Muqtti

A zero-dependency MQTT v5.0 broker implementation in Java 11.

Features

In-memory storage of QoS 1 messages
Certificate-based Mutual TLS
Most MQTT v5.0 features like:
- Shared subscriptions
- Message properties
- Flow control
- NACK packets

Not Supported

QoS 2
Retained messages
AUTH packet
Websockets

Use cases

As a lightweight broker for local development
Embedded broker for unit tests (JUnit rule coming soon)
Extensible broker for custom authentication schemes

Getting Started

You'll need at least Java 11 to run Muqtti. The easiest way to get started is to download the latest release jar and run it:

java -jar muqtti-1.0.0.jar

Muqtti is a standard maven project, so you can also build from source using:

mvn -B package

Usage

Once you have Muqtti running, you can use pub/sub commands bundled with mosquitto as clients to Muqtti.

To subscribe:

mosquitto_pub -V 5 -t vehicles/teslas/1

To publish:

mosquitto_pub -V 5 -t vehicles/teslas/1 -m 'wake up'

NOTE: You'll need to specify -V 5 because mosquitto tries to connect using MQTT 3.1.1 by default, and Muqtti only supports MQTT v5.0

Shared subscriptions

An interesting addition to MQTT v5.0 is the concept of shared subscriptions. With shared subscriptions, you can have multiple clients subscribed to the same topic filter and have messages load-balanced among the clients. You can test this feature out by running two separate mosquitto_sub clients with the command line:

mosquitto_sub -V 5 -t '$share/group1/#'

Then publish messages to any topic - you should see the messages alternate between the clients. You can read more about shared subscriptions in the spec

Configuration

Muqtti is configured using Java system properties.

TLS

To enable TLS, pass it the following properties:

-Dmuqtti.port=8883 -Dmuqtti.certfile=/path/to/servercert.pem -Dmuqtti.keyfile=/path/to/server.pkcs8.private.key -Dmuqtti.cafile=/path/to/rootcacert.pem

These properties are equivalent to the certfile, keyfile and cafile configuration options in mosquitto

Note that only PKCS#8 RSA private keys are supported - if you have keys generated by openssl you can converted them to PKCS#8 using:

openssl pkcs8 -topk8 -nocrypt -in <openssl_generated_key> -out <pkcs8_key>

Logging

Muqtti uses java.util.logging (aka j.u.l) that comes with the JDK. You can specify a custom logging.properties file using:

-Djava.util.logging.config.file=/path/to/custom-logging.properties

A logging configuration file that enables verbose logging is included in the repo as an example (muqtti-logging-example.properties)

Motivation

I've written my fair share of networked services professionally. However, all my projects were built on top of open source networking libraries like the excellent Netty Project. My goal was to create a project using only the standard JDK so I could delve into lower level aspects like protocol encoding/decoding using ByteBuffers and asynchronous I/O using Selectors.

It also demonstrates the power of the Java platform and shows its relevance even when it is losing popularity among the trendier tech demographics. This is not to say that the platform doesn't have its rough edges (hello, SSLEngine), but overall, it provides a solid base for programmers to build software of moderate-to-high complexity without having to tear their hair out.

Scale-out MQTT v5.0 broker

If you're looking for a horizontally scalable, easily deployable (in the cloud or on-prem) MQTT v5.0 broker that plays well with Kubernetes, contact me at lokare.ameya@gmail.com

juggernaut/muqtti