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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:

{% tabs %} {% tab title="Go" %}

nc, err := nats.Connect("demo.nats.io")
if err != nil {
    log.Fatal(err)
}
defer nc.Close()

// Subscribe
sub, err := nc.SubscribeSync("updates")
if err != nil {
    log.Fatal(err)
}

// Wait for a message
msg, err := sub.NextMsg(10 * time.Second)
if err != nil {
    log.Fatal(err)
}

// Use the response
log.Printf("Reply: %s", msg.Data)

{% endtab %}

{% tab title="Java" %}

Connection nc = Nats.connect("nats://demo.nats.io:4222");

// Subscribe
Subscription sub = nc.subscribe("updates");

// Read a message
Message msg = sub.nextMessage(Duration.ZERO);

String str = new String(msg.getData(), StandardCharsets.UTF_8);
System.out.println(str);

// Close the connection
nc.close();

{% endtab %}

{% tab title="JavaScript" %}

// node-nats subscriptions are always async.

{% endtab %}

{% tab title="Python" %}

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

{% endtab %}

{% tab title="Ruby" %}

# The Ruby NATS client subscriptions are all async.

{% endtab %}

{% tab title="C" %}

natsConnection      *conn      = NULL;
natsSubscription    *sub       = NULL;
natsMsg             *msg       = NULL;
natsStatus          s          = NATS_OK;

s = natsConnection_ConnectTo(&conn, NATS_DEFAULT_URL);

// Subscribe
if (s == NATS_OK)
    s = natsConnection_SubscribeSync(&sub, conn, "updates");

// Wait for messages
if (s == NATS_OK)
    s = natsSubscription_NextMsg(&msg, sub, 10000);

if (s == NATS_OK)
{
    printf("Received msg: %s - %.*s\n",
            natsMsg_GetSubject(msg),
            natsMsg_GetDataLength(msg),
            natsMsg_GetData(msg));

    // Destroy message that was received
    natsMsg_Destroy(msg);
}

(...)

// Destroy objects that were created
natsSubscription_Destroy(sub);
natsConnection_Destroy(conn);

{% endtab %} {% endtabs %}