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RabbitMQ Cookbook

RabbitMQ Cookbook

By : Sigismondo Boschi, Gabriele Santomaggio
4.1 (9)
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RabbitMQ Cookbook

RabbitMQ Cookbook

4.1 (9)
By: Sigismondo Boschi, Gabriele Santomaggio

Overview of this book

RabbitMQ is an open source message broker software (sometimes called message-oriented middleware) that implements the Advanced Message Queuing Protocol (AMQP). The RabbitMQ server is written in the Erlang programming language and is built on the Open Telecom Platform framework for clustering and failover. Messaging enables software applications to connect and scale. Applications can connect to each other as components of a larger application or to user devices and data. RabbitMQ Cookbook touches on all the aspects of RabbitMQ messaging. You will learn how to use this enabling technology for the solution of highly scalable problems dictated by the dynamic requirements of Web and mobile architectures, based for example on cloud computing platforms. This is a practical guide with several examples that will help you to understand the usefulness and the power of RabbitMQ. This book helps you learn the basic functionalities of RabbitMQ with simple examples which describe the use of RabbitMQ client APIs and how a RabbitMQ server works. You will find examples of RabbitMQ deployed in real-life use-cases, where its functionalities will be exploited combined with other technologies. This book helps you understand the advanced features of RabbitMQ that are useful for even the most demanding programmer. Over the course of the book, you will learn about the usage of basic AMQP functionalities and use RabbitMQ to let decoupled applications exchange messages as per enterprise integration applications. The same building blocks are used to implement the architecture of highly scalable applications like today's social networks, and they are presented in the book with some examples. You will also learn how to extend RabbitMQ functionalities by implementing Erlang plugins. This book combines information with detailed examples coupled with screenshots and diagrams to help you create a messaging application with ease.
Table of Contents (19 chapters)
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RabbitMQ Cookbook
Credits
About the Authors
About the Reviewers
www.PacktPub.com
Preface
1
Index

Distributing messages to many consumers

In this example we are showing how to create a dynamic load balancer, and how to distribute messages to many consumers. We are going to create a file downloader.

You can find the source at Chapter01/Recipe10/Java_10/.

Getting ready

To use this recipe we need to set up the Java development environment as indicated in the Introduction section.

How to do it…

In order to let two or more RabbitMQ clients properly balance consuming messages, you need to follow the given steps:

  1. Declare a named queue, and specify the basicQos as follows:
    channel.queueDeclare(myQueue, false, false, false,null);
    channel.basicQos(1);
  2. Bind a consumer with explicit ack:
    channel.basicConsume(myQueue, false, consumer); 
  3. Send one or more messages using channel.basicPublish().
  4. Execute two or more consumers.

How it works...

The publisher sends a message with the URL to download:

String messageUrlToDownload=  "http://www.rabbitmq.com/releases/rabbitmq-dotnet-client/v3.0.2/rabbitmq-dotnet-client-3.0.2-user-guide.pdf";
channel.basicPublish("",MyQueue,null,messageUrlToDownload.getBytes());

The consumer gets the message and downloads the referenced URL:

System.out.println("Url to download:" + messageURL);
downloadUrl(messageURL);

Once the download is terminated, the consumer sends the ack back to the broker and is ready to download the next one:

getChannel().basicAck(envelope.getDeliveryTag(),false);
System.out.println("Ack sent!");
System.out.println("Wait for the next download...");

By default, messages are heavily prefetched. Messages are retrieved by the consumers in blocks, but are actually consumed and removed from the queue when the consumers send the ack, as already seen in the previous recipe.

On the other hand, using many consumers as in this recipe, the first one will prefetch the messages, and the other consumers started later won't find any available in the queue. In order to equally distribute the work among the active consumers, we need to use channel.basicQos(1), specifying to prefetch just one message at a time.

See also…

You can find more information about load balancing in Chapter 8, Performance Tuning for RabbitMQ.

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