rust-mqtt

rust-mqtt provides an MQTT client primarily for no_std environments. The library provides an async API depending on embedded_io_async's traits. As of now, only MQTT version 5.0 is supported.

The design goal is a strict yet flexible and explicit API that leverages Rust's type system to enforce the MQTT specification while exposing all protocol features transparently. Session state, configuration, and Quality of Service message delivery and retry behaviour remain fully under user control, giving complete freedom over protocol usage. Protocol-related errors are prevented by the client API and are modeled in a way that enables maximum recoverability. By avoiding opinionated design choices and making no assumptions about the runtime environment, rust-mqtt remains lightweight while providing a powerful MQTT client foundation.

rust-mqtt does not implement opinionated connection management — automatic reconnects, keepalive loops, retry policies, or background tasks are intentionally left to the user. Instead, the crate provides cancel-safe protocol primitives, suitable for higher-level clients, tooling, and resource-constrained embedded applications. In the future, the client will be extended with additional I/O traits such as ReadReady to further composability.

Library state

Supported MQTT features

Will
Bidirectional publications with Quality of Service 0, 1 and 2
Flow control
Configuration & session tracking
Session recovery
Client- & serverside maximum packet size
Subscription identifiers
Message expiry interval
Topic alias
Request/Response
Reason String in CONNACK & DISCONNECT packets

Currently unsupported MQTT features & limitations

AUTH packet
Properties: Authentication Method, Authentication Data, Request Problem Information, Reason String (PUBACK, PUBREC, PUBREL, PUBCOMP, SUBACK, UNSUBACK), User Property
Subscribing to multiple topics in a single packet

Extension plans (more or less by priority)

More versatile IO model allowing for more cancel-safety
Sync implementation
MQTT version 3.1.1

Feature flags

bump: Adds a simple bump allocator BufferProvider implementation
alloc: Adds an Owned(Box<[u8]>) variant to Bytes and a heap-allocation based BufferProvider implementation using the alloc crate
v3: Unused
v5: Enables MQTT version 5.0
Logging-related:
- log: Enables logging via the log crate
- defmt: Implements defmt::Format for crate items & enables logging via the defmt crate (version 1)
- log-level-*: Enables logs at the selected level and more severe levels (error, warn, info, debug, trace)
- log-verbose: Enables high-overhead IO traces at the trace log level and enables log-level-trace

Usage

It is recommended to use a buffering Write implementation, as the current IO model makes fragmented Write::write calls. The client also calls Read::read frequently; if your underlying implementation involves expensive syscalls, consider using a buffering reader as well.

Illustrative API example

Showing explicit session recovery and Quality of Service 2 retransmission after a network failure. The precise network and executor setup is omitted for brevity.

async fn main() {
    let mut buffer = AllocBuffer;
    let mut client = Client::new(&mut buffer);

    let transport = ...;    // Any Read/Write implementation (TCP, TLS, ...)

    let connect_options = ConnectOptions::new()
        .clean_start()
        .session_expiry_interval(SessionExpiryInterval::NeverEnd)
        .user_name(MqttString::from_str("user").unwrap())
        .password(MqttBinary::from_slice("pass").unwrap());

    client.connect(
        transport,
        &connect_options,
        Some(MqttString::from_str("rust-mqtt-demo").unwrap()),
    ).await.unwrap();

    let topic = TopicName::new(MqttString::from_str("demo/topic").unwrap()).unwrap();

    client.subscribe(
        topic.as_borrowed().into(),
        SubscriptionOptions::new().exactly_once(),
    ).await.unwrap();

    let packet_identifier = client.publish(
        &PublicationOptions::new(topic.as_borrowed().into()).exactly_once(),
        "Hello World!".into(),
    ).await.unwrap().unwrap();

    while let Ok(event) = client.poll().await {
        if let Event::PublishComplete(_) = event {
            // Publish succeeded, we can disconnect
            client.disconnect(&DisconnectOptions::new()).await.unwrap();
            return;
        }
    }

    // An error has occured (e.g. network failure)
    client.abort().await;

    let transport = ...;    // Open a fresh connection

    client.connect(
        transport,
        &connect_options,
        Some(MqttString::from_str("rust-mqtt-demo").unwrap()),
    ).await.unwrap();


    // Recover the in-flight Quality of Service 2 publish.

    match client.session().cpublish_flight_state(packet_identifier) {
        // - Republish if PUBLISH / PUBREC may have been lost
        Some(CPublishFlightState::AwaitingPubrec) => client.republish(
            packet_identifier,
            &PublicationOptions::new(topic.into()).exactly_once(),
            "Hello World!".into(),
        ).await.unwrap(),
        // - Re-release if PUBREL / PUBCOMP may have been lost
        Some(CPublishFlightState::AwaitingPubcomp) => client.rerelease().await.unwrap(),
        // - Flight state already completed
        _ => {}
    }
}

Examples

'demo' is a showcase of rust-mqtt's features over TCP. Note that the example usage is very strict and not really a good way of using the client.
'tls' connects the client to a broker over TLS with client certificate authentication and server certificate verification using embedded-tls.

Set up the broker for 'demo' by installing, configuring and running Mosquitto using the CI configuration:

cp .ci/mqtt_pass_plain.txt .ci/mqtt_pass_hashed.txt
chmod 700 .ci/mqtt_pass_hashed.txt
mosquitto_passwd -U .ci/mqtt_pass_hashed.txt
mosquitto -c .ci/mosquitto.conf -v

Set up the broker for 'tls' by running Mosquitto with the tls config file. The required PKI files have been generated using the .ci/pki/generate.sh script.

mosquitto -c .ci/mosquitto-tls.conf -v

Then you can run the examples with different logging configs and the bump/alloc features:

RUST_LOG=info cargo run --example demo
RUST_LOG=info cargo run --example tls
RUST_LOG=trace cargo run --example demo --no-default-features --features "v5 log bump log-level-trace"

Tests

Unit tests should be ran using both the 'alloc' and 'bump' features.

cargo test unit
cargo test unit --no-default-features --features "v5 bump"

For integration tests, you can set up the mosquitto broker as used in the CI pipeline. You should restart the broker after every run of the integration test suite as it carries non-idempotent state that will impact the tests.

cp .ci/mqtt_pass_plain.txt .ci/mqtt_pass_hashed.txt
chmod 700 .ci/mqtt_pass_hashed.txt
mosquitto_passwd -U .ci/mqtt_pass_hashed.txt
mosquitto -c .ci/mosquitto.conf [-d]

Then you can run integration tests with the alloc feature.

cargo test integration

It can be helpful to see logging output when running tests.

RUST_LOG=trace cargo test unit --no-default-features --features "v5 bump log-verbose log" -- --show-output
RUST_LOG=warn cargo test -- --show-output
RUST_LOG=info cargo test integration --features "log" -- --show-output

The full test suite can run with the alloc feature, just make sure a fresh broker is up and running.

cargo test

Acknowledgment

This project could not be in state in which currently is without Ulf Lilleengen and the rest of the community from Drogue IoT.

Contact

For any information, open an issue if your matter could be helpful or interesting for others or should be documented. Otherwise contact us on email julian.jg.graf@gmail.com, ond.babec@gmail.com.

rust-mqtt 0.5.1