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0.0.3 • Public • Published


Build Status npm version

middleware based components to build a custom mqtt broker

In development, not yet stable.


mqtt-stack is available in npm, first you need to install

npm install mqtt-stack --save

require it in your project

var mqttStack = require('mqtt-stack');

instantiate stack

var stack = new mqttStack.Stack();

register any middleware to be used by calling use method

stack.use(new mqttStack.MemoryBackend());


In a broad terms, mqtt-stack middlewares are components that listen mqtt connection stream to perform actions according to their specific broker functionality.


Middleware base class implementation is available in module exports, developpers are encouraged to inherit from that base class. mqtt-stack middlewares may implement following interface methods.

constructor (config)

Standard object constructor function takes configuration object as argument. If middleware is inherited from Middleware base class super(config, defaults) call sets up config member attribute of middleware object.

class MyMiddleware {
  constructor(config) {
    let defaults = {some: 'value'};
    /* calling super function sets this.config and
     * attributes */
    super(config, defaults);

install (client)

Method is called once a new connection is established.

handle (client, packet, next, done)

Method is called once a packet is received. Once middleware finishes its action, it should either call next function to propagate to next middleware or call done function to terminate propagation.

callback handlers (ctx, store, next, done)

Other than these interface methods, middleware may handle a stack callback by exposing a method function with callback name. For instance, please check OutboundManager middleware (path: src/middlewares/outbound_manager.js) to see forwardMessage callback handler. ctx argument is an object which contains any relevant data required for callback handling. store is an output argument, that is updated by callback handlers. done terminates callback chain and returns callback.

Built-in Middlewares

mqtt-stack provide some built-in middlewares to provide basic MQTT Broker functionality. Keep in mind that those middlewares are not mandatory, on contrary they are designed to be easily replacible.


Simple authentication binder middleware that executes authenticateConnection callback handler with {client, packet, username, password} context if client is not authenticated.


Simple authorization binder middleware that executes authorizePacket callback handler with {client, packet} context for every received packet.


Simple connection management middleware. It observes connection status.

When connection is closed gracefully it executes cleanDisconnect callback handler with {client} context. When connection is closed unexpectedly it executes uncleanDisconnect callback handler with {client} context.

It exposes closeClient callback handler that will terminate client connection.


Handles client's PUBLISH command by executing relayMessage callback handler with {client, packet, topic, payload} context. Once callback handler finishes and it sends PUBACK message to client if its QoS is 1.


Manages client connection's life span. Once client's CONNECT command is received, if it contains keepalive duration, middleware bounds life time of connection with this duration and resets the time on every received packet. It executes closeClient callback handler with {client} context if no packet is received within keepalive time frame.

It also responds received PINGREQ commands with PINGRESP.


Sends last will packet if client is disconnected unexpectedly. Once client's CONNECT command is received if it contains last will packet, will packet is stored. This middleware exposes uncleanDisconnect callback handler that sends will packet.


Simple non-persistent backend storage middleware. It stores clients' subscription list and topics' retained messages in memory. It exposes following callback handlers

  • storeSubscription stores that ctx.client is subscribed to ctx.topic with ctx.qos QoS level.
  • removeSubscription removes subscription record of ctx.client for topic ctx.topic.
  • clearSubscriptions removes all stored subscription data for ctx.client.
  • lookupSubscriptions returns all stored subscription data for ctx.client in store argument.
  • storeRetainedMessage clears previous retained message of ctx.topic and if ctx.packet.payload is not empty stores ctx.packet as new retained message.
  • lookupRetainedMessages returns stored retained message of ctx.topic in store argument.
  • relayMessage relays ctx.packet to subscribers of ctx.packet.topic by executing forwardMessage callback handler with context {client, packet}.
  • subscribeTopic subscribes ctx.client to ctx.topic with QoS level defined by ctx.qos.
  • unsubscribeTopic unsubscribes ctx.client from ctx.topic.
  • storeOfflineMessage stores ctx.packet for offline ctx.client with ctx.messageId.
  • lookupOfflineMessages returns all messages stored for ctx.client in store argument.
  • removeOfflineMessages removes messages with id's in the list ctx.messageIds stored for client with id ctx.clientId.
  • clearOfflineMessages removes all messages stored for client with id ctx.clientId.


Manages outgoing messages. Handles client's PUBACK command. Exposes forwardMessage that publishes message to client.


Simple event bridge that establishes connection with an eventemitter and connection. Event emitter should be set by calling setClientHandler method before it is used.


Manages retained messages for topics. If client's PUBLISH command has flag retain it executes storeRetainedMessage callback handler with {client, topic of packet, packet} context.

It exposes subscribeTopic callback handler that first executes lookupRetainedMessages callback handler with {topic} then if topic has retained message executes forwardMessage handler with {client, retained packet}


Manages the clients session and calls callbacks to manage the stored subscriptions for clean and unclean clients. Once client's CONNECT command is received,

  • if it contains clean flag, session manager does not store its subscriptions for later connections and also executes clearSubscriptions callback handler with {client, packet, clientId} context to destroy clients previous session. then sends CONNACK to client.
  • it it clean flag is false or not exists, session manager first executes lookupSubscriptions callback handler with {client, packet, clientId} context to retrieve old subscription list, then executes subscribeTopic callback handler for each subscription in list with {client, packet, topic, subscription QoS} context to restore old subscriptions. After session is restored, CONNACK is sent to client.


Manages client's SUBSCRIBE and UNSUBSCRIBE commands. For subscibe, it executes subscribeTopic callback handler with {client, packet, topic, QoS level} context, then SUBACK is sent to client. For unsubscribe it executes unsubscribeTopic callback handler with {client, packet, topic} context, then UNSUBACK is sent to client.


Unit test are available in test folder. Project includes mqtt-spec as git submodule. Stack is tested for mqtt specifications using mqtt-spec module. Very primitive benchmarking results +20k message per second for loopback network.


mqtt-stack is an OPEN Open Source Project. This means that:

Individuals making significant and valuable contributions are given commit-access to the project to contribute as they see fit. This project is more like an open wiki than a standard guarded open source project.

See the file for more details.


mqtt-stack is only possible due to the excellent work of the following contributors:

Joël GähwilerGitHub/256dpiTwitter/@256dpi
Matteo CollinaGitHub/mcollinaTwitter/@matteocollina
M Kamil SulubulutGitHub/kokeksibirTwitter/@kokeksibir






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