RabbitMQ tutorial - Work Queues

Work Queues

(using the AMQP 1.0 Go client)

info

Prerequisites

This tutorial assumes RabbitMQ is installed and running on localhost on the standard port (5672). 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 GitHub Discussions or RabbitMQ community Discord.

In the first tutorial we wrote programs to send and receive messages from a named queue. In this one we'll create a Work Queue that will be used to distribute time-consuming tasks among multiple workers.

The main idea behind Work Queues (aka: Task Queues) is to avoid doing a resource-intensive task immediately and having to wait for it to complete. Instead we schedule the task to be done later. We encapsulate a task as a message and send it to a queue. A worker process running in the background will pop the tasks and eventually execute the job. When you run many workers the tasks will be shared between them.

This concept is especially useful in web applications where it's impossible to handle a complex task during a short HTTP request window.

This tutorial uses the RabbitMQ AMQP 1.0 Go client (rabbitmq-amqp-go-client). It requires RabbitMQ 4.0 or later.

Preparation

In the previous part of this tutorial we sent a message containing "Hello World!". Now we'll be sending strings that stand for complex tasks. We don't have a real-world task, like images to be resized or pdf files to be rendered, so let's fake it by just pretending we're busy - by using the time.Sleep function. We'll take the number of dots in the string as its complexity; every dot will account for one second of "work". For example, a fake task described by Hello... will take three seconds.

We will slightly modify the send.go code from our previous example, to allow arbitrary messages to be sent from the command line. This program will schedule tasks to our work queue, so let's name it new_task.go:

body := bodyFrom(os.Args)
res, err := publisher.Publish(ctx, rmq.NewMessage([]byte(body)))
if err != nil {
	log.Panicf("Failed to publish a message: %v", err)
}
switch res.Outcome.(type) {
case *rmq.StateAccepted:
default:
	log.Panicf("Unexpected publish outcome: %v", res.Outcome)
}
log.Printf(" [x] Sent %s", body)

func bodyFrom(args []string) string {
	var s string
	if (len(args) < 2) || args[1] == "" {
		s = "hello"
	} else {
		s = strings.Join(args[1:], " ")
	}
	return s
}

Our old consumer program also requires some changes: it needs to fake a second of work for every dot in the message body. It will handle delivered messages and perform the task, so let's call it worker.go:

consumer, err := conn.NewConsumer(ctx, "task_queue", &rmq.ConsumerOptions{InitialCredits: 1})
if err != nil {
	log.Panicf("Failed to create consumer: %v", err)
}
defer func() { _ = consumer.Close(context.Background()) }()

log.Printf(" [*] Waiting for messages. To exit press CTRL+C")
for {
	delivery, err := consumer.Receive(ctx)
	if err != nil {
		if errors.Is(err, context.Canceled) {
			return
		}
		log.Panicf("Failed to receive a message: %v", err)
	}
	msg := delivery.Message()
	var payload []byte
	if len(msg.Data) > 0 {
		payload = msg.Data[0]
	}
	log.Printf("Received a message: %s", payload)
	dotCount := bytes.Count(payload, []byte("."))
	t := time.Duration(dotCount)
	time.Sleep(t * time.Second)
	log.Printf("Done")
	err = delivery.Accept(ctx)
	if err != nil {
		log.Panicf("Failed to accept message: %v", err)
	}
}

Note the use of ConsumerOptions{InitialCredits: 1} to limit messages in flight (the AMQP 1.0 analog to prefetch).

Run each program using go run:

go run new_task.go hello world
go run worker.go

Round-robin dispatching

One of the advantages of using a Task Queue is the ability to easily parallelise work. If we are building up a backlog of work, we can just add more workers and that way, scale easily.

First, let's try to run two worker instances at the same time. They will both get messages from the queue, but how exactly? Let's see.

You need three terminals open. Two will run the worker program. These terminals will be our two consumers - C1 and C2.

# shell 1
go run worker.go
# => [*] Waiting for messages. To exit press CTRL+C

# shell 2
go run worker.go
# => [*] Waiting for messages. To exit press CTRL+C

In the third one we'll publish new tasks. Once you've started the consumers you can publish a few messages:

# shell 3
go run new_task.go First message.
# => [x] Sent 'First message.'
go run new_task.go Second message..
# => [x] Sent 'Second message..'
go run new_task.go Third message...
# => [x] Sent 'Third message...'

Let's see what is delivered to our workers:

go run worker.go
# => [*] Waiting for messages. To exit press CTRL+C
# => Received a message: First message.
# => Done
# => Received a message: Third message...
# => Done

go run worker.go
# => [*] Waiting for messages. To exit press CTRL+C
# => Received a message: Second message..
# => Done

By default, RabbitMQ will send each message to the next consumer, in sequence. On average every consumer will get the same number of messages. This way of distributing messages is called round-robin. Try this out with three or more workers.

Message acknowledgment

Doing a task can take a few seconds, you may wonder what happens if a consumer starts a long task and it terminates before it completes. With AMQP 1.0, the consumer must settle each message (Accept, Discard, or Requeue). Until you settle, the broker can redeliver the message if the worker stops.

In order to make sure a message is not lost when a worker dies after receiving it but before finishing processing, settle only after the task is done. Here we call delivery.Accept(ctx) after time.Sleep completes.

If a consumer dies without settling, RabbitMQ will redeliver the message to another consumer. That way you can be sure that no message is lost, even if the workers occasionally die.

The complete worker.go file uses delivery.Accept after work completes.

Forgotten settlement

It's a common mistake to omit delivery.Accept() (or to call it before work finishes). Messages will be redelivered when your client quits (which may look like random redelivery), and unacknowledged messages accumulate on the broker.

You can use rabbitmqctl to inspect queues:
sudo rabbitmqctl list_queues name messages_ready messages_unacknowledged
On Windows, drop the sudo:
rabbitmqctl.bat list_queues name messages_ready messages_unacknowledged

Fair dispatch

You might have noticed that the dispatching still doesn't work exactly as we want. For example in a situation with two workers, when all odd messages are heavy and even messages are light, one worker will be constantly busy and the other one will do hardly any work. Well, RabbitMQ doesn't know anything about that and will still dispatch messages evenly.

This happens because the broker may deliver multiple messages before earlier ones are settled.

With the AMQP 1.0 Go client, limit how many messages are in flight per consumer by setting InitialCredits to 1 on the consumer builder. This is the analogue of prefetch 1 in AMQP 0-9-1:

consumer, err := conn.NewConsumer(ctx, "task_queue", &rmq.ConsumerOptions{InitialCredits: 1})

This ensures that RabbitMQ won't prefetch messages; instead, it will only deliver a new message after the current one is settled.

Note about queue size

If all the workers are busy, your queue can fill up. You will want to keep an eye on that, and maybe add more workers, or have some other strategy.

Putting it all together

Final outline of new_task.go:

package main

import (
	"context"
	"log"
	"os"
	"strings"

	rmq "github.com/rabbitmq/rabbitmq-amqp-go-client/pkg/rabbitmqamqp"
)

const brokerURI = "amqp://guest:guest@localhost:5672/"

func main() {
	ctx := context.Background()
	env := rmq.NewEnvironment(brokerURI, nil)
	conn, err := env.NewConnection(ctx)
	if err != nil {
		log.Panicf("Failed to connect to RabbitMQ: %v", err)
	}
	defer func() {
		_ = env.CloseConnections(context.Background())
	}()

	_, err = conn.Management().DeclareQueue(ctx, &rmq.QuorumQueueSpecification{Name: "task_queue"})
	if err != nil {
		log.Panicf("Failed to declare a queue: %v", err)
	}

	publisher, err := conn.NewPublisher(ctx, &rmq.QueueAddress{Queue: "task_queue"}, nil)
	if err != nil {
		log.Panicf("Failed to create publisher: %v", err)
	}
	defer func() { _ = publisher.Close(context.Background()) }()

	body := bodyFrom(os.Args)
	res, err := publisher.Publish(ctx, rmq.NewMessage([]byte(body)))
	if err != nil {
		log.Panicf("Failed to publish a message: %v", err)
	}
	switch res.Outcome.(type) {
	case *rmq.StateAccepted:
	default:
		log.Panicf("Unexpected publish outcome: %v", res.Outcome)
	}
	log.Printf(" [x] Sent %s", body)
}

func bodyFrom(args []string) string {
	var s string
	if (len(args) < 2) || args[1] == "" {
		s = "hello"
	} else {
		s = strings.Join(args[1:], " ")
	}
	return s
}

(new_task.go source)

And worker.go:

package main

import (
	"bytes"
	"context"
	"errors"
	"log"
	"time"

	rmq "github.com/rabbitmq/rabbitmq-amqp-go-client/pkg/rabbitmqamqp"
)

const brokerURI = "amqp://guest:guest@localhost:5672/"

func main() {
	ctx := context.Background()
	env := rmq.NewEnvironment(brokerURI, nil)
	conn, err := env.NewConnection(ctx)
	if err != nil {
		log.Panicf("Failed to connect to RabbitMQ: %v", err)
	}
	defer func() {
		_ = env.CloseConnections(context.Background())
	}()

	_, err = conn.Management().DeclareQueue(ctx, &rmq.QuorumQueueSpecification{Name: "task_queue"})
	if err != nil {
		log.Panicf("Failed to declare a queue: %v", err)
	}

	consumer, err := conn.NewConsumer(ctx, "task_queue", &rmq.ConsumerOptions{InitialCredits: 1})
	if err != nil {
		log.Panicf("Failed to create consumer: %v", err)
	}
	defer func() { _ = consumer.Close(context.Background()) }()

	log.Printf(" [*] Waiting for messages. To exit press CTRL+C")
	for {
		delivery, err := consumer.Receive(ctx)
		if err != nil {
			if errors.Is(err, context.Canceled) {
				return
			}
			log.Panicf("Failed to receive a message: %v", err)
		}
		msg := delivery.Message()
		var payload []byte
		if len(msg.Data) > 0 {
			payload = msg.Data[0]
		}
		log.Printf("Received a message: %s", payload)
		dotCount := bytes.Count(payload, []byte("."))
		t := time.Duration(dotCount)
		time.Sleep(t * time.Second)
		log.Printf("Done")
		err = delivery.Accept(ctx)
		if err != nil {
			log.Panicf("Failed to accept message: %v", err)
		}
	}
}

(worker.go source)

Using explicit settlement and InitialCredits: 1 you can set up a work queue. Quorum queues ensure tasks survive broker restarts.

Now we can move on to tutorial 3 and learn how to deliver the same message to many consumers.

Work Queues​

(using the AMQP 1.0 Go client)​

Prerequisites​

Where to get help​

Preparation​

Round-robin dispatching​

Message acknowledgment​

Forgotten settlement​

Fair dispatch​

Note about queue size​

Putting it all together​