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Event-Driven ConsumerEvent-Driven Consumer

Messaging Patterns

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An application needs to consume Messages as soon as they’re delivered.

How can an application automatically consume messages as they become available?

The application should use an Event-Driven Consumer, one that is automatically handed messages as they’re delivered on the channel.

This is also known as an asynchronous receiver, because the receiver does not have a running thread until a callback thread delivers a message. We call it an Event-Driven Consumer because the receiver acts like the message delivery is an event that triggers the receiver into action.

...

Example: RabbitMQ Consumer (NEW)

RabbitMQ consumers are by default event-driven. When consuming messages off a Channel, you have to provide an implementation of the Consumer interface. The following code example sets up a very simple consumer that simply prints incoming messages as strings:

Consumer consumer = new DefaultConsumer(channel) {
  @Override
  public void handleDelivery(
        String consumerTag, 
        Envelope envelope, 
        AMQP.BasicProperties properties, 
        byte[] body) throws IOException {
    String message = new String(body, "UTF-8");
    System.out.println("Received '" + message + "'");
  }
};

boolean autoAck = true;
channel.basicConsume(queueName, autoAck, consumer);	

This implementation is not a Transactional Client because the client acknowledges messages automatically on receipt but before processing.

The complete source code for this example can be found on Github.

Related patterns: Competing Consumers, Durable Subscriber, Message, Message Channel, Message Dispatcher, Selective Consumer, Polling Consumer, Transactional Client


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Enterprise Integration Patterns Find the full description of this pattern in:
Enterprise Integration Patterns
Gregor Hohpe and Bobby Woolf
ISBN 0321200683
650 pages
Addison-Wesley

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Table of Contents
Preface
Introduction
Solving Integration Problems using Patterns
Integration Styles
File Transfer
Shared Database
Remote Procedure Invocation
Messaging
Messaging Systems
Message Channel
Message
Pipes and Filters
Message Router
Message Translator
Message Endpoint
Messaging Channels
Point-to-Point Channel
Publish-Subscribe Channel
Datatype Channel
Invalid Message Channel
Dead Letter Channel
Guaranteed Delivery
Channel Adapter
Messaging Bridge
Message Bus
Message Construction
Command Message
Document Message
Event Message
Request-Reply
Return Address
Correlation Identifier
Message Sequence
Message Expiration
Format Indicator
Interlude: Simple Messaging
JMS Request/Reply Example
.NET Request/Reply Example
JMS Publish/Subscribe Example
Message Routing
Content-Based Router
Message Filter
Dynamic Router
Recipient List
Splitter
Aggregator
Resequencer
Composed Msg. Processor
Scatter-Gather
Routing Slip
Process Manager
Message Broker
Message Transformation
Envelope Wrapper
Content Enricher
Content Filter
Claim Check
Normalizer
Canonical Data Model
Interlude: Composed Messaging
Synchronous (Web Services)
Asynchronous (MSMQ)
Asynchronous (TIBCO)
Messaging Endpoints
Messaging Gateway
Messaging Mapper
Transactional Client
Polling Consumer
Event-Driven Consumer
Competing Consumers
Message Dispatcher
Selective Consumer
Durable Subscriber
Idempotent Receiver
Service Activator
System Management
Control Bus
Detour
Wire Tap
Message History
Message Store
Smart Proxy
Test Message
Channel Purger
Interlude: Systems Management Example
Instrumenting Loan Broker
Integration Patterns in Practice
Case Study: Bond Trading System
Concluding Remarks
Emerging Standards
Appendices
Bibliography
Revision History