Synchronous Subscriptions

Synchronous subscriptions require the application to wait for messages. This type of subscription is easy to set-up and use, but requires the application to deal with looping if multiple messages are expected. For situations where a single message is expected, synchronous subscriptions are sometimes easier to manage, depending on the language.

For example, to subscribe to the subject updates and receive a single message you could do:

Go

  1. nc, err := nats.Connect("demo.nats.io")
  2. if err != nil {
  3.     log.Fatal(err)
  4. }
  5. defer nc.Close()
  7. // Subscribe
  8. sub, err := nc.SubscribeSync("updates")
  9. if err != nil {
  10.     log.Fatal(err)
  11. }
  13. // Wait for a message
  14. msg, err := sub.NextMsg(10 * time.Second)
  15. if err != nil {
  16.     log.Fatal(err)
  17. }
  19. // Use the response
  20. log.Printf("Reply: %s", msg.Data)

Java

  1. Connection nc = Nats.connect("nats://demo.nats.io:4222");
  3. // Subscribe
  4. Subscription sub = nc.subscribe("updates");
  6. // Read a message
  7. Message msg = sub.nextMessage(Duration.ZERO);
  9. String str = new String(msg.getData(), StandardCharsets.UTF_8);
  10. System.out.println(str);
  12. // Close the connection
  13. nc.close();

JavaScript

  1. // node-nats subscriptions are always async.

Python

  1. # Asyncio NATS client currently does not have a sync subscribe API

Ruby

  1. # The Ruby NATS client subscriptions are all async.

C

  1. natsConnection      *conn      = NULL;
  2. natsSubscription    *sub       = NULL;
  3. natsMsg             *msg       = NULL;
  4. natsStatus          s          = NATS_OK;
  6. s = natsConnection_ConnectTo(&conn, NATS_DEFAULT_URL);
  8. // Subscribe
  9. if (s == NATS_OK)
  10.     s = natsConnection_SubscribeSync(&sub, conn, "updates");
  12. // Wait for messages
  13. if (s == NATS_OK)
  14.     s = natsSubscription_NextMsg(&msg, sub, 10000);
  16. if (s == NATS_OK)
  17. {
  18.     printf("Received msg: %s - %.*s\n",
  19.             natsMsg_GetSubject(msg),
  20.             natsMsg_GetDataLength(msg),
  21.             natsMsg_GetData(msg));
  23.     // Destroy message that was received
  24.     natsMsg_Destroy(msg);
  25. }
  27. (...)
  29. // Destroy objects that were created
  30. natsSubscription_Destroy(sub);
  31. natsConnection_Destroy(conn);

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