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EMQ (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 EMQ 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 EMQ 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
  • MQTT-SN Protocol
  • CoAP 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
  • Local subscription
  • Shared subscription

Quick Start

Download and Install

The EMQ 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


The EMQ 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 EMQ 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


The EMQ broker could be extended by Plugins. A plugin is an Erlang application to extend the EMQ broker:

emq_auth_clientid Authentication with ClientId
emq_auth_username Authentication with Username and Password
emq_plugin_template Plugin template and demo
emq_dashboard Web Dashboard
emq_auth_ldap LDAP Auth Plugin
emq_auth_http Authentication/ACL with HTTP API
emq_auth_mysql Authentication with MySQL
emq_auth_pgsql Authentication with PostgreSQL
emq_auth_redis Authentication with Redis
emq_mod_rewrite Topics rewrite like HTTP rewrite module
emq_mod_retainer Retainer Module
emq_mod_presence Presence Module
emq_mod_subscription Subscription Module
`emq_mod_mongo`_ Authentication with MongoDB
emq_sn MQTT-SN Protocol Plugin
emq_coap CoAP Protocol Plugin
emq_stomp STOMP Protocol Plugin
emq_sockjs SockJS(Stomp) Plugin
emq_recon Recon Plugin
emq_reloader Reloader Plugin

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

For example, enable ‘emq_auth_pgsql’ plugin:

./bin/emqttd_ctl plugins load emq_auth_pgsql

One Million Connections

Latest release of the EMQ 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


## Erlang Process Limit
node.process_limit = 2097152

## Sets the maximum number of simultaneously existing ports for this system
node.max_ports = 1048576

Max Allowed Connections

emqttd/etc/emq.conf ‘listeners’:

## Size of acceptor pool
mqtt.listener.tcp.acceptors = 64

## Maximum number of concurrent clients
mqtt.listener.tcp.max_clients = 1000000

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

Eclipse Paho: