by Example
Pull Consumers in JetStream
A pull consumer allows for the application to fetch one or more messages on-demand using a subscription bound to the consumer. This allows the application to control the flow of the messages coming in so it can process and ack them in an appropriate amount of time.
A consumer can either be durable or ephemeral. A durable consumer will have its state tracked on the server, most importantly, the last acknowledged message from the client.
Ephemeral consumers are useful as one-off needs and are a bit cheaper in terms of resources and management. However, ephemerals do not (of course) persist after the primary subscriber unsubscribes. The server will automatically clean up (delete) the consumer after a period of time.
Since each subscription is fetching messages on-demand, multiple subscriptions can be create bound to the same pull consumer without any additional configuration. Each subscriber can fetch batches of messages and process them concurrently.
It is important to note that the messages in a given batch are ordered with respect to each other, but each subscriber will be handling a batch independently. If there is a need to have determinstic partitioning for scalable order processing, learn more here.
$ nbe run jetstream/pull-consumer/csharpView the source code or learn how to run this example yourself
Code
using System.Diagnostics;
using NATS.Client.JetStream;
using NATS.Client.JetStream.Models;
using NATS.Net;
NATS_URL environment variable can be used to pass the locations of the NATS servers.
var url = Environment.GetEnvironmentVariable("NATS_URL") ?? "nats://127.0.0.1:4222";
Connect to NATS server. Since connection is disposable at the end of our scope, we should flush our buffers and close the connection cleanly.
await using var nc = new NatsClient(url);
Access JetStream for managing streams and consumers as well as for publishing and consuming messages to and from the stream.
var js = nc.CreateJetStreamContext();
var streamName = "EVENTS";
Declare a simple limits-based stream.
var stream = await js.CreateStreamAsync(new StreamConfig(streamName, subjects: ["events.>"]));
Publish a few messages, for example.
await js.PublishAsync(subject: "events.1", data: "event-data-1");
await js.PublishAsync(subject: "events.2", data: "event-data-2");
await js.PublishAsync(subject: "events.3", data: "event-data-3");
Create the consumer bound to the previously created stream.
If a durable name is not supplied, consumer will be removed after InactiveThreshold
(it defaults to 5 seconds) is reached when not actively consuming messages.
Name is optional, if not provided, it will be auto-generated.
For this example, let’s use the consumer with no options, which will
be ephemeral with auto-generated name.
var consumer = await stream.CreateOrUpdateConsumerAsync(new ConsumerConfig());
Messages can be consumed continuously in a loop using Consume
method. Consume can be supplied with various options, but for this
example we will use the default ones.break is used as part of this
example to make sure to stop processing after we process 3 messages (so
that it does not interfere with other examples).
var count = 0;
await foreach (var msg in consumer.ConsumeAsync<string>())
{
await msg.AckAsync();
Console.WriteLine($"received msg on {msg.Subject} with data {msg.Data}");
if (++count == 3)
break;
}
Publish more messages.
await js.PublishAsync(subject: "events.1", data: "event-data-1");
await js.PublishAsync(subject: "events.2", data: "event-data-2");
await js.PublishAsync(subject: "events.3", data: "event-data-3");
We can fetch messages in batches. The first argument is the batch size, which is the maximum number of messages that should be returned. For this first fetch, we ask for two, and we will get those since they are in the stream.
var fetchCount = 0;
await foreach (var msg in consumer.FetchAsync<string>(opts: new NatsJSFetchOpts { MaxMsgs = 2 }))
{
await msg.AckAsync();
fetchCount++;
}
Console.WriteLine($"Got {fetchCount} messages");
Fetch puts messages on the returned Messages() channel. This channel
will only be closed when the requested number of messages has been
received or the operation times out. If we do not want to wait for the
rest of the messages and want to quickly return as many messages as there
are available (up to the provided batch size), we can use FetchNoWait
instead.
Here, because we have already received two messages, we will only get
one more.
NOTE: FetchNoWait usage is discouraged since it can cause an unnecessary load
if not used correctly e.g., in a loop without a backoff it will continuously
try to get messages even if there are no new messages in the stream.
fetchCount = 0;
await foreach (var msg in ((NatsJSConsumer)consumer).FetchNoWaitAsync<string>(opts: new NatsJSFetchOpts { MaxMsgs = 100 }))
{
await msg.AckAsync();
fetchCount++;
}
Console.WriteLine($"Got {fetchCount} messages");
Finally, if we are at the end of the stream, and we call fetch, the call will be blocked until the “max wait” time which is 30 seconds by default, but this can be set explicitly as an option.
var fetchStopwatch = Stopwatch.StartNew();
fetchCount = 0;
await foreach (var msg in consumer.FetchAsync<string>(opts: new NatsJSFetchOpts { MaxMsgs = 100, Expires = TimeSpan.FromSeconds(1) }))
{
await msg.AckAsync();
fetchCount++;
}
Console.WriteLine($"Got {fetchCount} messages in {fetchStopwatch.Elapsed}");
Durable consumers can be created by specifying the Durable name.
Durable consumers are not removed automatically regardless of the
InactiveThreshold. They can be removed by calling DeleteConsumer.
var durable = await stream.CreateOrUpdateConsumerAsync(new ConsumerConfig("processor"));
Consume and fetch work the same way for durable consumers.
await foreach (var msg in durable.FetchAsync<string>(opts: new NatsJSFetchOpts { MaxMsgs = 1 }))
{
Console.WriteLine($"Received {msg.Subject} from durable consumer");
}
While ephemeral consumers will be removed after InactiveThreshold, durable consumers have to be removed explicitly if no longer needed.
await stream.DeleteConsumerAsync("processor");
Let’s try to get the consumer to make sure it’s gone.
try
{
await stream.GetConsumerAsync("processor");
}
catch (NatsJSApiException e)
{
if (e.Error.Code == 404)
{
Console.WriteLine("Consumer is gone");
}
}
That’s it!
Console.WriteLine("Bye!");Output
received msg on events.1 with data event-data-1 received msg on events.2 with data event-data-2 received msg on events.3 with data event-data-3 Got 2 messages Got 1 messages Got 0 messages in 00:00:01.0032504 Received events.1 from durable consumer Consumer is gone Bye!
Install NuGet package
NATS.Net