Event-Driven ConsumerMessaging Patterns » Messaging Endpoints

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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 ConsumerNEW

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

Integration Pattern Language

Message Construct.

Message

Command Message

Document Message

Event Message

Request-Reply

Return Address

Correlation Identifier

Message Sequence

Message Expiration

Format Indicator

Message Routing

Pipes-and-Filters

Message Router

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

Message Translator

Envelope Wrapper

Content Enricher

Content Filter

Claim Check

Normalizer

Canonical Data Model

Messaging Endpoints

Message Endpoint

Messaging Gateway

Messaging Mapper

Transactional Client

Polling Consumer

Event-driven Consumer

Competing Consumers

Message Dispatcher

Selective Consumer

Durable Subscriber

Idempotent Receiver

Service Activator

Messaging Channels

Message Channel

Point-to-Point Channel

Publish-Subscr. Channel

Datatype Channel

Invalid Message Channel

Dead Letter Channel

Guaranteed Delivery

Channel Adapter

Messaging Bridge

Message Bus

Systems Mgmt.

Control Bus

Detour

Wire Tap

Message History

Message Store

Smart Proxy

Test Message

Channel Purger