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
Java
JavaScript
Python
Ruby
TypeScript
C
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)
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();
JavaScript
// node-nats subscriptions are always async.
Python
# Asyncio NATS client currently does not have a sync subscribe API
Ruby
# The Ruby NATS client subscriptions are all async.
TypeScript
/ Typescript NATS subscriptions are always async.
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);
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