Need help with
Click the “chat” button below for chat support from the developer who created it, or find similar developers for support.

About the developer

133 Stars 42 Forks Apache License 2.0 185 Commits 15 Opened issues


The official NATS .NET C# Streaming Client

Services available


Need anything else?

Contributors list

NATS .NET C# Streaming Client

NATS Streaming is an extremely performant, lightweight reliable streaming platform powered by NATS.

License Apache 2.0 API Documentation Build Status NuGet

NATS Streaming provides the following high-level feature set: - Log based persistence - At-Least-Once Delivery model, giving reliable message delivery - Rate matched on a per subscription basis - Replay/Restart - Last Value Semantics


  • Please raise issues or ask questions via the Issue Tracker. For general discussion, visit our slack channel. Requests to join can be made here.

Getting started

The easiest and recommended way to start using the NATS streaming client in your .NET projects, is to use the NuGet package. For examples on how to use the client, see below or in any of the included sample projects.

Get up and running with the source code

First, download the source code:

git clone [email protected]:nats-io/

Project files

The repository contains several projects, all located under

  • STAN.Client - The NATS.Client assembly
  • Tests
    • IntegrationTests - XUnit tests, verifying the client integration with
      (ensure you have both in your path to run these).
    • UnitTests - XUnit tests that requires no dependencies
  • Samples
    • StanPub - A sample publisher.
    • StanSub - A samble subscriber.


.NET Core SDK style projects are used, so ensure your environment (command line, VSCode, Visual Studio, etc) supports the targetted .NET Core SDK in

as well as .NET Framework 4.6 or greater.

Visual Studio

The recommendation is to load

into Visual Studio 2019 (Visual Studio 2017 works as well). .NET Core SDK style projects are used to multitarget different frameworks, so when working with the source code (debugging, running tets etc) you might need to mind the "context" of the current framework.

XML documentation is generated (in

), so code completion, context help, etc, will be available in the editor.

Command line

Since .NET Core SDK style projects are used, you can use the .NET SDK to build, run tests, pack etc.

E.g. to build:

dotnet build src\STAN.sln -c Release

This will build the respective

, samples etc in Release mode, with only requiring the .NET Core SDK and the .NET Platform.

API Documentation

Current API Documentation

Doxygen is used for building the API documentation. To build the API documentation, change directories to

and run the following command:

Doxygen will build the NATS .NET Streaming Client API documentation, placing it in the

directory. Doxygen is required to be installed and in the PATH. Version 1.8 is known to work.

Basic Usage

var cf = new StanConnectionFactory();
using (var c = cf.CreateConnection("test-cluster", "appname"))
    using (c.Subscribe("foo", (obj, args) =>
        c.Publish("foo", System.Text.Encoding.UTF8.GetBytes("hello"));

Subscription Start (i.e. Replay) Options

NATS Streaming subscriptions are similar to NATS subscriptions, but clients may start their subscription at an earlier point in the message stream, allowing them to receive messages that were published before this client registered interest.

The options are described with examples below:

// Subscribe starting with most recently published value
var opts = StanSubscriptionOptions.GetDefaultOptions();
var s = c.Subscribe("foo", opts, (obj, args) =>
    Console.WriteLine("Received a message: {0}",

// Receive all stored values in order var opts = StanSubscriptionOptions.GetDefaultOptions(); opts.DeliverAllAvailable(); var s = c.Subscribe("foo", opts, (obj, args) => { Console.WriteLine("Received a message: {0}", System.Text.Encoding.UTF8.GetString(args.Message.Data)); });

// Receive messages starting at a specific sequence number var opts = StanSubscriptionOptions.GetDefaultOptions(); opts.StartAt(22); var s = c.Subscribe("foo", opts, (obj, args) => { Console.WriteLine("Received a message: {0}", System.Text.Encoding.UTF8.GetString(args.Message.Data)); });

// Subscribe starting at a specific time var opts = StanSubscriptionOptions.GetDefaultOptions(); opts.StartAt(new DateTime(2016, 07, 28, 5, 35, 04, 570)); var s = c.Subscribe("foo", opts, (obj, args) => { Console.WriteLine("Received a message: {0}", System.Text.Encoding.UTF8.GetString(args.Message.Data)); });

// Subscribe starting a specific amount of time in the past (e.g. 30 seconds ago) var opts = StanSubscriptionOptions.GetDefaultOptions(); opts.StartAt(new TimeSpan(0, 0, 30)); var s = c.Subscribe("foo", opts, (obj, args) => { Console.WriteLine("Received a message: {0}", System.Text.Encoding.UTF8.GetString(args.Message.Data)); });

Durable Subscriptions

Replay of messages offers great flexibility for clients wishing to begin processing at some earlier point in the data stream. However, some clients just need to pick up where they left off from an earlier session, without having to manually track their position in the stream of messages. Durable subscriptions allow clients to assign a durable name to a subscription when it is created. Doing this causes the NATS Streaming server to track the last acknowledged message for that clientID + durable name, so that only messages since the last acknowledged message will be delivered to the client.

var cf = new StanConnectionFactory();
var c = cf.CreateConnection("test-cluster", "client-123");

// Subscribe with a durable name var opts = StanSubscriptionOptions.GetDefaultOptions(); opts.DurableName = "my-durable";

var s = c.Subscribe("foo", opts, (obj, args) => { Console.WriteLine("Received a message: {0}", System.Text.Encoding.UTF8.GetString(args.Message.Data)); }); ... // client receives message sequence 1-40 ... // client disconnects for an hour ... // client reconnects with same clientID "client-123" c = cf.CreateConnection("test-cluster", "client-123");

// client re-subscribes to "foo" with same durable name "my-durable" (set above) var s = c.Subscribe("foo", opts, (obj, args) => { Console.WriteLine("Received a message: {0}", System.Text.Encoding.UTF8.GetString(args.Message.Data)); }); ... // client receives messages 41-current

Wildcard Subscriptions

NATS Streaming subscriptions do not support wildcards.

Advanced Usage

Connection Status

The fact that the NATS Streaming server and clients are not directly connected poses a challenge when it comes to know if a client is still valid. When a client disconnects, the streaming server is not notified, hence the importance of calling

. The server sends heartbeats to the client's private inbox and if it misses a certain number of responses, it will consider the client's connection lost and remove it from its state.

Before version

, the client library was not sending PINGs to the streaming server to detect connection failure. This was problematic especially if an application was never sending data (had only subscriptions for instance). Picture the case where a client connects to a NATS Server which has a route to a NATS Streaming server (either connecting to a standalone NATS Server or the server it embeds). If the connection between the streaming server and the client's NATS Server is broken, the client's NATS connection would still be ok, yet, no communication with the streaming server is possible. This is why relying on
to check the status is not helpful.

Starting version

of this library and server
, the client library will now send PINGs at regular intervals (default is 5 seconds) and will close the streaming connection after a certain number of PINGs have been sent without any response (default is 3). When that happens, a callback - if one is registered - will be invoked to notify the user that the connection is permanently lost, and the reason for the failure.

Here is how you would specify your own PING values and the callback:

    // Send PINGs every 10 seconds, and fail after 5 PINGs without any response.
    StanOptions so = StanOptions.GetDefaultOptions();
    so.PingInterval = 10000;
    so.PingMaxOutstanding = 5;
    so.ConnectionLostEventHandler = (obj, args) =>
        // handle the case where a connection has been lost

var sc = new StanConnectionFactory().CreateConnection(CLUSTER_ID, CLIENT_ID, so);

Note that the only way to be notified is to set the callback. If the callback is not set, PINGs are still sent and the connection will be closed if needed, but the application won't know if it has only subscriptions.

When the connection is lost, your application would have to re-create it and all subscriptions if any.

When no NATS connection is provided, the library creates its own NATS connection and will now set the reconnect attempts to "infinite", which was not the case before. It should therefore be possible for the library to always reconnect, but this does not mean that the streaming connection will not be closed, even if you set a very high threshold for the PINGs max out value. Keep in mind that while the client is disconnected, the server is sending heartbeats to the clients too, and when not getting any response, it will remove that client from its state. When the communication is restored, the PINGs sent to the server will allow to detect this condition and report to the client that the connection is now closed.

Also, while a client is "disconnected" from the server, another application with connectivity to the streaming server may connect and uses the same client ID. The server, when detecting the duplicate client ID, will try to contact the first client to know if it should reject the connect request of the second client. Since the communication between the server and the first client is broken, the server will not get a response and therefore will replace the first client with the second one.

Prior to client

and server
, if the communication between the first client and server were to be restored, and the application would send messages, the server would accept those because the published messages client ID would be valid, although the client is not. With client at
and server
, additional information is sent with each message to allow the server to reject messages from a client that has been replaced by another client.

Asynchronous Publishing

The basic publish API (

Publish(subject, payload)
) is synchronous; it does not return control to the caller until the NATS Streaming server has acknowledged receipt of the message. To accomplish this, a unique ID is generated for the message on creation, and the client library waits for a publish acknowledgement from the server with a matching NUID before it returns control to the caller, possibly with an error indicating that the operation was not successful due to some server problem or authorization error.

Advanced users may wish to process these publish acknowledgements manually to achieve higher publish throughput by not waiting on individual acknowledgements during the publish operation. An asynchronous publish API is provided for this purpose:

The NATS streaming .NET client supports async/await usage with a publisher. The task returned to await upon contains the GUID of the published message. The publish API will still block when the maximum outstanding acknowledgements has been reached to allow flow control in your application.

// all in one call.
var guid = await c.PublishAsync("foo", null);

// alternatively, one can work in some application code. var t = c.PublishAsync("foo", null);

// your application can do work here

guid = await t;

A more traditional method is provided as well (an event handler).

var cf = new StanConnectionFactory();
var c = cf.CreateConnection("test-cluster", "client-123");

// when the server responds with an acknowledgement, this // handler will be invoked. EventHandler ackHandler = (obj, args) => { if (!string.IsNullOrEmpty(args.Error)) { Console.WriteLine("Published Msg {0} failed: {1}", args.GUID, args.Error); }

// handle success - correlate the send with the guid..
Console.WriteLine("Published msg {0} was stored on the server.");


// returns immediately string guid = c.Publish("foo", null, ackHandler); Console.WriteLine("Published msg {0} was stored on the server.", guid);

Message Acknowledgements and Redelivery

NATS Streaming offers At-Least-Once delivery semantics, meaning that once a message has been delivered to an eligible subscriber, if an acknowledgement is not received within the configured timeout interval, NATS Streaming will attempt redelivery of the message. This timeout interval is specified by the subscription option

, which defaults to 30 seconds.

By default, messages are automatically acknowledged by the NATS Streaming client library after the subscriber's message handler is invoked. However, there may be cases in which the subscribing client wishes to accelerate or defer acknowledgement of the message. To do this, the client must set manual acknowledgement mode on the subscription, and invoke

on the
. ex:
var sOpts = StanSubscriptionOptions.GetDefaultOptions();
sOpts.ManualAcks = true;
sOpts.AckWait = 60000;
var s = c.Subscribe("foo", sOpts, (obj, args) =>
    // ack message before performing I/O intensive operation

// Perform long operation.  Note, the message will not be redelivered
// in an application crash, unless requested.


Rate limiting/matching

A classic problem of publish-subscribe messaging is matching the rate of message producers with the rate of message consumers. Message producers can often outpace the speed of the subscribers that are consuming their messages. This mismatch is commonly called a "fast producer/slow consumer" problem, and may result in dramatic resource utilization spikes in the underlying messaging system as it tries to buffer messages until the slow consumer(s) can catch up.

Publisher rate limiting

NATS Streaming provides a connection option called

that effectively limits the number of unacknowledged messages that a publisher may have in-flight at any given time. When this maximum is reached, further
calls will block until the number of unacknowledged messages falls below the specified limit. ex:
var cf = new StanConnectionFactory();

var opts = StanOptions.GetDefaultOptions(); opts.MaxPubAcksInFlight = 25;

var c = cf.CreateConnection("test-cluster", "client-123", opts);

EventHandler ackHandler = (obj, args) => { // process the ack. };

for (int i = 0; i < 1000; i++) { // If the server is unable to keep up with the publisher, the number of oustanding acks will eventually // reach the max and this call will block string guid = c.Publish("foo", null, ackHandler); }

Subscriber rate limiting

Rate limiting may also be accomplished on the subscriber side, on a per-subscription basis, using a subscription option called

. This option specifies the maximum number of outstanding acknowledgements (messages that have been delivered but not acknowledged) that NATS Streaming will allow for a given subscription. When this limit is reached, NATS Streaming will suspend delivery of messages to this subscription until the number of unacknowledged messages falls below the specified limit. ex:
var sOpts = StanSubscriptionOptions.GetDefaultOptions();
sOpts.ManualAcks = true;
sOpts.MaxInflight = 25;
var s = c.Subscribe("foo", sOpts, (obj, args) =>
    Console.WriteLine("Received a message: {0}",
   // Does not ack, or takes a very long time to ack
   // Message delivery will suspend when the number of unacknowledged messages reaches 25

Enabling TLS

Establishing secure connections for use in the NATS streaming client is relatively straightforward. Create a core NATS connection with TLS enabled then configure the streaming client to use that secure connection through options.

Here is example code to establish a secure connection:

using System;
using System.Net.Security;
using System.Security.Cryptography.X509Certificates;
using NATS.Client;
using STAN.Client;

namespace NATSStreamingExamples { class StanTLSExample { // This is an override for convenience. NEVER blindly ignore TLS // errors in production code. static bool verifyServerCert(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors) { Console.WriteLine("WARN: Ignoring TLS Certificate validation error: {0}", sslPolicyErrors); return true; // throw verification out the window for this example. }

    // This method returns an established secure core NATS connection
    // for the streaming client to use.
    static IConnection createSecureNATSConnection()
        // get the default NATS options
        var natsOptions = ConnectionFactory.GetDefaultOptions();

        // Setup NATS to be "secure", then load and add a certificate into the NATS options.
        // See for additional information.
        natsOptions.Secure = true;
        natsOptions.AddCertificate(new X509Certificate2("client.pfx", "password"));
        natsOptions.TLSRemoteCertificationValidationCallback += verifyServerCert;

        return new ConnectionFactory().CreateConnection(natsOptions);

    // To use TLS with a NATS streaming client, create a secure core NATS
    // connection to pass into the NATS streaming client.
    static void Main(string[] args)
        // Get default STAN options, then create and assign a secure
        // core NATS connection for the streaming client to use.
        var stanOptions = StanOptions.GetDefaultOptions();
        stanOptions.NatsConn = createSecureNATSConnection();

        // Finally create a STAN logical connection using the
        // secure NATS connection we just created.
        var connection = new StanConnectionFactory().CreateConnection("test-cluster", "test-client-001", stanOptions);


RX Usage

Importing the namespace

you will be able to use an extension method
to turn the connection to an observable.

You can now import the namespace

. After this you get builtin support for: - Subscribe - SubscribeSafe (will not fail an observer if it misbehaves) - Where - Select

If you want, you could instead take an external dependency on

and use that instead of
using System;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using NATS.Client.Rx.Ops; //Can be replaced with using System.Reactive.Linq;
using STAN.Client;
using STAN.Client.Rx;

namespace RxSample { class Program { static void Main(string[] args) { var clusterId = "test-cluster"; var clientId = Guid.NewGuid().ToString("N");

        using (var cn = new StanConnectionFactory().CreateConnection(clusterId, clientId))
            var cts = new CancellationTokenSource();

            Task.Run(() =&gt;
                var temperatures =
                        .Where(m =&gt; m.Data?.Any() == true)
                        .Select(m =&gt; BitConverter.ToInt32(m.Data, 0));

                temperatures.Subscribe(t =&gt; Console.WriteLine($"{t}C"));

                temperatures.Subscribe(t =&gt; Console.WriteLine($"{(t * 9 / 5) + 32}F"));
            }, cts.Token);

            Task.Run(async () =&gt;
                var rnd = new Random();

                while (!cts.IsCancellationRequested)
                    cn.Publish("temperatures", BitConverter.GetBytes(rnd.Next(-10, 40)));

                    await Task.Delay(1000, cts.Token);
            }, cts.Token);

            Console.WriteLine("Hit any key to exit");


Unit Testing Applications

Testing in a distributed environment is difficult at best, so to facilitate unit testing, the NATS streaming client library has provided some constructors for objects normally created by the NATS client library. These are not recommended for use outside of a test environment.

There are constructors available for

, and
. See the documentation or code for more information.

Let's look at message handling as an example. You have two options, either generate the

and call your event handler directly, or construct a
to test with.
// Application code to process a message
void processMessageEventHandler(object o, StanMsgHandlerArgs args)
    // Your application code does something here.

Testing would look something like this:

public void TestProcessMessageEventHandler()
    // Create a test args object
    var mhArgs = new StanMsgHandlerArgs(
        true, "foo", 10000, 1234, null);

processMessageEventHandler(this, mhArgs);

// Check the results here


Alternatively, you can create a

public void TestHandleDataMessage()
    StanMsg msg = new StanMsg(
        false, "bar", 0, 1234, null);

// Call your application code that handles a data message.


While these faciliate unit testing, we always recommend testing your application code end to end.


  • [ ] Robust Benchmark Testing

We use cookies. If you continue to browse the site, you agree to the use of cookies. For more information on our use of cookies please see our Privacy Policy.