Consumer Details
Consumers are how client applications get the messages stored in the streams. You can have many consumers on a single stream. Consumers are like a view on a stream, can filter messages and have some state (maintained by the servers) associated with them.
Consumers can be 'durable' or 'ephemeral'.
Durable versus ephemeral consumers
Durable consumer persist message delivery progress on the server side. A durable consumer can be retrieved by name and shared between client instance for load balancing. It can me made high available through replicas
An ephemeral consumer does not persist delivery progress and will automatically be deleted when there are no more client instances connected.
Durable consumers
Durable consumers are meant to be used by multiple instances of an application, either to distribute and scale out the processing, or to persist the position of the consumer over the stream between runs of an application.
Durable consumers as the name implies are meant to last 'forever' and are typically created and deleted administratively rather than by the application code which only needs to specify the durable's well known name to use it.
You create a durable consumer using the nats consumer add
CLI tool command, or programmatically by passing a durable name option to the subscription creation call.
Ephemeral consumers
Ephemeral consumers are meant to be used by a single instance of an application (e.g. to get its own replay of the messages in the stream).
Ephemeral consumers are not meant to last 'forever', they are defined automatically at subscription time by the client library and disappear after the application disconnect.
You (automatically) create an ephemeral consumer when you call the js.Subscribe function without specifying the Durable or Bind subscription options. Calling Drain on that subscription automatically deletes the underlying ephemeral consumer. You can also explicitly create an ephemeral consumer by not passing a durable name option to the jsm.AddConsumer call.
Ephemeral consumers otherwise have the same control over message acknowledged and re-delivery as durable consumers.
Push and Pull consumers
Clients implement two implementations of consumers identified as 'push' or 'pull'.
Push consumers
Push consumers receive messages on a specific subject where message flow is controlled by the server. Load balancing is supported through NATS core queue groups. The messages from the stream are distributed automatically between the subscribing clients to the push consumers.
Pull consumers
Pull consumers request messages explicitly from the server in batches, giving the client full control over dispatching, flow control, pending (unacknowledged) messages and load balancing. Pull consuming client make fetch()
calls in a dispatch loop.
We recommend using pull consumers for new projects. In particular when scalability, detailed flow control or error handling are a design focus. Most client API have been updated to provide convenient interfaces for consuming messages through callback handler or iterators without the need to manage message retrieval.
fetch()
calls can be immediate or have a defined timeout, allowing for either controlled (1 by 1) consumption or realtime
delivery with minimal polling overhead.
Pull consumers create less CPU load on the NATS servers and therefore scale better (note that the push consumers are still quite fast and scalable, you may only notice the difference between the two if you have sustained high message rates).
Pull
func ExampleJetStream() {
nc, err := nats.Connect("localhost")
if err != nil {
log.Fatal(err)
}
// Use the JetStream context to produce and consumer messages
// that have been persisted.
js, err := nc.JetStream(nats.PublishAsyncMaxPending(256))
if err != nil {
log.Fatal(err)
}
js.AddStream(&nats.StreamConfig{
Name: "FOO",
Subjects: []string{"foo"},
})
js.Publish("foo", []byte("Hello JS!"))
// Publish messages asynchronously.
for i := 0; i < 500; i++ {
js.PublishAsync("foo", []byte("Hello JS Async!"))
}
select {
case <-js.PublishAsyncComplete():
case <-time.After(5 * time.Second):
fmt.Println("Did not resolve in time")
}
// Create Pull based consumer with maximum 128 inflight.
sub, _ := js.PullSubscribe("foo", "wq", nats.PullMaxWaiting(128))
ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
defer cancel()
for {
select {
case <-ctx.Done():
return
default:
}
// Fetch will return as soon as any message is available rather than wait until the full batch size is available, using a batch size of more than 1 allows for higher throughput when needed.
msgs, _ := sub.Fetch(10, nats.Context(ctx))
for _, msg := range msgs {
msg.Ack()
}
}
}
package io.nats.examples.jetstream.simple;
import io.nats.client.*;
import io.nats.client.api.ConsumerConfiguration;
import io.nats.examples.jetstream.ResilientPublisher;
import java.io.IOException;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.atomic.AtomicInteger;
import static io.nats.examples.jetstream.NatsJsUtils.createOrReplaceStream;
/**
* This example will demonstrate simplified consume with a handler
*/
public class MessageConsumerExample {
private static final String STREAM = "consume-stream";
private static final String SUBJECT = "consume-subject";
private static final String CONSUMER_NAME = "consume-consumer";
private static final String MESSAGE_PREFIX = "consume";
private static final int STOP_COUNT = 500;
private static final int REPORT_EVERY = 100;
private static final String SERVER = "nats://localhost:4222";
public static void main(String[] args) {
Options options = Options.builder().server(SERVER).build();
try (Connection nc = Nats.connect(options)) {
JetStreamManagement jsm = nc.jetStreamManagement();
createOrReplaceStream(jsm, STREAM, SUBJECT);
//Utility for filling the stream with some messages
System.out.println("Starting publish...");
ResilientPublisher publisher = new ResilientPublisher(nc, jsm, STREAM, SUBJECT).basicDataPrefix(MESSAGE_PREFIX).jitter(10);
Thread pubThread = new Thread(publisher);
pubThread.start();
// get stream context, create consumer and get the consumer context
StreamContext streamContext;
ConsumerContext consumerContext;
CountDownLatch latch = new CountDownLatch(1);
AtomicInteger atomicCount = new AtomicInteger();
long start = System.nanoTime();
streamContext = nc.getStreamContext(STREAM);
streamContext.createOrUpdateConsumer(ConsumerConfiguration.builder().durable(CONSUMER_NAME).build());
consumerContext = streamContext.getConsumerContext(CONSUMER_NAME);
MessageHandler handler = msg -> {
msg.ack();
int count = atomicCount.incrementAndGet();
if (count % REPORT_EVERY == 0) {
System.out.println("Handler" + ": Received " + count + " messages in " + (System.nanoTime() - start) / 1_000_000 + "ms.");
}
if (count == STOP_COUNT) {
latch.countDown();
}
};
// create the consumer and install handler
MessageConsumer consumer = consumerContext.consume(handler);
//Waiting for the handler signalling us to stop
latch.await();
// When stop is called, no more pull requests will be made, but messages already requested
// will still come across the wire to the client.
System.out.println("Stopping the consumer...");
consumer.stop();
// wait until the consumer is finished processing backlog
while (!consumer.isFinished()) {
Thread.sleep(10);
}
System.out.println("Final" + ": Received " + atomicCount.get() + " messages in " + (System.nanoTime() - start) / 1_000_000 + "ms.");
publisher.stop(); // otherwise the ConsumerContext background thread will complain when the connection goes away
pubThread.join();
}
catch (JetStreamApiException | IOException e) {
// JetStreamApiException:
// 1. the stream or consumer did not exist
// 2. api calls under the covers theoretically this could fail, but practically it won't.
// IOException:
// likely a connection problem
System.err.println("Exception should not handled, exiting.");
System.exit(-1);
}
catch (Exception e) {
System.err.println("Exception should not handled, exiting.");
System.exit(-1);
}
}
}
import { AckPolicy, connect, nanos } from "../../src/mod.ts";
import { nuid } from "../../nats-base-client/nuid.ts";
const nc = await connect();
const stream = nuid.next();
const subj = nuid.next();
const durable = nuid.next();
const jsm = await nc.jetstreamManager();
await jsm.streams.add({ name: stream, subjects: [subj] });
const js = nc.jetstream();
await js.publish(subj);
await js.publish(subj);
await js.publish(subj);
await js.publish(subj);
const psub = await js.pullSubscribe(subj, {
mack: true,
// artificially low ack_wait, to show some messages
// not getting acked being redelivered
config: {
durable_name: durable,
ack_policy: AckPolicy.Explicit,
ack_wait: nanos(4000),
},
});
(async () => {
for await (const m of psub) {
console.log(
`[${m.seq}] ${
m.redelivered ? `- redelivery ${m.info.redeliveryCount}` : ""
}`
);
if (m.seq % 2 === 0) {
m.ack();
}
}
})();
const fn = () => {
console.log("[PULL]");
psub.pull({ batch: 1000, expires: 10000 });
};
// do the initial pull
fn();
// and now schedule a pull every so often
const interval = setInterval(fn, 10000); // and repeat every 2s
setTimeout(() => {
clearInterval(interval);
nc.drain();
}, 20000);
import asyncio
import nats
from nats.errors import TimeoutError
async def main():
nc = await nats.connect("localhost")
# Create JetStream context.
js = nc.jetstream()
# Persist messages on 'foo's subject.
await js.add_stream(name="sample-stream", subjects=["foo"])
for i in range(0, 10):
ack = await js.publish("foo", f"hello world: {i}".encode())
print(ack)
# Create pull based consumer on 'foo'.
psub = await js.pull_subscribe("foo", "psub")
# Fetch and ack messagess from consumer.
for i in range(0, 10):
msgs = await psub.fetch(1)
for msg in msgs:
print(msg)
await nc.close()
if __name__ == '__main__':
asyncio.run(main())
#include "examples.h"
static const char *usage = ""\
"-gd use global message delivery thread pool\n" \
"-sync receive synchronously (default is asynchronous)\n" \
"-pull use pull subscription\n" \
"-fc enable flow control\n" \
"-count number of expected messages\n";
static void
onMsg(natsConnection *nc, natsSubscription *sub, natsMsg *msg, void *closure)
{
if (print)
printf("Received msg: %s - %.*s\n",
natsMsg_GetSubject(msg),
natsMsg_GetDataLength(msg),
natsMsg_GetData(msg));
if (start == 0)
start = nats_Now();
// We should be using a mutex to protect those variables since
// they are used from the subscription's delivery and the main
// threads. For demo purposes, this is fine.
if (++count == total)
elapsed = nats_Now() - start;
// Since this is auto-ack callback, we don't need to ack here.
natsMsg_Destroy(msg);
}
static void
asyncCb(natsConnection *nc, natsSubscription *sub, natsStatus err, void *closure)
{
printf("Async error: %u - %s\n", err, natsStatus_GetText(err));
natsSubscription_GetDropped(sub, (int64_t*) &dropped);
}
int main(int argc, char **argv)
{
natsConnection *conn = NULL;
natsStatistics *stats = NULL;
natsOptions *opts = NULL;
natsSubscription *sub = NULL;
natsMsg *msg = NULL;
jsCtx *js = NULL;
jsErrCode jerr = 0;
jsOptions jsOpts;
jsSubOptions so;
natsStatus s;
bool delStream = false;
opts = parseArgs(argc, argv, usage);
printf("Created %s subscription on '%s'.\n",
(pull ? "pull" : (async ? "asynchronous" : "synchronous")), subj);
s = natsOptions_SetErrorHandler(opts, asyncCb, NULL);
if (s == NATS_OK)
s = natsConnection_Connect(&conn, opts);
if (s == NATS_OK)
s = jsOptions_Init(&jsOpts);
if (s == NATS_OK)
s = jsSubOptions_Init(&so);
if (s == NATS_OK)
{
so.Stream = stream;
so.Consumer = durable;
if (flowctrl)
{
so.Config.FlowControl = true;
so.Config.Heartbeat = (int64_t)1E9;
}
}
if (s == NATS_OK)
s = natsConnection_JetStream(&js, conn, &jsOpts);
if (s == NATS_OK)
{
jsStreamInfo *si = NULL;
// First check if the stream already exists.
s = js_GetStreamInfo(&si, js, stream, NULL, &jerr);
if (s == NATS_NOT_FOUND)
{
jsStreamConfig cfg;
// Since we are the one creating this stream, we can delete at the end.
delStream = true;
// Initialize the configuration structure.
jsStreamConfig_Init(&cfg);
cfg.Name = stream;
// Set the subject
cfg.Subjects = (const char*[1]){subj};
cfg.SubjectsLen = 1;
// Make it a memory stream.
cfg.Storage = js_MemoryStorage;
// Add the stream,
s = js_AddStream(&si, js, &cfg, NULL, &jerr);
}
if (s == NATS_OK)
{
printf("Stream %s has %" PRIu64 " messages (%" PRIu64 " bytes)\n",
si->Config->Name, si->State.Msgs, si->State.Bytes);
// Need to destroy the returned stream object.
jsStreamInfo_Destroy(si);
}
}
if (s == NATS_OK)
{
if (pull)
s = js_PullSubscribe(&sub, js, subj, durable, &jsOpts, &so, &jerr);
else if (async)
s = js_Subscribe(&sub, js, subj, onMsg, NULL, &jsOpts, &so, &jerr);
else
s = js_SubscribeSync(&sub, js, subj, &jsOpts, &so, &jerr);
}
if (s == NATS_OK)
s = natsSubscription_SetPendingLimits(sub, -1, -1);
if (s == NATS_OK)
s = natsStatistics_Create(&stats);
if ((s == NATS_OK) && pull)
{
natsMsgList list;
int i;
for (count = 0; (s == NATS_OK) && (count < total); )
{
s = natsSubscription_Fetch(&list, sub, 1024, 5000, &jerr);
if (s != NATS_OK)
break;
if (start == 0)
start = nats_Now();
count += (int64_t) list.Count;
for (i=0; (s == NATS_OK) && (i<list.Count); i++)
s = natsMsg_Ack(list.Msgs[i], &jsOpts);
natsMsgList_Destroy(&list);
}
}
else if ((s == NATS_OK) && async)
{
while (s == NATS_OK)
{
if (count + dropped == total)
break;
nats_Sleep(1000);
}
}
else if (s == NATS_OK)
{
for (count = 0; (s == NATS_OK) && (count < total); count++)
{
s = natsSubscription_NextMsg(&msg, sub, 5000);
if (s != NATS_OK)
break;
if (start == 0)
start = nats_Now();
s = natsMsg_Ack(msg, &jsOpts);
natsMsg_Destroy(msg);
}
}
if (s == NATS_OK)
{
printStats(STATS_IN|STATS_COUNT, conn, sub, stats);
printPerf("Received");
}
if (s == NATS_OK)
{
jsStreamInfo *si = NULL;
// Let's report some stats after the run
s = js_GetStreamInfo(&si, js, stream, NULL, &jerr);
if (s == NATS_OK)
{
printf("\nStream %s has %" PRIu64 " messages (%" PRIu64 " bytes)\n",
si->Config->Name, si->State.Msgs, si->State.Bytes);
jsStreamInfo_Destroy(si);
}
if (delStream)
{
printf("\nDeleting stream %s: ", stream);
s = js_DeleteStream(js, stream, NULL, &jerr);
if (s == NATS_OK)
printf("OK!");
printf("\n");
}
}
else
{
printf("Error: %u - %s - jerr=%u\n", s, natsStatus_GetText(s), jerr);
nats_PrintLastErrorStack(stderr);
}
// Destroy all our objects to avoid report of memory leak
jsCtx_Destroy(js);
natsStatistics_Destroy(stats);
natsSubscription_Destroy(sub);
natsConnection_Destroy(conn);
natsOptions_Destroy(opts);
// To silence reports of memory still in used with valgrind
nats_Close();
return 0;
}
A push consumer can also be used in some other use cases such as without a queue group, or with no acknowledgement or cumulative acknowledgements.
Push
func ExampleJetStream() {
nc, err := nats.Connect("localhost")
if err != nil {
log.Fatal(err)
}
// Use the JetStream context to produce and consumer messages
// that have been persisted.
js, err := nc.JetStream(nats.PublishAsyncMaxPending(256))
if err != nil {
log.Fatal(err)
}
js.AddStream(&nats.StreamConfig{
Name: "FOO",
Subjects: []string{"foo"},
})
js.Publish("foo", []byte("Hello JS!"))
// Publish messages asynchronously.
for i := 0; i < 500; i++ {
js.PublishAsync("foo", []byte("Hello JS Async!"))
}
select {
case <-js.PublishAsyncComplete():
case <-time.After(5 * time.Second):
fmt.Println("Did not resolve in time")
}
// Create async consumer on subject 'foo'. Async subscribers
// ack a message once exiting the callback.
js.Subscribe("foo", func(msg *nats.Msg) {
meta, _ := msg.Metadata()
fmt.Printf("Stream Sequence : %v\n", meta.Sequence.Stream)
fmt.Printf("Consumer Sequence: %v\n", meta.Sequence.Consumer)
})
// Async subscriber with manual acks.
js.Subscribe("foo", func(msg *nats.Msg) {
msg.Ack()
}, nats.ManualAck())
// Async queue subscription where members load balance the
// received messages together.
// If no consumer name is specified, either with nats.Bind()
// or nats.Durable() options, the queue name is used as the
// durable name (that is, as if you were passing the
// nats.Durable(<queue group name>) option.
// It is recommended to use nats.Bind() or nats.Durable()
// and preferably create the JetStream consumer beforehand
// (using js.AddConsumer) so that the JS consumer is not
// deleted on an Unsubscribe() or Drain() when the member
// that created the consumer goes away first.
// Check Godoc for the QueueSubscribe() API for more details.
js.QueueSubscribe("foo", "group", func(msg *nats.Msg) {
msg.Ack()
}, nats.ManualAck())
// Subscriber to consume messages synchronously.
sub, _ := js.SubscribeSync("foo")
msg, _ := sub.NextMsg(2 * time.Second)
msg.Ack()
// We can add a member to the group, with this member using
// the synchronous version of the QueueSubscribe.
sub, _ = js.QueueSubscribeSync("foo", "group")
msg, _ = sub.NextMsg(2 * time.Second)
msg.Ack()
// ChanSubscribe
msgCh := make(chan *nats.Msg, 8192)
sub, _ = js.ChanSubscribe("foo", msgCh)
select {
case msg := <-msgCh:
fmt.Println("[Received]", msg)
case <-time.After(1 * time.Second):
}
}
package io.nats.examples.jetstream;
import io.nats.client.*;
import io.nats.client.api.PublishAck;
import io.nats.examples.ExampleArgs;
import io.nats.examples.ExampleUtils;
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.time.Duration;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;
import static io.nats.examples.jetstream.NatsJsUtils.createStreamExitWhenExists;
/**
* This example will demonstrate JetStream push subscribing using a durable consumer and a queue
*/
public class NatsJsPushSubQueueDurable {
static final String usageString =
"\nUsage: java -cp <classpath> NatsJsPushSubQueueDurable [-s server] [-strm stream] [-sub subject] [-q queue] [-dur durable] [-mcnt msgCount] [-scnt subCount]"
+ "\n\nDefault Values:"
+ "\n [-strm stream] qdur-stream"
+ "\n [-sub subject] qdur-subject"
+ "\n [-q queue] qdur-queue"
+ "\n [-dur durable] qdur-durable"
+ "\n [-mcnt msgCount] 100"
+ "\n [-scnt subCount] 5"
+ "\n\nUse tls:// or opentls:// to require tls, via the Default SSLContext\n"
+ "\nSet the environment variable NATS_NKEY to use challenge response authentication by setting a file containing your private key.\n"
+ "\nSet the environment variable NATS_CREDS to use JWT/NKey authentication by setting a file containing your user creds.\n"
+ "\nUse the URL in the -s server parameter for user/pass/token authentication.\n";
public static void main(String[] args) {
ExampleArgs exArgs = ExampleArgs.builder("Push Subscribe, Durable Consumer, Queue", args, usageString)
.defaultStream("qdur-stream")
.defaultSubject("qdur-subject")
.defaultQueue("qdur-queue")
.defaultDurable("qdur-durable")
.defaultMsgCount(100)
.defaultSubCount(5)
.build();
try (Connection nc = Nats.connect(ExampleUtils.createExampleOptions(exArgs.server, true))) {
// Create a JetStreamManagement context.
JetStreamManagement jsm = nc.jetStreamManagement();
// Use the utility to create a stream stored in memory.
createStreamExitWhenExists(jsm, exArgs.stream, exArgs.subject);
// Create our JetStream context
JetStream js = nc.jetStream();
System.out.println();
// Setup the subscribers
// - the PushSubscribeOptions can be re-used since all the subscribers are the same
// - use a concurrent integer to track all the messages received
// - have a list of subscribers and threads so I can track them
PushSubscribeOptions pso = PushSubscribeOptions.builder().durable(exArgs.durable).build();
AtomicInteger allReceived = new AtomicInteger();
List<JsQueueSubscriber> subscribers = new ArrayList<>();
List<Thread> subThreads = new ArrayList<>();
for (int id = 1; id <= exArgs.subCount; id++) {
// setup the subscription
JetStreamSubscription sub = js.subscribe(exArgs.subject, exArgs.queue, pso);
// create and track the runnable
JsQueueSubscriber qs = new JsQueueSubscriber(id, exArgs, js, sub, allReceived);
subscribers.add(qs);
// create, track and start the thread
Thread t = new Thread(qs);
subThreads.add(t);
t.start();
}
nc.flush(Duration.ofSeconds(1)); // flush outgoing communication with/to the server
// create and start the publishing
Thread pubThread = new Thread(new JsPublisher(js, exArgs));
pubThread.start();
// wait for all threads to finish
pubThread.join();
for (Thread t : subThreads) {
t.join();
}
// report
for (JsQueueSubscriber qs : subscribers) {
qs.report();
}
System.out.println();
// delete the stream since we are done with it.
jsm.deleteStream(exArgs.stream);
}
catch (Exception e) {
e.printStackTrace();
}
}
static class JsPublisher implements Runnable {
JetStream js;
ExampleArgs exArgs;
public JsPublisher(JetStream js, ExampleArgs exArgs) {
this.js = js;
this.exArgs = exArgs;
}
@Override
public void run() {
for (int x = 1; x <= exArgs.msgCount; x++) {
try {
PublishAck pa = js.publish(exArgs.subject, ("Data # " + x).getBytes(StandardCharsets.US_ASCII));
} catch (IOException | JetStreamApiException e) {
// something pretty wrong here
e.printStackTrace();
System.exit(-1);
}
}
}
}
static class JsQueueSubscriber implements Runnable {
int id;
int thisReceived;
List<String> datas;
ExampleArgs exArgs;
JetStream js;
JetStreamSubscription sub;
AtomicInteger allReceived;
public JsQueueSubscriber(int id, ExampleArgs exArgs, JetStream js, JetStreamSubscription sub, AtomicInteger allReceived) {
this.id = id;
thisReceived = 0;
datas = new ArrayList<>();
this.exArgs = exArgs;
this.js = js;
this.sub = sub;
this.allReceived = allReceived;
}
public void report() {
System.out.printf("Sub # %d handled %d messages.\n", id, thisReceived);
}
@Override
public void run() {
while (allReceived.get() < exArgs.msgCount) {
try {
Message msg = sub.nextMessage(Duration.ofMillis(500));
while (msg != null) {
thisReceived++;
allReceived.incrementAndGet();
String data = new String(msg.getData(), StandardCharsets.US_ASCII);
datas.add(data);
System.out.printf("QS # %d message # %d %s\n", id, thisReceived, data);
msg.ack();
msg = sub.nextMessage(Duration.ofMillis(500));
}
} catch (InterruptedException e) {
// just try again
}
}
System.out.printf("QS # %d completed.\n", id);
}
}
}
import { AckPolicy, connect } from "../../src/mod.ts";
import { nuid } from "../../nats-base-client/nuid.ts";
const nc = await connect();
// create a regular subscription - this is plain nats
const sub = nc.subscribe("my.messages", { max: 5 });
const done = (async () => {
for await (const m of sub) {
console.log(m.subject);
m.respond();
}
})();
const jsm = await nc.jetstreamManager();
const stream = nuid.next();
const subj = nuid.next();
await jsm.streams.add({ name: stream, subjects: [`${subj}.>`] });
// create a consumer that delivers to the subscription
await jsm.consumers.add(stream, {
ack_policy: AckPolicy.Explicit,
deliver_subject: "my.messages",
});
// publish some old nats messages
nc.publish(`${subj}.A`);
nc.publish(`${subj}.B`);
nc.publish(`${subj}.C`);
nc.publish(`${subj}.D.A`);
nc.publish(`${subj}.F.A.B`);
await done;
await nc.close();
import asyncio
import nats
from nats.errors import TimeoutError
async def main():
nc = await nats.connect("localhost")
# Create JetStream context.
js = nc.jetstream()
# Persist messages on 'foo's subject.
await js.add_stream(name="sample-stream", subjects=["foo"])
for i in range(0, 10):
ack = await js.publish("foo", f"hello world: {i}".encode())
print(ack)
# Create pull based consumer on 'foo'.
psub = await js.pull_subscribe("foo", "psub")
# Fetch and ack messagess from consumer.
for i in range(0, 10):
msgs = await psub.fetch(1)
for msg in msgs:
print(msg)
# Create single push based subscriber that is durable across restarts.
sub = await js.subscribe("foo", durable="myapp")
msg = await sub.next_msg()
await msg.ack()
# Create deliver group that will be have load balanced messages.
async def qsub_a(msg):
print("QSUB A:", msg)
await msg.ack()
async def qsub_b(msg):
print("QSUB B:", msg)
await msg.ack()
await js.subscribe("foo", "workers", cb=qsub_a)
await js.subscribe("foo", "workers", cb=qsub_b)
for i in range(0, 10):
ack = await js.publish("foo", f"hello world: {i}".encode())
print("\t", ack)
await nc.close()
if __name__ == '__main__':
asyncio.run(main())
#include "examples.h"
static const char *usage = ""\
"-gd use global message delivery thread pool\n" \
"-sync receive synchronously (default is asynchronous)\n" \
"-pull use pull subscription\n" \
"-fc enable flow control\n" \
"-count number of expected messages\n";
static void
onMsg(natsConnection *nc, natsSubscription *sub, natsMsg *msg, void *closure)
{