Eclipse Paho MQTT Rust Client Library

The Eclipse Paho MQTT Rust client library on memory-managed operating systems such as Linux/Posix, Mac, and Windows.

The Rust crate is a safe wrapper around the Paho C Library.

Features

The initial version of this crate is a wrapper for the Paho C library, and includes all of the features available in that library, including:

• Supports MQTT v5, 3.1.1, and 3.1
• Network Transports:
  • Standard TCP support
  • SSL / TLS (with optional ALPN protocols)
  • WebSockets (secure and insecure), and optional Proxies
• QoS 0, 1, and 2
• Last Will and Testament (LWT)
• Message Persistence
  • File or memory persistence
  • User-defined key/value persistence (including example for Redis)
• Automatic Reconnect
• Offline Buffering
• High Availability
• Several API's:
  • Async/Await with Rust Futures and Streams for asynchronous operations.
  • Traditional asynchronous (token/wait) API
  • Synchronous/blocking API

Requires Paho C v1.3.8, or possibly later.

Latest News

To keep up with the latest announcements for this project, follow:

Twitter: @eclipsepaho and @fmpagliughi

EMail: Eclipse Paho Mailing List

Mattermost: Eclipse Mattermost Paho Channel

Unreleased Features in this Branch

• Switched consumers/streams to use crossbeam channels and async_channel's, respectively.
• Finally ran rustfmt on source files.
• #120, #121 Fixed subscribe_many_with_options() outbound opts.
• #122 Some clippy-recommended fixes

What's new in v0.9.1

• #101 Token::try_wait() to check for the result of a Token without blocking.
• #101 A try_publish() function for the AsyncClient and Topic which return a synchronous result that the message was created and queued for transmission successfully.
• #28 Some instructions for using the "cross" tool for cross-compiling.

Using the Crate

To use the library, simply add this to your application's Cargo.toml dependencies list:

paho-mqtt = "0.9"

By default it enables the features "bundled" and "ssl" meaning it will attempt to compile the Paho C library for the target, using the pre-built bindings, and will enable secure sockets capabilities.

Note that this default behavior requires a C compiler for the target and CMake to be installed.

Also note that the build will use pre-generated bindings by default to speed up compile times. If you experience segfaults or other hard crashes, the first thing to do is try using the "build_bindgen" feature in your crate to regenerate the bindings for your target. If that doesn't fix it, then please submit an issue on GitHub.

Configurable Features

The default behaviour can be altered by enabling or disabling the features:

• "bundled" - Whether to build the Paho C library contained in the Git submodule under the contained paho-mqtt-sys crate. This is similar to the "vendored" feature in other Rust projects.
• "build_bindgen" - Whether to build the bindings for the target using bindgen. If not set, the build will attempt to find and use pre-built bindings for the target.
• "ssl" - Whether to enable the use of secure sockets and secure websocket connections.
• "vendored-ssl" - Whether to build OpenSSL. This passes the "vendored" option to the openssl-sys crate.

Version 0.9 has started using the openssl-sys crate which allows for further modification of the behavior through environment variables, such as specifying the location of the OpenSSL library or linking it statically. See below for details, or get more information from that crate.

In particular, if you are using a pre-built OpenSSL library, you may now need to set the specific location of the library with an environment variable. For example, on Windows, you may need to do something like this:

set OPENSSL_DIR=C:\OpenSSL-Win64

Developing the Crate

The library is a standard Rust "crate" using the Cargo build tool. It uses the standard cargo commands for building:

$ cargo build

Builds the library, and also builds the -sys subcrate and the bundled Paho C library. It includes SSL, as it is defined as a default feature.

$ cargo build --examples

Builds the library and sample applications in the examples subdirectory.

$ cargo test

Builds and runs the unit tests.

$ cargo doc

Generates reference documentation.

The Paho C Library and paho-mqtt-sys

The Paho Rust crate is a wrapper around the Paho C library. This version is specifically matched to Paho C v 1.3.x, and is currently using version 1.3.8. It will generally not build against newer versions of the C library, as the C lib expands functionality by extending structures, thus breaking the Rust build.

The project includes a Rust -sys crate, called paho-mqtt-sys, which provides unsafe bindings to the C library. The repository contains a Git submodule pointing to the specific version of the C library that the Rust crate requires, and by default, it will automatically build and link to that library, using pre-generated C bindings that are also included in the repo.

When building, the user has several options:

• Build the bundled library using the pre-generated bindings and SSL (default).
• Build the bundled library and compile a copy of OpenSSL to statically link to.
• Build the bundled library, but regenerate the bindings at build time.
• Use an external library, with the location specified by environment variables, generating the bindings at build time.
• Use the pre-installed library with the pre-generated bindings.

These are chosen with cargo features, explained below.

Building the bundled Paho C library

This is the default:

$ cargo build

This will initialize and update the C library sources from Git, then use the cmake crate to build the static version of the C library, and link it in. By default, the build will use the pre-generated bindings in bindings/bindings_paho_mqtt_X_Y_Z.rs, where X_Y_Z is the currently supported library version.

The default features for the build are: ["bundled", "ssl"]

When building the bundled libraries, the bindings can also be regenerated at build-time. This is especially useful when building on uncommon/untested platforms to ensure proper bindings for that system. This is done adding the "build_bindgen" feature:

$ cargo build --features "build_bindgen"

In this case it will generate bindings based on the header files in the bundled C repository.

The cached versions of the bindings are target-specific. If the pre-generated version doesn't exist for the target, it will need to be generated.

Building the Paho C library with or without SSL/TLS

To build the Paho C library with SSL/TLS we depend on the openssl-sys crate. The openssl-sys crate supports automatically detecting OpenSSL installations, manually pointing towards an OpenSSL installation using environment variables or building and statically linking to a vendored copy of OpenSSL (see the openssl-sys documentation for all available options). To use the vendored option, please use the vendored-ssl feature which also enables the bundled and ssl features.

Building with SSL happens automatically as ssl is a default feature. It requires the OpenSSL libraries be installed for the target. If they are in a non-standard place, then the OPENSSL_DIR environment variable should be set, pointing at the top-level install path, with the .lib, .a and other library files in a lib/ directory just under the root. Use like:

$ export OPENSSL_DIR=/home/myacct/openssl

or wherever the library was installed.

The crate can also be build without SSL by using --no-default-features. For example, to build the bundled Paho C library without secure sockets:

$ cargo build --no-default-features --features "bundled"

Linking OpenSSL Statically

Enable the --vendored-ssl feature to build the crate with a compiled and statically linked copy of OpenSSL. The --vendored-ssl feature also enables the bundled and ssl features, so either of these command will work:

$ cargo build --features "vendored-ssl"
$ cargo build --no-default-features --features "vendored-ssl"

Linking to an external Paho C library

The crate can generate bindings to a copy of the Paho C library in a different location in the local file system, and link to that library.

$ cargo build --no-default-features --features "build_bindgen,ssl"

The ssl feature can be omitted if it is not desired.

The location of the C library is specified through an environment variable:

PAHO_MQTT_C_DIR= ...path to install directory...

It's assumed that the headers are in an include/ directory below the one specified, and the library is in lib/ under it. This would be the case with a normal install.

Alternately, this can be expressed with individual environment variables for each of the header and library directories:

PAHO_MQTT_C_INCLUDE_DIR= ...path to headers...
PAHO_MQTT_C_LIB_DIR= ...path to library...

In this case, the headers and library can be found independently. This was necessary when building against a development tree for Paho C that used GNU Make build. This doesn't seem as necessary now that CMake is used everywhere.

Linking to an installed Paho C library

If the correct version of the Paho C library is expected to be installed on the target system, the simplest solution is to use the pre-generated bindings and specify a link to the shared Paho C library.

$ cargo build --no-default-features --features "ssl"

This is especially useful in a production environment where the system is well controlled, such as when working with full-system build tools like yocto or buildroot. It could be easier to build or cross-compile the packages separately.

Again, the ssl feature can be omitted if it is not desired.

This option should be used with caution when building an application that will ship independently of the target system, since it assumes a very specific version of the C library and will fail if that is not the one on the target.

Rust-C Bindings

As described above, the crate can optionally use bindgen to create the bindings to the Paho C library.

https://rust-lang-nursery.github.io/rust-bindgen/

Generating bindings each time you build the Rust crate is time consuming and uses a lot of resources. This is especially noticeable when building natively on a small target like an ARM board, or similar.

But each release of the Rust crate is build against a specific version of the Paho C library, which means that for a specific target, the bindings never change from build to build. Therefore, we can create the bindings once for a target and then use them for a speedy build after that.

The crate comes with a number of pre-built bindings for several popular targets in: paho-mqtt-sys/bindings. These are files with names in the form:

bindings_paho_mqtt_c_<version>-<target>.rs

Some of these include:

bindings_paho_mqtt_c_1.3.8-x86_64-unknown-linux-gnu.rs
bindings_paho_mqtt_c_1.3.8-x86_64-pc-windows-msvc.rs
bindings_paho_mqtt_c_1.3.8-aarch64-unknown-linux-gnu.rs
bindings_paho_mqtt_c_1.3.8-armv7-unknown-linux-gnueabihf.rs
bindings_paho_mqtt_c_1.3.8-x86_64-apple-darwin.rs
bindings_paho_mqtt_c_1.3.8-default-32.rs
bindings_paho_mqtt_c_1.3.8-default-64.rs

Bindings can be created for new versions of the Paho C library or for different target platforms using the command-line bindgen tool. For example on an x86 version of Windows using MSVC, you can re-generate the bindings like this:

$ cd paho-mqtt-sys
$ bindgen wrapper.h -o bindings/bindings_paho_mqtt_c_1.3.8-x86_64-pc-windows-msvc.rs -- -Ipaho.mqtt.c/src

To create bindings for a different target, use the TARGET environment variable. For example, to build the 32-bit MSVC bindings for Windows on a 64-bit host, use the i686-pc-windows-msvc target:

$ TARGET=i686-pc-windows-msvc bindgen wrapper.h -o bindings/bindings_paho_mqtt_c_1.3.8-i686-pc-windows-msvc.rs -- -Ipaho.mqtt.c/src

Bindgen linker issue

Bindgen requires a relatively recent version of the Clang library installed on the system - recommended v3.9 or later. The bindgen dependencies seem, however, to seek out the oldest Clang version if multiple ones are installed on the system. On Ubuntu 14.04 or 16.04, the Clang v3.6 default might give some problems, although as the Paho builder is currently configured, it should work.

But the safest thing would be to set the LIBCLANG_PATH environment variable to point to a supported version, like:

export LIBCLANG_PATH=/usr/lib/llvm-3.9/lib

Cross-Compiling

The cmake crate automatically handles cross-compiling libraries. You'll need a C cross-compiler installed on your system. See here for more info about cross-compiling Rust, in general:

https://github.com/japaric/rust-cross

The Rust Book

For example, to do a full build for ARMv7, which includes Raspberry Pi's, BeagleBones, UDOO Neo's, and lots of other ARM maker boards:

$ cargo build --target=armv7-unknown-linux-gnueabihf --examples

This builds the main crate, the -sys crate, and it cross-compiles the Paho C library. It uses SSL, so it requires you to have a version of the SSL development library installed with the cross-compiler. If the SSL libraries are not available you can compile and link them as part of the Rust build using the --vendored-ssl feature:

$ cargo build --target=armv7-unknown-linux-gnueabihf --features="vendored-ssl" --examples

If you don't want to use SSL with the cross-compiler:

$ cargo build --target=armv7-unknown-linux-gnueabihf --no-default-features --features="bundled" --examples

If the triplet of the installed cross-compiler doesn't exactly match that of the Rust target, you might also need to correct the CC environment variable:

$ CC_armv7-unknown-linux-gnueabihf=armv7-unknown-linux-gnueabihf-gcc cargo build --target=armv7-unknown-linux-gnueabihf --features="vendored-ssl" --examples

Cross-Compiling with the "cross" project.

The cross project is a cross-compilation build tool that utilizes docker containers pre-loaded with the build tools for a number of targets. It requires Docker to be installed and running on your system.

Then build/install the cross tool:

$ cargo install cross

After that, you should be able to build the project for any of the supported targets. Just use the cross command instead of cargo.

$ cross build --target=armv7-unknown-linux-gnueabihf \
    --features=vendored-ssl --examples

Fully Static Builds with musl

With the v0.9 release and beyond, it should be fairly easy to create fully static builds of applications that use the Paho crate using the musl library and tools.

On a recent Ubuntu/Mint Linux host it should work as follows, but should be similar on any development host once the tools are installed.

First install the Rust compiler for musl and the tools:

$ rustup target add x86_64-unknown-linux-musl
$ sudo apt install musl-tools

Check the musl compiler:

$ musl-gcc --version
cc (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0
...

Building without SSL is like this:

$  cargo build --no-default-features --features="bundled" \
    --target=x86_64-unknown-linux-musl --examples 

Logging

The Rust library uses the log crate to output debug and trace information. Applications can choose to use one of the available logger implementations or define one of their own. More information is available at:

https://docs.rs/log/0.4.0/log/

The sample applications use the environment log crate, env_logger to configure output via the RUST_LOG environment variable. To use this, the following call is specified in the samples before using any of the Rust MQTT API:

env_logger::init().unwrap();

And then the library will output information as defined by the environment. Use like:

$ RUST_LOG=debug ./async_publish
DEBUG:paho_mqtt::async_client: Creating client with persistence: 0, 0x0
DEBUG:paho_mqtt::async_client: AsyncClient handle: 0x7f9ae2eab004
DEBUG:paho_mqtt::async_client: Connecting handle: 0x7f9ae2eab004
...

In addition, the underlying Paho C library has its own logging capabilities which can be used to trace network and protocol transactions. It is configured by the environment variables MQTT_C_CLIENT_TRACE and MQTT_C_CLIENT_TRACE_LEVEL. The former names the log file, with the special value "ON" to log to stdout. The latter specifies one of the levels: ERROR, PROTOCOL, MINIMUM, MEDIUM and MAXIMUM.

export MQTT_C_CLIENT_TRACE=ON
export MQTT_C_CLIENT_TRACE_LEVEL=PROTOCOL

Example

Several small sample applications can be found in the examples directory. Here is what a small MQTT publisher might look like:

use std::process;

extern crate paho_mqtt as mqtt;

fn main() {
    // Create a client & define connect options
    let cli = mqtt::Client::new("tcp://localhost:1883").unwrap_or_else(|err| {
        println!("Error creating the client: {:?}", err);
        process::exit(1);
    });

    let conn_opts = mqtt::ConnectOptionsBuilder::new()
        .keep_alive_interval(Duration::from_secs(20))
        .clean_session(true)
        .finalize();

    // Connect and wait for it to complete or fail
    if let Err(e) = cli.connect(conn_opts).wait() {
        println!("Unable to connect:\n\t{:?}", e);
        process::exit(1);
    }

    // Create a message and publish it
    let msg = mqtt::Message::new("test", "Hello world!");
    let tok = cli.publish(msg);

    if let Err(e) = tok.wait() {
        println!("Error sending message: {:?}", e);
    }

    // Disconnect from the broker
    let tok = cli.disconnect();
    tok.wait().unwrap();
}

External Libraries and Utilities

Several external projects are under development which use or enhance the Paho MQTT Rust library. These can be used in a system with the Rust library or serve as further examples of it's use.

Redis Persistence

The mqtt-redis create allows the use of Redis as a persistence store. It also provides a good example of creating a user-defined persistence which implements the ClientPersistence trait. It can be found at:

https://github.com/fpagliughi/mqtt.rust.redis