This component provides a NATS client for reading and sending messages from/to a https://nats.io server. The client supports both Core NATS as well as JetStream.
The nats-java-vertx-client is a Java client library for connecting to the NATS messaging system. It is built on top of the Vert.x event-driven framework and provides an asynchronous, non-blocking API for sending and receiving messages over NATS.
Current Release: 2.0.3 Â Current Snapshot: 2.0.4-SNAPSHOT
To use this component, add the following dependency to the dependencies section of your build descriptor:
<dependency>
<groupId>io.nats</groupId>
<artifactId>nats-vertx-interface</artifactId>
<version>${maven.version}</version>
</dependency>compile io.nats:nats-vertx-interface:${maven.version}
To connect to NATS, you must first create a NatsClient object by specifying the NATS server URL and any optional configuration options. Here is an example:
// Set options
final NatsOptions natsOptions = new NatsOptions();
natsOptions.getNatsBuilder().servers(new String[]{"localhost:" + port});
// Create client
final NatsClient natsClient = NatsClient.create(natsOptions);
final Future<Void> connect = natsClient.connect();This code creates a NatsClient object, sets the configuration options, and connects to a NATS server.
The first two lines create a new NatsOptions object and set the NATS server to connect to using the servers method of the NatsBuilder object. In this case, the server is set to "localhost" and the port variable is used to specify the port number.
The following two lines create a new NatsClient object using the create method and passing in the NatsOptions object as a parameter.
The connect method is then called on the NatsClient object to establish a connection to the NATS server.
This method returns a Future object, representing the connection attempt's asynchronous result.
NatsClient client = NatsClient.create(config.setVertx(vertx));
// Connect
client.connect()
.onSuccess(v ->
System.out.println("NATS successfully connected!"))
.onFailure(err ->
System.out.println("Fail to connect to NATS " + err.getMessage()));This code creates a NatsClient object, and sets the Vert.x instance, and connects to a NATS server using the Vert.x Future interface to handle the asynchronous result.
Using the create method, the first line creates a new NatsClient object and passes in a NatsOptions configuration
object as a parameter. In this case, the configuration object is created by setting the Vert.x instance to use with
the NATS client. The vertx is an instance of the Vert.x class.
The following few lines establish a connection to the NATS server using the connect method of the NatsClient object.
This method returns a Future<Void> object, representing the connection attempt's asynchronous result.
The onSuccess and onFailure methods of the Future object are then called (one or the other depending on outcome) to handle the result of the connection attempt.
In this example, we just print a message to the console to indicate whether the connection succeeded or failed.
Once connected, publishing is accomplished via one of three methods:
- With a subject and message body:
client
.publish("subject", "hello world".getBytes(StandardCharsets.UTF_8))
.onSuccess(v ->
System.out.println("Message published!"))
.onFailure(err ->
System.out.println("Something went wrong " + err.getMessage()));This code publishes a message to a NATS subject using the publish method of a NatsClient object, and handles the asynchronous
result of the operation using the Vert.x Future interface.
The publish method takes two parameters: the subject to publish the message to, and the message data as a byte array.
The onSuccess and onFailure methods are called on the Future object returned by the publish method, and are used to
handle the asynchronous result of the operation. In this example, we're just printing a message to the console to
indicate whether the operation succeeded or failed.
If the operation succeeds, the onSuccess method is called with a Void parameter. In this case, we're just printing the
message "Message published!" to the console.
If the operation fails, the onFailure method is called with a Throwable parameter. In this case, we're just printing a message to the console that includes the error message returned by the Throwable object.
- With a subject and message body, as well as a subject for the receiver to reply to:
client
.publish("subject", "replyto", "hello world".getBytes(StandardCharsets.UTF_8))
.onSuccess(v -> System.out.println("Message published!"))
.onFailure(err -> System.out.println("Something went wrong " + err.getMessage()));This code publishes a message to a NATS subject and specifies a reply-to subject to use for any responses received in reply to the message. It also handles the asynchronous result of the operation using the Vert.x Future interface.
The publish method takes three parameters: the subject to publish the message to, the reply-to subject to use for any responses, and the message data as a byte array.
The onSuccess and onFailure methods are called on the Future object returned by the publish method, and are used to
handle the asynchronous result of the operation. In this example, we're just printing a message to the console to
indicate whether the operation succeeded or failed.
If the operation succeeds, the onSuccess method is called with a Void parameter. In this case, we're just printing the
message "Message published!" to the console.
If the operation fails, the onFailure method is called with a Throwable parameter. In this case, we're just printing
a message to the console that includes the error message returned by the Throwable object.
- When a request expects a reply, a response is provided. Under the covers as request/reply pair is the same as a publish/subscribe only the library manages the subscription for you.
client
.request("subject", "hello world".getBytes(StandardCharsets.UTF_8))
.onSuccess(response ->
System.out.println("Received response " + response.getData()))
.onFailure(err ->
System.out.println("Something went wrong " + err.getMessage()));When a request is made and a reply is expected, the NATS messaging system provides a response. This is achieved through a request/reply pair. This is the same as a publish/subscribe pair, except the library handles the subscription for you.
This code makes a request to a NATS subject and handles a response using the request method of a NatsClient object,
and handles the asynchronous result of the operation using the Vert.x Future interface.
The request method takes two parameters: the subject to send the request to, and the message data as a byte array. When a response is received, the onSuccess method is called with a Message parameter, which contains the response data as a byte array.
If the operation fails, the onFailure method is called with a Throwable parameter. In this case, we're just printing a
message to the console that includes the error message returned by the Throwable object.
All of these methods, as well as the incoming message code use byte arrays for maximum flexibility. Applications can send JSON, Strings, YAML, Protocol Buffers, or any other format through NATS to applications written in a wide range of languages.
The Java NATS library also provides a mechanism to listen to messages.
natsClient.subscribe(SUBJECT_NAME, "FOO", event -> {
doSomethingWithTheEvent(event);
});Unsubscribing from messages.
client.unsubscribe(SUBJECT_NAME)
.onFailure(Throwable::printStackTrace)
.onSuccess(event -> System.out.println("Success"));Publishing and subscribing to JetStream enabled servers is straightforward. A JetStream enabled application will connect to a server, establish a JetStream context, and then publish or subscribe. This can be mixed and matched with standard NATS subject, and JetStream subscribers, depending on configuration, receive messages from both streams and directly from other NATS producers.
After establishing a connection as described above, create a JetStream Context.
final Future<NatsStream> streamFuture = natsClient.jetStream();
streamFuture.onSuccess(natsStream -> {
doSomethingWithStream(natsStream)
}).onFailure(error -> {
handleTheStreamFailed(error);
});You can pass options to configure the JetStream client, although the defaults should suffice for most users. See the JetStreamOptions class.
There is no limit to the number of contexts used, although normally, one would only require a single context.
To publish messages, use the publish method.
final NatsStream jetStreamPub = ...
final NatsStream jetStreamSub = ...
final String data = "data";
jetStreamSub.subscribe(SUBJECT_NAME, event -> {
doSomethingWithMessage(event.message());
}, true, PushSubscribeOptions.builder().build());
// Send a message
final NatsMessage message = NatsMessage.builder()
.subject(SUBJECT_NAME)
.data(data + i, StandardCharsets.UTF_8).build();
jetStreamPub.publish(message).onSuccess(event -> ...).onError(error -> ...);
To unsubscribe from JetStream the interface is similar to unsubscribing to a NATS subscription.
jetStreamSub.unsubscribe(SUBJECT_NAME).onSuccess(event -> System.out.println("Unsubscribed"))
.onFailure(Throwable::printStackTrace);There are a variety of publish options that can be set when publishing. When duplicate checking has been enabled on the stream, a message ID should be set. One set of options is expectations. You can set a publish expectation such as a particular stream name, previous message ID, or previous sequence number. These are hints to the server that it should reject messages where these are not met, primarily for enforcing your ordering or ensuring messages are not stored on the wrong stream.
void publish(Message data, Handler<AsyncResult<Void>> handler);
Future<Void> publish(Message data);
Future<Void> publish(String subject, String replyTo, String message);
Future<Void> publish(String subject, String message);There are two methods of subscribing, Push and Pull with each variety having its own set of options and abilities.
Push subscriptions are asynchronous. The server pushes messages to the client.
PushSubscribeOptions so = PushSubscribeOptions.builder()
.durable("optional-durable-name")
.build();
boolean autoAck = ...
js.subscribe("my-subject", (msg) -> {
// Process the message.
// Ack the message depending on the ack model
}, autoAck, so)
.onSuccess(done ->
System.out.println("Subscribe success."))
.onFailure(err ->
System.out.println("Something went wrong " + err.getMessage()));Pull subscriptions are always synchronous. The server organizes messages into a batch that it sends when requested.
PullSubscribeOptions pullOptions = PullSubscribeOptions.builder()
.durable("durable-name-is-required")
.build();
js.subscribe("subject", pullOptions)
.onSuccess(done ->
System.out.println("Subscribe success.")
JetStreamSubscription sub = done.result()
sub
.fetch(100, Duration.ofSeconds(1))
.onSuccess(messages ->
for (Message m : messages) {
// process message
m.ackAsync().onSuccess(e -> ...).onError(err -> ...);
}
)
.onFailure(err ->
System.out.println("Something went wrong " + err.getMessage()))
.onFailure(err ->
System.out.println("Something went wrong " + err.getMessage()));The fetch pull is a macro pull that uses advanced pulls under the covers to return a list of messages. The list may be empty or contain at most the batch size. All status messages are handled for you. The client can provide a timeout to wait for the first message in a batch. The timeout may be exceeded if the server sends messages very near the end of the timeout period.
See https://github.com/nats-io/nats.java#ordered-push-subscription-option
See https://github.com/nats-io/nats.java#subscription-creation-checks
There are multiple types of acknowledgments in JetStream:
Message.ackAsync(): Acknowledges a message.Message.ackSync(Duration): Acknowledges a message and waits for a confirmation. When used with deduplications, this creates exactly once delivery guarantees (within the deduplication window). This deduplication may significantly impact the performance of the system.Message.nakAsync(): A negative acknowledgment indicating processing failed, and the message should be resent later.Message.termAsync(): Never send this message again, regardless of configuration.Message.inProgressAsync(): The message is being processed, and reset the redelivery timer in the server. The message must be acknowledged later when processing is complete.
Note that exactly once delivery guarantee can be achieved by using a consumer with explicit ack mode attached to stream setup with a deduplication window and using the ackSync to acknowledge messages. The guarantee is only valid for the duration of the deduplication window.
You should always use the async versions of the methods when running in the vert.x event loop.
The nats-java-vertx-client library provides a simple and easy-to-use API for connecting to NATS messaging system from Java applications, using the Vert.x interface. With the asynchronous, non-blocking API and Vert.x event-driven framework, it is well-suited for building high-performance, scalable messaging applications.