RabbitMQ Stream tutorial - "Hello World!"

Introduction

info

Prerequisites

This tutorial assumes RabbitMQ is installed, running on localhost and the stream plugin enabled. The standard stream port is 5552. In case you use a different host, port or credentials, connections settings would require adjusting.

Where to get help

If you're having trouble going through this tutorial you can contact us through the mailing list or discord community server.

Stream was introduced in RabbitMQ 3.9. You can find more information here.

"Hello World"

(using the Rust Stream Client)

In this part of the tutorial we'll write two programs in Rust; a producer that sends a single message, and a consumer that receives messages and prints them out. We'll gloss over some of the detail in the Rust client API, concentrating on this very simple thing just to get started. It's a "Hello World" of messaging.

The Rust stream client library

RabbitMQ speaks multiple protocols. This tutorial uses RabbitMQ stream protocol which is a dedicated protocol for RabbitMQ streams. There are a number of clients for RabbitMQ in many different languages, see the stream client libraries for each language. We'll use the Rust stream client provided by RabbitMQ.

RabbitMQ Rust stream client 0.4.2 and later versions are distributed via crates.io.

This tutorial assumes you are using powershell on Windows. On MacOS and Linux nearly any shell will work.

Setup

First let's verify that you have Rust toolchain in PATH:

rustc --version
cargo --version

should produce a help message.

Now let's generate two projects, one for the publisher and one for the consumer:

cargo new rust-stream
cd rust-stream

This will create two new files named send.rs and receive.rs inside the dir src/bin.

Now we have the Rust project set up we can write some code.

Sending

We'll call our message producer (sender) send.rs and our message consumer (receiver) receive.rs. The producer will connect to RabbitMQ, send a single message, then exit.

In send.rs, we need to use some namespaces:

use rabbitmq_stream_client::error::StreamCreateError;
use rabbitmq_stream_client::types::{ByteCapacity, Message, ResponseCode};

then we can create a connection to the server:

use rabbitmq_stream_client::Environment;
let environment = Environment::builder().build().await?;
...

The entry point of the stream GO client is the Environment. It deals with stream management and the creation of publisher and consumer instances.

It abstracts the socket connection, and takes care of protocol version negotiation and authentication and so on for us. Here we connect to a RabbitMQ node on the local machine - hence the localhost. If we wanted to connect to a node on a different machine we'd simply specify its hostname or IP address on the Environment::builder().

Next we create a producer.

To send, we must declare a stream for us to send to; then we can publish a message to the stream:

    let stream = "hello-rust-stream";
    let create_response = environment
        .stream_creator()
        .max_length(ByteCapacity::GB(5))
        .create(stream)
        .await;

    if let Err(e) = create_response {
        if let StreamCreateError::Create { stream, status } = e {
            match status {
                // we can ignore this error because the stream already exists
                ResponseCode::StreamAlreadyExists => {}
                err => {
                    println!("Error creating stream: {:?} {:?}", stream, err);
                }
            }
        }
    }

    let producer = environment.producer().build(stream).await?;

    producer
        .send_with_confirm(Message::builder().body("Hello, World!").build())
        .await?;
    println!("Sent message to stream: {}", stream);
    producer.close().await?;
    Ok(())

Declaring a stream is idempotent - it will only be created if it doesn't exist already. In this case, we ignore the error StreamAlreadyExists because we don't care if the stream already exists.

Streams model an append-only log of messages that can be repeatedly read until they expire. It is a good practice to always define the retention policy, 5Gb in this case.

The message content is a byte array, so you can encode whatever you like there.

When the code above finishes running, the producer connection and stream-system connection will be closed. That's it for our producer.

Each time you run the producer, it will send a single message to the server and the message will be appended to the stream.

Here's the whole send.rs struct.

Sending doesn't work!

If this is your first time using RabbitMQ and you don't see the "Sent" message then you may be left scratching your head wondering what could be wrong. Maybe the broker was started without enough free disk space (by default it needs at least 50 MB free) and is therefore refusing to accept messages. Check the broker logfile to confirm and reduce the limit if necessary. The configuration file documentation will show you how to set disk_free_limit.

Receiving

As for the consumer, it is listening for messages from RabbitMQ. So unlike the producer which publishes a single message, we'll keep the consumer running continuously to listen for messages and print them out.

The code (in receive.rs) needs some import

use std::io::stdin;
use rabbitmq_stream_client::error::StreamCreateError;
use rabbitmq_stream_client::types::{ByteCapacity, OffsetSpecification, ResponseCode};
use futures::{StreamExt};
use tokio::task;

Setting up is the same as the producer; we create a, consumer, and declare the stream from which we're going to consume. Note this matches up with the stream that send publishes to.

 use rabbitmq_stream_client::Environment;
    let environment = Environment::builder().build().await?;
    let stream = "hello-rust-stream";
    let create_response = environment
        .stream_creator()
        .max_length(ByteCapacity::GB(5))
        .create(stream)
        .await;

    if let Err(e) = create_response {
        if let StreamCreateError::Create { stream, status } = e {
            match status {
                // we can ignore this error because the stream already exists
                ResponseCode::StreamAlreadyExists => {}
                err => {
                    println!("Error creating stream: {:?} {:?}", stream, err);
                }
            }
        }
    }
...

Note that we declare the stream here as well. Because we might start the consumer before the producer, we want to make sure the stream exists before we try to consume messages from it.

We need to use Consumer struct to create the consumer.

We're about to tell the server to deliver us the messages from the queue. The Consumer provides the method next() to get the next message from the stream.

offset defines the starting point of the consumer. In this case, we start from the first message.

    let mut consumer = environment
        .consumer()
        .offset(OffsetSpecification::First)
        .build(stream)
        .await
        .unwrap();

    let handle = consumer.handle();
    task::spawn(async move {
        while let Some(delivery) = consumer.next().await {
            let d = delivery.unwrap();
            println!("Got message: {:#?} with offset: {}",
                     d.message().data().map(|data| String::from_utf8(data.to_vec()).unwrap()),
                     d.offset(),);
        }
    });


    println!("Press any key to close the consumer");
     _ = stdin().read_line(&mut "".to_string());

Here's the whole receive.rs struct.

Putting It All Together

Open two terminals.

You can run the clients in any order, as both declare the stream. We will run the consumer first so you can see it waiting for and then receiving the message:

 cargo run --bin receive

Then run the producer:

cargo run --bin send

The consumer will print the message it gets from the publisher via RabbitMQ. The consumer will keep running, waiting for messages, so try restarting the publisher several times.

Streams are different from queues in that they are append-only logs of messages. So you can run the different consumers and they will always start from the first message.