paho-mqtt

The Official Eclipse Paho MQTT Rust Client Library. This is a wrapper around the Paho C library.


Keywords
messaging, iot, mqtt, paho
License
EPL-1.0

Documentation

Eclipse Paho MQTT Rust Client Library

This repository contains the source code for the Eclipse Paho MQTT Rust client library on memory-managed operating systems such as Linux/Posix and Windows.

Pre-release notes

The Rust crate is a safe wrapper around the Paho C Library. This version is specifically matched to Paho C v 1.3.x, and is currently being tested with version 1.3.0. It will not build against newer versions of the C library, as the C lib expands functionality by extending structures, thus breaking the Rust build.

This is a pre-release version of the library for the development and testing of an MQTT API for the Rust language.

The API is guaranteed to change repeatedly and often while the code is being developed prior to a formal release. Use it with caution.

Initial development is being done on Linux. That is currently the only system known to work.

It is hoped that a full, stable, release should be ready by early 2019.

Latest News

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

Twitter: @eclipsepaho and @fmpagliughi

EMail: Eclipse Paho Mailing List

Unreleased Features

  • Bundled Paho C Library upgraded to v1.3.0
  • WebSocket support

Features

The initial version of the library is a wrapper for the Paho C library, similar to the implementation for the current Paho C++ library. It targets MQTT v3.1 and 3.1.1, and includes all of the features available in the C library for those versions, including:

  • Standard TCP support
  • SSL / TLS
  • WebSockets
  • QoS 0, 1, and 2
  • Last Will and Testament (LWT)
  • Message Persistence
    • File or memory persistence
    • User-defined persistence
  • Automatic Reconnect
  • Offline Buffering
  • High Availability
  • Asynchronous (Non-blocking) API
  • Synchronous (Blocking) API

Future Release

This crate was initially started with the intention of being a quick re-write of the Paho C++ crate, which was used as a template for a rough API and implementation. The hope was more to introduce Rust to the the MQTT community than the other way around. Thus, this initial version lacks tight integration into some of the de facto libraries used in Rust for this type of library, such as futures and tokio.

This will, hopefully, be remidied very soon.

As soon as the current version is stabilized and released, work will immediately begin on a version to wrap the recently-released Paho C v1.3 to bring in support for MQTT v5 and WebSockets. At that time, we will also introduce integration with futures and optional tokio support.

Building 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. 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, 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.

Currently the Rust library is only linking to the SSL version of the library, libpaho-mqtt3as.

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.

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 using the "buildtime_bindgen" feature:

$ cargo build --features "build_bindgen"

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

Linking to an exteral 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,

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 independetly 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.

Bindgen linker issue

The crate can optionally use the Rust bindgen library to create the bindings to the Paho C library.

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

Bindgen requires a relatively recent version of the Clang library installed on the system - recommended v3.9 or 4.0. 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

I was pleasently surprised to discover that the cmake crate seems to automatically handle 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

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 you don't have SSL for the cross-compiler

$ cargo build --target=armv7-unknown-linux-gnueabihf --no-default-features \
    --features="bundled" --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 enviroment 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 an example of a small MQTT publisher:

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