nats

What is nats?

The 'nats' npm package is a client library for the NATS messaging system, which is a high-performance, lightweight, and open-source messaging system for cloud-native applications, IoT messaging, and microservices architectures. It provides publish-subscribe, request-reply, and distributed queueing functionalities.

What are nats's main functionalities?

Publish-Subscribe

This feature allows you to publish messages to a subject and have multiple subscribers receive those messages. The code sample demonstrates how to set up a simple publish-subscribe system where a message is published to the 'updates' subject and received by a subscriber.

const { connect, StringCodec } = require('nats');

(async () => {
  const nc = await connect({ servers: 'demo.nats.io:4222' });
  const sc = StringCodec();

  // Subscriber
  const sub = nc.subscribe('updates');
  (async () => {
    for await (const m of sub) {
      console.log(`Received a message: ${sc.decode(m.data)}`);
    }
  })();

  // Publisher
  nc.publish('updates', sc.encode('Hello, NATS!'));
})();

Request-Reply

This feature allows you to send a request and receive a reply, enabling synchronous communication between services. The code sample shows how to set up a responder that listens for requests on the 'help' subject and a requester that sends a request and waits for a reply.

const { connect, StringCodec } = require('nats');

(async () => {
  const nc = await connect({ servers: 'demo.nats.io:4222' });
  const sc = StringCodec();

  // Responder
  nc.subscribe('help', {
    callback: (err, msg) => {
      if (err) {
        console.error(err);
      } else {
        msg.respond(sc.encode('I can help!'));
      }
    }
  });

  // Requester
  const msg = await nc.request('help', sc.encode('Need assistance'), { timeout: 1000 });
  console.log(`Received reply: ${sc.decode(msg.data)}`);
})();

Distributed Queueing

This feature allows you to distribute tasks among multiple workers, ensuring that each task is processed by only one worker. The code sample demonstrates how to set up two workers that listen on the 'tasks' subject and a publisher that sends tasks to be processed.

const { connect, StringCodec } = require('nats');

(async () => {
  const nc = await connect({ servers: 'demo.nats.io:4222' });
  const sc = StringCodec();

  // Worker 1
  nc.subscribe('tasks', { queue: 'workers' }, (err, msg) => {
    if (err) {
      console.error(err);
    } else {
      console.log(`Worker 1 received: ${sc.decode(msg.data)}`);
    }
  });

  // Worker 2
  nc.subscribe('tasks', { queue: 'workers' }, (err, msg) => {
    if (err) {
      console.error(err);
    } else {
      console.log(`Worker 2 received: ${sc.decode(msg.data)}`);
    }
  });

  // Publisher
  nc.publish('tasks', sc.encode('Task 1'));
  nc.publish('tasks', sc.encode('Task 2'));
})();

Other packages similar to nats

amqplib

The 'amqplib' package is a client for RabbitMQ, a widely-used message broker that supports multiple messaging protocols. Compared to NATS, RabbitMQ offers more advanced features like message persistence, complex routing, and transactions, but it is generally heavier and more complex to set up and manage.

kafka-node

The 'kafka-node' package is a client for Apache Kafka, a distributed streaming platform. Kafka is designed for high-throughput, fault-tolerant, and scalable messaging. It is more suitable for large-scale data streaming and log aggregation compared to NATS, which is more lightweight and easier to use for simple messaging needs.

mqtt

The 'mqtt' package is a client for the MQTT protocol, which is designed for lightweight, low-bandwidth, and low-latency communication, often used in IoT applications. While NATS is also lightweight, MQTT is specifically optimized for constrained environments and offers features like last will and testament (LWT) messages.

README

NATS.js - Node.js Client

A Node.js client for the NATS messaging system.

Installation

npm install nats

# to install current dev version:
npm install nats@next

Basic Usage

const NATS = require('nats')

const nc = NATS.connect()

// Simple publisher
nc.publish('foo', 'Hello World!')

// Simple Subscriber - error is set if there was some error
nc.subscribe('foo', (err, msg) => {
  if (err) {
    console.error(err)
    return
  }
  console.log('Received a message: ' + msg)
})

// Unsubscribing
const sub = nc.subscribe('foo', (err, m) => {})
sub.unsubscribe()


// Subscription/Request are given two arguments:
// - an error (undefined if no error)
// - a message object
//   - the message has 4 properties:
//   - data - the message payload (can be undefined)
//   - subject - subject where the message was sent
//   - reply - the reply subject if this is a request (can be undefined)
//   - sid - the subscription id associated with the handler (same value as the return of subscribe())
nc.subscribe('foo', (_, m) => {
  if (m.reply) {
    nc.publish(m.reply, 'got ' + m.data + ' on ' + m.subject + ' in subscription id ' + m.sid)
    return
  }
  console.log('Received a message: ' + m.data + " it wasn't a request.")
})

// Request, creates a subscription to handle any replies to the request
// subject, and publishes the request with an optional payload. This usage
// allows you to collect responses from multiple services
nc.request('request', (_, m) => {
  console.log('Got a response in msg stream: ' + m.data)
})

// Request with a max option will unsubscribe after
// the first max messages are received. You can also specify the number
// of milliseconds you are willing to wait for the response - when a timeout
// is specified, you can receive an error
nc.request('help', (err, m) => {
  if (err && err.code === NATS.REQ_TIMEOUT) {
    console.log('request timed out')
  } else if (err) {
    console.error('request got error', err)
  } else {
    console.log('Got a response for help: ' + m.data)
  }
}, null, { max: 1, timeout: 1000 })

// Replies
nc.subscribe('help', (_, m) => {
  nc.publish(m.reply, 'I can help!')
})

// Close connection
nc.close()

JSON

The json connect property makes it easier to exchange JSON data with other clients.

const nc = NATS.connect({ json: true })
nc.on('connect', () => {
  nc.on('error', (err) => {
    console.log(err)
  })

  nc.subscribe('greeting', (_, m) => {
    // msg is a parsed JSON object object
    if (m.data && m.data.name && m.reply) {
      nc.publish(m.reply, { greeting: 'hello ' + m.data.name })
    }
  })

  // As with all inputs from unknown sources, if you don't trust the data
  // you should verify it prior to accessing it. While JSON is safe because
  // it doesn't export functions, it is still possible for a client to
  // cause issues to a downstream consumer that is not written carefully
  nc.subscribe('unsafe', (_, m) => {
    // for example a client could inject a bogus `toString` property
    // which could cause your client to crash should you try to
    // concatenation with the `+` like this:
    // console.log("received", msg + "here");
    // `TypeError: Cannot convert object to primitive value`
    // Note that simple `console.log(msg)` is fine.
    if (Object.hasOwnProperty.call(m, 'toString')) {
      console.log('tricky - trying to crash me:', m.toString)
      return
    }

    // of course this is no different than using a value that is
    // expected in one format (say a number), but the client provides
    // a string:
    if (isNaN(m.data.amount) === false) {
      // do something with the number
    }
    // ...
  })

  // the bad guy
  nc.publish('unsafe', { toString: 'no good' })

  nc.flush(function () {
    nc.close()
  })
})

Wildcard Subscriptions

const nc = NATS.connect({ json: true })
// "*" matches any token, at any level of the subject.
nc.subscribe('foo.*.baz', (_, m) => {
  console.log('Msg received on [' + m.subject + '] : ' + m.data)
})

nc.subscribe('foo.bar.*', (_, m) => {
  console.log('Msg received on [' + m.subject + '] : ' + m.data)
})

// ">" matches any length of the tail of a subject, and can only be
// the last token E.g. 'foo.>' will match 'foo.bar', 'foo.bar.baz',
// 'foo.foo.bar.bax.22'
nc.subscribe('foo.>', (_, m) => {
  console.log('Msg received on [' + m.subject + '] : ' + m.data)
})

Queue Groups

// All subscriptions with the same queue name will form a queue group.
// Each message will be delivered to only one subscriber per queue group,
// queuing semantics. You can have as many queue groups as you wish.
// Normal subscribers will continue to work as expected.
let received = 0
nc.subscribe('foo', () => {
  received++
}, { queue: 'job.workers' })

Clustered Usage

const servers = ['nats://nats.io:4222', 'nats://nats.io:5222', 'nats://nats.io:6222']

// Randomly connect to a server in the cluster group.
// Note that because `url` is not specified, the default connection is called first
// (nats://localhost:4222). If you don't want default connection, specify one of
// the above the above servers as `url`: `nats.connect(servers[0], {'servers': servers});`
let nc = NATS.connect({ servers: servers })

// currentServer is the URL of the connected server.
nc.on('connect', () => {
  console.log('Connected to ' + nc.currentServer.url.host)
})

// Preserve order when connecting to servers.
nc = NATS.connect({ noRandomize: true, servers: servers })

Draining Connections and Subscriptions

// Unsubscribing removes the subscription handler for a subscription
// and cancels the subscription. Any pending messages on the client's
// buffer are discarded.
//
// Draining is similar to unsubscribe, but the client instead
// sends the unsubscribe request followed by a flush. When the flush
// returns, the subscription handler is removed. Thus the client is
// able to process all messages sent by the server before the subscription
// handler is removed.
//
// Draining is particularly valuable with queue subscriptions preventing
// messages from being lost.

let c1 = 0
const sub = nc.subscribe('foo', () => {
  c1++
  if (c1 === 1) {
    sub.drain((err) => {
      if (err) {
        console.error(err)
        return
      }
      console.log(`subscription ${sub.sid} drained`)
    })
  }
}, { queue: 'q1' })

// It is possible to drain a connection, draining a connection:
// - drains all subscriptions
// - after calling drain it is impossible to make subscriptions or requests
// - when all subscriptions are drained, it is impossible to publish
// messages and drained connection is closed.
// - finally, the callback handler is called (with possibly an error).

let c2 = 0
nc.subscribe('foo', () => {
  c2++
  if (c2 === 1) {
    nc.drain((err) => {
      if (err) {
        console.error('error draining', err)
        return
      }
      console.log('connection drained')
    })
  }
}, { queue: 'q1' })

TLS

const NATS = require('nats')
const fs = require('fs')

// Simple TLS connect
let nc = NATS.connect({ tls: true })

// Overriding and not verifying the server
let tlsOptions = {
  rejectUnauthorized: false
}
nc = NATS.connect({ tls: tlsOptions })
// nc.stream.authorized will be false

// Use a specified CA for self-signed server certificates
tlsOptions = {
  ca: [fs.readFileSync('./test/certs/ca.pem')]
}
nc = NATS.connect({ tls: tlsOptions })
// nc.stream.authorized should be true

// Use a client certificate if the server requires
tlsOptions = {
  key: fs.readFileSync('./test/certs/client-key.pem'),
  cert: fs.readFileSync('./test/certs/client-cert.pem'),
  ca: [fs.readFileSync('./test/certs/ca.pem')]
}
nc = NATS.connect({ tls: tlsOptions })

Basic Authentication

// Connect with username and password in the url
let nc = NATS.connect('nats://foo:bar@localhost:4222')

// Connect with username and password inside object
nc = NATS.connect({ url: 'nats://localhost:4222', user: 'foo', pass: 'bar' })

// Connect with token in url
nc = NATS.connect('nats://mytoken@localhost:4222')

// Connect with token inside object
nc = NATS.connect({ url: 'nats://localhost:4222', token: 'mytoken' })

New Authentication (Nkeys and User Credentials)

See examples for more usage.

const nkeys = require('ts-nkeys')

// Simple connect using credentials file. This loads JWT and signing key
// each time that NATS connects.
let nc = NATS.connect('connect.ngs.global', NATS.creds('./myid.creds'))

// Manually, you need to specify the JWT, and seed and sign the challenge
const jwt = 'eyJ0eXAiOiLN1...'
const nkeySeed = 'SUAIBDPBAUTWCWBKIO6XHQNINK5FWJW4OHLXC3HQ2KFE4PEJUA44CNHTC4'
const sk = nkeys.fromSeed(Buffer.from(nkeySeed))

// Setting nkey and signing callback directly.
nc = NATS.connect('nats://localhost:4222', {
  nkey: 'UAH42UG6PV552P5SWLWTBP3H3S5BHAVCO2IEKEXUANJXR75J63RQ5WM6',
  nonceSigner: function (nonce) {
    return sk.sign(nonce)
  }
})

// Setting user JWT statically.
nc = NATS.connect({
  userJWT: jwt,
  nonceSigner: function (nonce) {
    return sk.sign(nonce)
  }
})

// Having user JWT be a function that returns the JWT. Can be useful for
// loading a new JWT.
nc = NATS.connect({
  userJWT: function () {
    return jwt
  },
  nonceSigner: function (nonce) {
    return sk.sign(nonce)
  }
})

Advanced Usage

// Flush connection to server, callback fires when all messages have
// been processed.
nc.flush((err) => {
  if (err) {
    console.error('error flushing', err)
    return
  }
  console.log('round trip to the server done')
})

// If you want to make sure NATS yields during the processing
// of messages, you can use an option to specify a yieldTime in ms.
// During the processing of the inbound stream, NATS will yield if it
// spends more than yieldTime milliseconds processing.
nc = NATS.connect({ port: 4222, yieldTime: 10 })

// Timeout a subscription unless a certain number of messages have been received
// When a subscription times out, it automatically cancels. You can specify more
// messages in the `expected` option. However that count of messages must be
// received before the timeout specified. If `expected` is not specified, it
// defaults to '1'.
NATS.subscribe('foo', (err) => {
  if (err && err.code === TIMEOUT_ERR) {
    // didn't get the message
  }
  // do something
}, {timeout: 1000, expected: 1})


// Auto-unsubscribe after max messages received
nc.subscribe('foo', () => {}, { max: 100 })
// or
const sub = nc.subscribe('foo', () => {})
sub.unsubscribe(100)



// Encodings

// By default messages received will be decoded using UTF8. To change that,
// set the encoding option on the connection.

NATS.connect({ encoding: 'ascii' })

// PreserveBuffers

// To prevent payload conversion from a Buffer to a string, set the
// preserveBuffers option to true. Message payload return will be a Buffer.

NATS.connect({ preserveBuffers: true })

// Reconnect Attempts and Time between reconnects

// By default a NATS connection will try to reconnect to all servers 10 times
// waiting 2 seconds between the previous reconnect to the server. If the 
// maximum number of retries is reached, the client will close the connection.
// To change the default behaviour specify the max number of connection
// attempts in `maxReconnectAttempts` (set to -1 to retry forever), and the
// time in milliseconds between reconnects in `reconnectTimeWait`.

NATS.connect({ maxReconnectAttempts: -1, reconnectTimeWait: 250 })

Events

The nats client is an event emitter, you can listen to several kinds of events.

// emitted whenever there's an error. if you don't implement at least
// the error handler, your program will crash if an error is emitted.
nc.on('error', (err) => {
  console.log(err)
})

// connect callback provides a reference to the connection as an argument
nc.on('connect', (nc) => {
  console.log(`connect to ${nc.currentServer.url.host}`)
})

// emitted whenever the client disconnects from a server
nc.on('disconnect', () => {
  console.log('disconnect')
})

// emitted whenever the client is attempting to reconnect
nc.on('reconnecting', () => {
  console.log('reconnecting')
})

// emitted whenever the client reconnects
// reconnect callback provides a reference to the connection as an argument
nc.on('reconnect', (nc) => {
  console.log(`reconnect to ${nc.currentServer.url.host}`)
})

// emitted when the connection is closed - once a connection is closed
// the client has to create a new connection.
nc.on('close', () => {
  console.log('close')
})

// emitted whenever the client unsubscribes
nc.on('unsubscribe', (sid, subject) => {
  console.log('unsubscribed subscription', sid, 'for subject', subject)
})

// emitted whenever the server returns a permission error for
// a publish/subscription for the current user. This sort of error
// means that the client cannot subscribe and/or publish/request
// on the specific subject
nc.on('permission_error', (err) => {
  console.error('got a permissions error', err.message)
})

See examples and benchmarks for more information.

Connect Options

The following is the list of connection options and default values.

Option	Default	Description
encoding	"utf8"	Encoding specified by the client to encode/decode data
json	false	If true, message payloads are converted to/from JSON
maxPingOut	2	Max number of pings the client will allow unanswered before raising a stale connection error
maxReconnectAttempts	10	Sets the maximum number of reconnect attempts. The value of -1 specifies no limit
name		Optional client name
nkey	``	See NKeys/User Credentials
noEcho	false	Subscriptions receive messages published by the client. Requires server support (1.2.0). If set to true, and the server does not support the feature, an error with code NO_ECHO_NOT_SUPPORTED is emitted, and the connection is aborted. Note that it is possible for this error to be emitted on reconnect when the server reconnects to a server that does not support the feature.
noRandomize	false	If set, the order of user-specified servers is randomized.
nonceSigner	``	See NKeys/User Credentials. A function that takes a Buffer and returns a nkey signed signature.
pass		Sets the password for a connection
pedantic	false	Turns on strict subject format checks
pingInterval	120000	Number of milliseconds between client-sent pings
preserveBuffers	false	If true, data for a message is returned as Buffer
reconnectTimeWait	2000	If disconnected, the client will wait the specified number of milliseconds between reconnect attempts
reconnect	true	If false server will not attempt reconnecting
servers		Array of connection urls
timeout	node default - no timeout	Number of milliseconds the client will wait for a connection to be established. If it fails it will emit a connection_timeout event with a NatsError that provides the hostport of the server where the connection was attempted.
tls	false	This property can be a boolean or an Object. If true the client requires a TLS connection. If false a non-tls connection is required. The value can also be an object specifying TLS certificate data. The properties ca, key, cert should contain the certificate file data. ca should be provided for self-signed certificates. key and cert are required for client provided certificates. rejectUnauthorized if true validates server's credentials
token		Sets a authorization token for a connection
tokenHandler		A function returning a token used for authentication.
url	"nats://localhost:4222"	Connection url
useOldRequestStyle	false	If set to true calls to request() and requestOne() will create an inbox subscription per call.
user		Sets the username for a connection
userCreds	``	See NKeys/User Credentials. Set with NATS.creds().
userJWT	``	See NKeys/User Credentials. The property can be a JWT or a function that returns a JWT.
verbose	false	Turns on +OK protocol acknowledgements
waitOnFirstConnect	false	If true the server will fall back to a reconnect mode if it fails its first connection attempt.
yieldTime		If set, processing will yield at least the specified number of milliseconds to IO callbacks before processing inbound messages

Tools

The examples, node-pub, node-sub, node-req, node-reply are now bound to bin entries on the npm package. You can use these while developing your own tools. After you install the nats npm package, you'll need to add a dependency on minimist before you can use the tools:

npm install nats
npm install minimist
...
% npx node-sub hello &
[1] 9208
% Listening on [hello]
% npx node-pub hello world
Received "world"
Published [hello] : "world"

Supported Node Versions

Our support policy for Nodejs versions follows Nodejs release support. We will support and build node-nats on even-numbered Nodejs versions that are current or in LTS.

Running Tests

To run the tests, you need to have a nats-server executable in your path. Refer to the server installation guide in the NATS.io documentation. With that in place, you can run npm test to run all tests.

License

Unless otherwise noted, the NATS source files are distributed under the Apache Version 2.0 license found in the LICENSE file.