Overview

This module provides the capability to send and receive messages by connecting to the NATS server.

NATS messaging enables the communication of data that is segmented into messages among computer applications and services. Data is encoded and framed as a message and sent by a publisher. The message is received, decoded, and processed by one or more subscribers. NATS makes it easy for programs to communicate across different environments, languages, cloud providers, and on-premise systems. Clients connect to the NATS system usually via a single URL and then subscribe or publish messages to a subject.

Basic usage

Set up the connection

First, you need to set up the connection with the NATS Basic server. The following ways can be used to connect to a NATS Basic server.

1. Connect to a server using the default URL:

nats:Client natsClient = check new(nats:DEFAULT_URL);

2. Connect to a server using the URL:

nats:Client natsClient = check new("nats://serverone:4222");

3. Connect to one or more servers with custom configurations:

nats:ConnectionConfiguration config = {
    connectionName: "my-nats",
    noEcho: true
};
nats:Client natsClient = check new(["nats://serverone:4222",  "nats://servertwo:4222"],  config);

Publish messages

Publish messages to the NATS basic server

Once connected, publishing is accomplished via one of the three methods below.

1. Publish with the subject and the message content:

string message = "hello world";
nats:Error? result = 
    natsClient->publishMessage({ content: message.toBytes(), subject: "demo.nats.basic"});

2. Publish as a request that expects a reply:

string message = "hello world";
nats:Message|nats:Error reqReply = 
    natsClient->requestMessage({ content: message.toBytes(), subject: "demo.nats.basic"}, 5);

3. Publish messages with a replyTo subject:

string message = "hello world";
nats:Error? result = natsClient->publish({ content: message.toBytes(), subject: "demo.nats.basic",
                                                    replyTo: "demo.reply" });

Listen to incoming messages

Listen to messages from a NATS server

1. Listen to incoming messages with the onMessage remote method:

// Binds the consumer to listen to the messages published to the 'demo.example.*' subject
@nats:ServiceConfig {
    subject: "demo.example.*"
}
service nats:Service on new nats:Listener(nats:DEFAULT_URL) {

    remote function onMessage(nats:Message message) {
    }
}

2. Listen to incoming messages and reply directly with the onRequest remote method:

// Binds the consumer to listen to the messages published to the 'demo.example.*' subject
@nats:ServiceConfig {
    subject: "demo.example.*"
}
service nats:Service on new nats:Listener(nats:DEFAULT_URL) {

    // The returned message will be published to the replyTo subject of the consumed message
    remote function onRequest(nats:Message message) returns string? {
        return "Reply Message";
    }
}

Advanced usage

Set up TLS

The Ballerina NATS module allows the use of TLS in communication. This setting expects a secure socket to be set in the connection configuration as shown below.

Configure TLS in the nats:Listener

nats:SecureSocket secured = {
    cert: {
        path: "<path>/truststore.p12",
        password: "password"
    },
    key: {
        path: "<path>/keystore.p12",
        password: "password"
    }
};
nats:Listener natsListener = check new("nats://serverone:4222", secureSocket = secured);

Configure TLS in the nats:Client

nats:SecureSocket secured = {
    cert: {
        path: "<path>/truststore.p12",
        password: "password"
    },
    key: {
        path: "<path>/keystore.p12",
        password: "password"
    }
};
nats:Client natsClient = check new("nats://serverone:4222", secureSocket = secured);