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Mastering Apache Camel
Mastering Apache Camel

Mastering Apache Camel: An advanced guide to Enterprise Integration using Apache Camel

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Profile Icon Jean-Baptiste Onofré Profile Icon Bilgin Ismet Ibryam
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Profile Icon Jean-Baptiste Onofré Profile Icon Bilgin Ismet Ibryam
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Full star icon Full star icon Full star icon Half star icon Empty star icon 3.4 (7 Ratings)
Paperback Jun 2015 238 pages 1st Edition
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Mastering Apache Camel

Chapter 2. Core Concepts

This chapter introduces the core concepts of Camel. These concepts are the key basis of all functionalities provided by Camel. We will use them in the next chapters. As we have seen in the previous chapter, Camel is an integration framework. This means that it provides everything to implement your mediation logic: messaging, routing, transformation, and connectivity.

We will look at the following concepts:

  • Messages
  • Exchanges
  • Camel contexts

Messages

Messages transport the data between the different parts of the mediation logic. Your mediation logic will define the flow of messages between different nodes.

A message flows in one direction, from a sender to a receiver. It's not possible to use the same message to answer the sender, we will have to use another message. A message is described in the org.apache.camel.Message interface.

The javadoc is available at http://camel.apache.org/maven/camel-2.13.0/camel-core/apidocs/org/apache/camel/Message.html.

A message contains the following:

  • ID: A message ID of type String. Camel creates an ID for you. This ID identifies the message and can be used for correlation or storage. For instance, we will see that the message ID is used in the idempotent consumer pattern to identify the message in a store.
  • Header: A set of headers, allowing you to store any kind of data associated with a message. The headers are stored as org.apache.camel.util.CaseInsensitiveMap by default. The CaseInsensitiveMap...

Exchange

Camel doesn't transport a message directly. The main reason is that a message flows only in one direction. When dealing with messaging, there are many Message Exchange Patterns (MEP) that we can use.

Depending on the use cases, we can send a message without expecting any return from the destination: this pattern is named event message and uses InOnlyMEP. For instance, when you read a file from the filesystem, you just process the file content, without returning anything to the endpoint that read the file. In that case, the component responsible for reading the filesystem will define an InOnlyMEP.

On the other hand, you may want to implement a request reply pattern: a response message should be returned to the sender of the request message, and so it uses an InOutMEP. For instance, you receive a SOAP Request from a WebService component, so you should return a SOAP Response (or SOAP Fault) to the message sender.

In Camel, MEP are described in the org.apache.camel.ExchangePattern...

Camel context

The Camel context is the runtime system and the loading container of all resources required for the execution of the routing. It keeps everything together to allow the user to execute the routing logic. When the context starts, it also starts various components and endpoints, and activates the routing rules.

The Camel context is described by the org.apache.camel.CamelContext interface (http://camel.apache.org/maven/current/camel-core/apidocs/org/apache/camel/CamelContext.html).

Camel context

A Camel context contains the following:

  • The components and endpoints used in the routing (see later for the details about components and endpoints)
  • The type converters used to transform a message of one type to another
  • The data formats used to define the format of a message body
  • The registry where Camel will look for the beans used in the routing
  • The languages describing expressions and predicates used in the routing by a language (xpath, xquery, PHP, and so on)
  • The routes definition itself allowing you to...

Processor

A processor is a node in the routing which is able to use, create, or modify an incoming exchange. During routing, the exchanges flow from one processor to another. This means all Enterprise Integration Patterns (EIP) are implemented using processors in Camel. The exchanges get in and out of a processor by using components and endpoints, as we will see later in this chapter.

A processor is described using the org.apache.camel.Processor interface. To create your own processor, you just have to implement the Processor interface and override the process() method:

Tip

Downloading the example code

You can download the example code files from your account at http://www.packtpub.com for all the Packt Publishing books you have purchased. If you purchased this book elsewhere, you can visit http://www.packtpub.com/support and register to have the files e-mailed directly to you.

public class MyProcessor implements Processor {

  public void process(Exchange exchange) {
System.out.println(&quot...

Routes

The Camel route is the routing definition. It's a graph of processors. The routes (routing definition) are loaded in the Camel context. The execution and flow of the exchange in a route is performed by the routing engine. The routes are used to decouple clients from servers, and producers from consumers: an exchange consumer doesn't know where the exchange comes from, and on the other hand an exchange producer doesn't know the destination of the exchange. Thanks to that, it provides a flexible way to add extra processing or change the routing with limited impact on the logic.

Each route has a unique identifier that you can specify (or Camel will create one for you). This identifier is used to easily find the route, especially when you want to log, debug, monitor, or manage a route (start or stop).

A route has exactly one input source (the input endpoint). A route has a life cycle similar to the Camel context with the same states: started, stopped, and suspended. To Camel...

Messages


Messages transport the data between the different parts of the mediation logic. Your mediation logic will define the flow of messages between different nodes.

A message flows in one direction, from a sender to a receiver. It's not possible to use the same message to answer the sender, we will have to use another message. A message is described in the org.apache.camel.Message interface.

The javadoc is available at http://camel.apache.org/maven/camel-2.13.0/camel-core/apidocs/org/apache/camel/Message.html.

A message contains the following:

  • ID: A message ID of type String. Camel creates an ID for you. This ID identifies the message and can be used for correlation or storage. For instance, we will see that the message ID is used in the idempotent consumer pattern to identify the message in a store.

  • Header: A set of headers, allowing you to store any kind of data associated with a message. The headers are stored as org.apache.camel.util.CaseInsensitiveMap by default. The CaseInsensitiveMap...

Exchange


Camel doesn't transport a message directly. The main reason is that a message flows only in one direction. When dealing with messaging, there are many Message Exchange Patterns (MEP) that we can use.

Depending on the use cases, we can send a message without expecting any return from the destination: this pattern is named event message and uses InOnlyMEP. For instance, when you read a file from the filesystem, you just process the file content, without returning anything to the endpoint that read the file. In that case, the component responsible for reading the filesystem will define an InOnlyMEP.

On the other hand, you may want to implement a request reply pattern: a response message should be returned to the sender of the request message, and so it uses an InOutMEP. For instance, you receive a SOAP Request from a WebService component, so you should return a SOAP Response (or SOAP Fault) to the message sender.

In Camel, MEP are described in the org.apache.camel.ExchangePattern enumeration...

Camel context


The Camel context is the runtime system and the loading container of all resources required for the execution of the routing. It keeps everything together to allow the user to execute the routing logic. When the context starts, it also starts various components and endpoints, and activates the routing rules.

The Camel context is described by the org.apache.camel.CamelContext interface (http://camel.apache.org/maven/current/camel-core/apidocs/org/apache/camel/CamelContext.html).

A Camel context contains the following:

  • The components and endpoints used in the routing (see later for the details about components and endpoints)

  • The type converters used to transform a message of one type to another

  • The data formats used to define the format of a message body

  • The registry where Camel will look for the beans used in the routing

  • The languages describing expressions and predicates used in the routing by a language (xpath, xquery, PHP, and so on)

  • The routes definition itself allowing you to design...

Processor


A processor is a node in the routing which is able to use, create, or modify an incoming exchange. During routing, the exchanges flow from one processor to another. This means all Enterprise Integration Patterns (EIP) are implemented using processors in Camel. The exchanges get in and out of a processor by using components and endpoints, as we will see later in this chapter.

A processor is described using the org.apache.camel.Processor interface. To create your own processor, you just have to implement the Processor interface and override the process() method:

Tip

Downloading the example code

You can download the example code files from your account at http://www.packtpub.com for all the Packt Publishing books you have purchased. If you purchased this book elsewhere, you can visit http://www.packtpub.com/support and register to have the files e-mailed directly to you.

public class MyProcessor implements Processor {

  public void process(Exchange exchange) {
System.out.println("Hello...

Routes


The Camel route is the routing definition. It's a graph of processors. The routes (routing definition) are loaded in the Camel context. The execution and flow of the exchange in a route is performed by the routing engine. The routes are used to decouple clients from servers, and producers from consumers: an exchange consumer doesn't know where the exchange comes from, and on the other hand an exchange producer doesn't know the destination of the exchange. Thanks to that, it provides a flexible way to add extra processing or change the routing with limited impact on the logic.

Each route has a unique identifier that you can specify (or Camel will create one for you). This identifier is used to easily find the route, especially when you want to log, debug, monitor, or manage a route (start or stop).

A route has exactly one input source (the input endpoint). A route has a life cycle similar to the Camel context with the same states: started, stopped, and suspended. To Camel, a context...

Channels


In every Camel route, there is a channel that sits between each processor in the route graph. It's responsible for the routing of an Exchange to the next Processor in the graph. The channel acts as a controller that monitors and controls the routing at runtime. It allows Camel to enrich the route with interceptors. For instance, the Camel tracer or the error handling are functionalities implemented using an interceptor on the channel.

The channel is described by the org.apache.camel.Channel interface. You can configure your own interceptor on the channels by describing it in Camel context.

Camel supports three kinds of interceptors on the channels:

  • Global interceptors: This intercepts all exchanges on the channels

  • Interceptors on the incoming exchanges: This has limited the scope of the interceptor only on the first channel (the one just after the first endpoint)

  • Interceptors on the exchanges going to one specific endpoint: This limits the interceptor to the channel just before a given...

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Description

This book is intended for all Camel users who want to get the best out of Camel, and who want to implement the most efficient integration logic using best practices.

Who is this book for?

This book is intended for all Camel users who want to get the best out of Camel, and who want to implement the most efficient integration logic using best practices.

What you will learn

  • Walk through the key features and core concepts of Apache Camel
  • Implement routing with Processor, using Camel Java DSL and Camel Blueprint DSL
  • Use beans with Camel to access to the POJO model, and leverage IoC frameworks like Spring or Blueprint
  • Get to grips with the EIPs supported by Camel and implement them in your projects
  • Create polling and eventdriven components, and learn how Camel uses components to create endpoints
  • Identify and deal with errors in your routing logic
  • Conduct unit tests and integration tests on your Camel routes

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Publication date : Jun 30, 2015
Length: 238 pages
Edition : 1st
Language : English
ISBN-13 : 9781782173151
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Length: 238 pages
Edition : 1st
Language : English
ISBN-13 : 9781782173151
Vendor :
Apache
Category :
Languages :
Concepts :

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Table of Contents

9 Chapters
1. Key Features Chevron down icon Chevron up icon
2. Core Concepts Chevron down icon Chevron up icon
3. Routing and Processors Chevron down icon Chevron up icon
4. Beans Chevron down icon Chevron up icon
5. Enterprise Integration Patterns Chevron down icon Chevron up icon
6. Components and Endpoints Chevron down icon Chevron up icon
7. Error Handling Chevron down icon Chevron up icon
8. Testing Chevron down icon Chevron up icon
Index Chevron down icon Chevron up icon

Customer reviews

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Rating distribution
Full star icon Full star icon Full star icon Half star icon Empty star icon 3.4
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5 star 42.9%
4 star 14.3%
3 star 0%
2 star 28.6%
1 star 14.3%
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Mitchell Trachtenberg Jan 21, 2024
Full star icon Full star icon Full star icon Full star icon Full star icon 5
An invaluable introduction to Apache Camel. I would hope it can be updated with all the developments since the second edition, since the evolution of Camel's ecosystem has been rapid.
Feefo Verified review Feefo
NJ Aug 21, 2015
Full star icon Full star icon Full star icon Full star icon Full star icon 5
This is really a great book for anyone who wants to learn Camel and use in their project. Book thoroughly explains all the enterprise integration patterns and how that's implemented in Camel. The book is very well written and explains the complex topics easily. Good for anyone who is learning Camel for first time or want to update his knowledge.
Amazon Verified review Amazon
"H Gasmi" Aug 06, 2020
Full star icon Full star icon Full star icon Full star icon Full star icon 5
This book is very good as an introduction to the framework before exploring it in detail. The topics are nicely and clearly covered with detailed and complete examples.
Amazon Verified review Amazon
S. Bryant Sep 08, 2015
Full star icon Full star icon Full star icon Full star icon Empty star icon 4
A nice mix of hands-on get-going quickly instruction with added discussion on why and how Camel works. You can pick and chose how in depth you want to be and/or revisit sections later for more in-depth reading. I am finding it very useful for second and third project - refreshing my memory by re-reading specific chapters that apply to my current problem space.
Amazon Verified review Amazon
Alsq Sep 23, 2015
Full star icon Full star icon Empty star icon Empty star icon Empty star icon 2
If you care for stream-of-consciousness grade school printed English, pages of (repetitive) listing of pom.xml files, and irritating print with little regard for proper form, use of pronouns or articles, or even trivial editing (e.g. PrefixerProcessor and PrefixerProcesser [sic] appear two lines away from each other [p. 24]), this is your book. The text could be a stenographical transcription of a recorded live classroom session, enriched with pro-forma screen captures of maven build output. A promise of concise explanation implied in the advertised restrained page count is completely squandered with content poorly organized and inadequate to the task of providing a coherent, well-explained introduction organized according to a readily recognizable path. The table of content suggests it might have been (I fell for it), but the implementation is truly disappointing. If you are already fluent in Camel, but dated, this could maybe fill in as a short partial read on the latest about this project, and how to integrate it with the Apache Karaf OSGi container. But then, you would not need an introductory tutorial like this book appear to me it claims to be. Alas, I will have to backtrack and look elsewhere. Caveat emptor, as the book is not that cheap either.
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