToOneRingBuffer，并通过 BusySpinIdleStrategy 每秒发送大约 1800 万条 4 字节信息，或通过 NoOpIdleStrategy 每秒发送超过 5000 万条信息。
六、Using the C Media Driver
要使用 Aeron 的 C Media Driver测试此示例，您需要执行以下操作：
首先，从源代码中构建 C Media Driver（说明因操作系统而异，此部分参考博客即可，建议翻墙进行操作，贴出cookbook只是帮助借阅）：
- Building the C Media Driver on macOS
- Building the C Media Driver on CentOS Linux 8
- Building the C Media Driver on Ubuntu 20.04
- Building the C Media Driver on Windows 10
Next, start the C Media Driver with default settings
./aeronmd(Linux/macOS)aeronmd(Windows)
Then, remove the Media Driver from StartHere.java, and reduce the Aeron context to defaults:
//construct Media Driver, cleaning up media driver folder on start/stop
//final MediaDriver.Context mediaDriverCtx = new MediaDriver.Context()
// .dirDeleteOnStart(true)
// .threadingMode(ThreadingMode.SHARED)
// .sharedIdleStrategy(new BusySpinIdleStrategy())
// .dirDeleteOnShutdown(true);
//final MediaDriver mediaDriver = MediaDriver.launchEmbedded(mediaDriverCtx);//construct Aeron, pointing at the media driver's folder
final Aeron.Context aeronCtx = new Aeron.Context();
final Aeron aeron = Aeron.connect(aeronCtx);Aeron 和Media Driver将默认使用同一目录。
最后，正常运行 StartHere.java。进程应正常运行，输出应包括类似内容：
14:30:00.293 [main] starting
14:30:00.758 [receiver] received: 10000000
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