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emqttd(Erlang MQTT Broker) is an open source MQTT broker written in Erlang/OTP. Erlang/OTP is a concurrent, fault-tolerant, soft-realtime and distributed programming platform. MQTT is an extremely lightweight publish/subscribe messaging protocol powering IoT, M2M and Mobile applications.

The emqttd project is aimed to implement a scalable, distributed, extensible open-source MQTT broker for IoT, M2M and Mobile applications that hope to handle millions of concurrent MQTT clients.

Highlights of the emqttd broker:

  • Full MQTT V3.1/3.1.1 Protocol Specifications Support
  • Easy to Install - Quick Install on Linux, FreeBSD, Mac and Windows
  • Massively scalable - Scaling to 1 million connections on a single server
  • Cluster and Bridge Support
  • Easy to extend - Hooks and plugins to customize or extend the broker
  • Pluggable Authentication - LDAP, MySQL, PostgreSQL, Redis Authentication Plugins


  • Full MQTT V3.1/V3.1.1 protocol specification support
  • QoS0, QoS1, QoS2 Publish and Subscribe
  • Session Management and Offline Messages
  • Retained Message
  • Last Will Message
  • TCP/SSL Connection
  • MQTT Over WebSocket(SSL)
  • HTTP Publish API
  • STOMP protocol
  • STOMP over SockJS
  • $SYS/# Topics
  • ClientID Authentication
  • IpAddress Authentication
  • Username and Password Authentication
  • Access control based on IpAddress, ClientID, Username
  • Authentication with LDAP, Redis, MySQL, PostgreSQL and HTTP API
  • Cluster brokers on several servers
  • Bridge brokers locally or remotely
  • mosquitto, RSMB bridge
  • Extensible architecture with Hooks, Modules and Plugins
  • Passed eclipse paho interoperability tests

Quick Start

Download and Install

The emqttd broker is cross-platform, which could be deployed on Linux, FreeBSD, Mac, Windows and even Raspberry Pi.

Download binary package from:

Installing on Mac, for example:

unzip && cd emqttd

# Start emqttd
./bin/emqttd start

# Check Status
./bin/emqttd_ctl status

# Stop emqttd
./bin/emqttd stop

Installing from Source


emqttd broker requires Erlang R18+ to build since 1.1 release.

git clone

cd emqttd && make && make dist

cd rel/emqttd && ./bin/emqttd console

Web Dashboard

A Web Dashboard will be loaded when the emqttd broker is started successfully.

The Dashboard helps check running status of the broker, monitor statistics and metrics of MQTT packets, query clients, sessions, topics and subscriptions.

Default Address http://localhost:18083
Default User admin
Default Password public

Modules and Plugins

The Authentication and Authorization(ACL) are usually implemented by a Module or Plugin.


emqttd_auth_clientid Authentication with ClientId
emqttd_auth_username Authentication with Username and Password
emqttd_auth_ldap Authentication with LDAP
emqttd_mod_presence Publish presence message to $SYS topics when client connected or disconnected
emqttd_mod_subscription Subscribe topics automatically when client connected
emqttd_mod_rewrite Topics rewrite like HTTP rewrite module

Configure the ‘auth’, ‘module’ paragraph in ‘etc/emqttd.config’ to enable a module.

Enable ‘emqttd_auth_username’ module:

{access, [
    %% Authetication. Anonymous Default
    {auth, [
        %% Authentication with username, password
        {username, []},


Enable ‘emqttd_mod_presence’ module:

{modules, [
    %% Client presence management module.
    %% Publish messages when client connected or disconnected
    {presence, [{qos, 0}]}


A plugin is an Erlang application to extend the emqttd broker.

emqttd_plugin_template Plugin template and demo
emqttd_dashboard Web Dashboard
emqttd_auth_http Authentication/ACL with HTTP API
emqttd_plugin_mysql Authentication with MySQL
emqttd_plugin_pgsql Authentication with PostgreSQL
emqttd_plugin_redis Authentication with Redis
emqttd_plugin_mongo Authentication with MongoDB
emqttd_stomp STOMP Protocol Plugin
emqttd_sockjs SockJS(Stomp) Plugin
emqttd_recon Recon Plugin

A plugin could be enabled by ‘bin/emqttd_ctl plugins load’ command.

For example, enable ‘emqttd_plugin_pgsql’ plugin:

./bin/emqttd_ctl plugins load emqttd_plugin_pgsql

One Million Connections

Latest release of emqttd broker is scaling to 1.3 million MQTT connections on a 12 Core, 32G CentOS server.


The emqttd broker only allows 512 concurrent connections by default, for ‘ulimit -n’ limit is 1024 on most platform.

We need tune the OS Kernel, TCP Stack, Erlang VM and emqttd broker for one million connections benchmark.

Linux Kernel Parameters

# 2M:
sysctl -w fs.file-max=2097152
sysctl -w fs.nr_open=2097152
echo 2097152 > /proc/sys/fs/nr_open

# 1M:
ulimit -n 1048576

TCP Stack Parameters

# backlog
sysctl -w net.core.somaxconn=65536

Erlang VM


## max process numbers
+P 2097152

## Sets the maximum number of simultaneously existing ports for this system
+Q 1048576

## Increase number of concurrent ports/sockets
-env ERL_MAX_PORTS 1048576

-env ERTS_MAX_PORTS 1048576

emqttd broker


{mqtt, 1883, [
    %% Size of acceptor pool
    {acceptors, 64},

    %% Maximum number of concurrent clients
    {max_clients, 1000000},

    %% Socket Access Control
    {access, [{allow, all}]},

    %% Connection Options
    {connopts, [
        %% Rate Limit. Format is 'burst, rate', Unit is KB/Sec
        %% {rate_limit, "100,10"} %% 100K burst, 10K rate

Test Client

sysctl -w net.ipv4.ip_local_port_range="500 65535"
echo 1000000 > /proc/sys/fs/nr_open
ulimit -n 100000

MQTT Client Libraries


emqttc Erlang MQTT Client
emqtt_benchmark MQTT benchmark Tool
CocoaMQTT Swift MQTT Client