NATS 持久消息 Java 客户端

Question

有没有人有将 NATS 流媒体服务器与 Java 客户端结合使用的经验？具体来说，我不知道如何使用 Java 客户端检索订阅者离线时发送的消息。

我可以看到使用 Go client that I can publish a message and later add a subscription to retrieve all published messages. This is in the NATS Streaming Getting Started 文档并且它像宣传的那样工作。

Publish several messages. For each publication you should get a result.

$ cd $GOPATH/src/github.com/nats-io/go-nats-streaming/examples
go run stan-pub.go foo "msg one"
Published [foo] : 'msg one'
$ go run stan-pub.go foo "msg two"
Published [foo] : 'msg two'
$ go run stan-pub.go foo "msg three"
Published [foo] : 'msg three'

Run the subscriber client. Use the --all flag to receive all published messages.

$ go run stan-sub.go --all -c test-cluster -id myID foo
Connected to nats://localhost:4222 clusterID: [test-cluster] clientID: [myID]
subscribing with DeliverAllAvailable
Listening on [foo], clientID=[myID], qgroup=[] durable=[]
[#1] Received on [foo]: 'sequence:1 subject:"foo" data:"msg one" timestamp:1465962202884478817 '
[#2] Received on [foo]: 'sequence:2 subject:"foo" data:"msg two" timestamp:1465962208545003897 '
[#3] Received on [foo]: 'sequence:3 subject:"foo" data:"msg three" timestamp:1465962215567601196

我正在尝试使用 NATS Java client 执行此操作。我不知道是不是我没有找到类似的方法调用，还是 Java 客户端中不存在该功能。

这是我试过的方法

    import io.nats.client.Connection;
import io.nats.client.ConnectionFactory;
import io.nats.client.Constants;
import io.nats.client.Message;
import io.nats.client.SyncSubscription;

import java.io.IOException;
import java.security.SecureRandom;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;

public class NatsTest2 {

  private static final SecureRandom random = new SecureRandom();

  public static void main(String... args) throws Exception {
    final ConnectionFactory factory = new ConnectionFactory(Constants.DEFAULT_URL);
    try (final Connection conn = factory.createConnection()) {
      // Simple Async Subscriber
      final String expectMessage = "Yum, cookies " + System.currentTimeMillis();
      works(conn, expectMessage);
      broken(conn, expectMessage);
    }
  }

  private static void works(Connection conn, String expectMessage) throws IOException, TimeoutException {
    final String queue = Long.toString(random.nextLong());
    System.out.print(queue + "=>");
    try (final SyncSubscription subscription = conn.subscribeSync(queue)) {
      conn.publish(queue, expectMessage.getBytes());
      subscribe(subscription);
    }
  }

  private static void broken(Connection conn, String expectMessage) throws IOException, TimeoutException {
    final String queue = Long.toString(random.nextLong());
    System.out.print(queue + "=>");
    conn.publish(queue, expectMessage.getBytes());
    try (final SyncSubscription subscription = conn.subscribeSync(queue)) {
      subscribe(subscription);
    }
  }

  private static void subscribe(SyncSubscription subscription) throws IOException, TimeoutException {
    final Message message = subscription.nextMessage(1, TimeUnit.SECONDS);
    System.out.println(new String(message.getData()));
  }
}

这给出了输出

-8522002637987832314=>Yum, cookies 1473462495040
-3024385525006291780=>Exception in thread "main" java.util.concurrent.TimeoutException: Channel timed out waiting for items

Answer 1

如果您使用 nats-streaming-server，则需要使用 java-nats-streaming 客户端。您正在寻找的功能（订阅历史消息）仅存在于该客户端。

无论如何，这就是为什么你看到了你对 jnats 客户端所做的事情：

nats-streaming-server 当前嵌入了一个 NATS 服务器 (gnatsd)，因此允许常规 NATS 客户端使用标准 NATS 功能，这就是您所看到的。

在您的示例代码中，works() 恰好有效，因为您的订阅在您发布消息之前已经创建（换句话说，您的 try-with-resources 块保证订阅在其他任何事情之前已经处于活动状态发生）。因此，您实际上并没有收到过去发布的消息；您收到的消息是在订阅开始后发布的。

broken()示例不工作，因为它实际上是在创建订阅之前发布消息，并且该消息被服务器丢弃，因为（还）没有兴趣。

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