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Chapter 3. Introducing Enterprise Integration Patterns
The Apache Camel's Enterprise Integration Patterns are inspired by a book of the same name written by Gregor Hohpe and Bobby Woolf. The patterns described by these authors provide an excellent toolbox for developing enterprise integration projects. In addition to providing a common language for discussing integration architectures, many of the patterns can be implemented directly using Apache Camel's programming interfaces and XML configuration.
3.1. Overview of the Patterns
Enterprise Integration Patterns book
Apache Camel supports most of the patterns from the book, Enterprise Integration Patterns by Gregor Hohpe and Bobby Woolf.
The messaging systems patterns, shown in Table 3.1, “Messaging Systems”, introduce the fundamental concepts and components that make up a messaging system.
Table 3.1. Messaging Systems
|Message||How can two applications connected by a message channel exchange a piece of information?|
|Message Channel||How does one application communicate with another application using messaging?|
|Message Endpoint||How does an application connect to a messaging channel to send and receive messages?|
|Pipes and Filters||How can we perform complex processing on a message while still maintaining independence and flexibility?|
|Message Router||How can you decouple individual processing steps so that messages can be passed to different filters depending on a set of defined conditions?|
|Message Translator||How do systems using different data formats communicate with each other using messaging?|
A messaging channel is the basic component used for connecting the participants in a messaging system. The patterns in Table 3.2, “Messaging Channels” describe the different kinds of messaging channels available.
Table 3.2. Messaging Channels
|Point to Point Channel||How can the caller be sure that exactly one receiver will receive the document or will perform the call?|
|Publish Subscribe Channel||How can the sender broadcast an event to all interested receivers?|
|Dead Letter Channel||What will the messaging system do with a message it cannot deliver?|
|Guaranteed Delivery||How does the sender make sure that a message will be delivered, even if the messaging system fails?|
|Message Bus||What is an architecture that enables separate, decoupled applications to work together, such that one or more of the applications can be added or removed without affecting the others?|
The message construction patterns, shown in Table 3.3, “Message Construction”, describe the various forms and functions of the messages that pass through the system.
Table 3.3. Message Construction
|Correlation Identifier||How does a requestor identify the request that generated the received reply?|
|Return Address||How does a replier know where to send the reply?|
The message routing patterns, shown in Table 3.4, “Message Routing”, describe various ways of linking message channels together, including various algorithms that can be applied to the message stream (without modifying the body of the message).
Table 3.4. Message Routing
|Content Based Router||How do we handle a situation where the implementation of a single logical function (e.g., inventory check) is spread across multiple physical systems?|
|Message Filter||How does a component avoid receiving uninteresting messages?|
|Recipient List||How do we route a message to a list of dynamically specified recipients?|
|Splitter||How can we process a message if it contains multiple elements, each of which might have to be processed in a different way?|
|Aggregator||How do we combine the results of individual, but related messages so that they can be processed as a whole?|
|Resequencer||How can we get a stream of related, but out-of-sequence, messages back into the correct order?|
|Composed Message Processor||How can you maintain the overall message flow when processing a message consisting of multiple elements, each of which may require different processing?|
|Scatter-Gather||How do you maintain the overall message flow when a message needs to be sent to multiple recipients, each of which may send a reply?|
|Routing Slip||How do we route a message consecutively through a series of processing steps when the sequence of steps is not known at design-time, and might vary for each message?|
|Throttler||How can I throttle messages to ensure that a specific endpoint does not get overloaded, or that we don't exceed an agreed SLA with some external service?|
|Delayer||How can I delay the sending of a message?|
|Load Balancer||How can I balance load across a number of endpoints?|
|Multicast||How can I route a message to a number of endpoints at the same time?|
|Loop||How can I repeat processing a message in a loop?|
|Sampling||How can I sample one message out of many in a given period to avoid downstream route does not get overloaded?|
The message transformation patterns, shown in Table 3.5, “Message Transformation”, describe how to modify the contents of messages for various purposes.
Table 3.5. Message Transformation
|Content Enricher||How do we communicate with another system if the message originator does not have all the required data items available?|
|Content Filter||How do you simplify dealing with a large message, when you are interested in only a few data items?|
|Claim Check||How can we reduce the data volume of message sent across the system without sacrificing information content?|
|Normalizer||How do you process messages that are semantically equivalent, but arrive in a different format?|
|Sort||How can I sort the body of a message?|
A messaging endpoint denotes the point of contact between a messaging channel and an application. The messaging endpoint patterns, shown in Table 3.6, “Messaging Endpoints”, describe various features and qualities of service that can be configured on an endpoint.
Table 3.6. Messaging Endpoints
|Messaging Mapper||How do you move data between domain objects and the messaging infrastructure while keeping the two independent of each other?|
|Event Driven Consumer||How can an application automatically consume messages as they become available?|
|Polling Consumer||How can an application consume a message when the application is ready?|
|Competing Consumers||How can a messaging client process multiple messages concurrently?|
|Message Dispatcher||How can multiple consumers on a single channel coordinate their message processing?|
|Selective Consumer||How can a message consumer select which messages it wants to receive?|
|Durable Subscriber||How can a subscriber avoid missing messages when it's not listening for them?|
|Idempotent Consumer||How can a message receiver deal with duplicate messages?|
|Transactional Client||How can a client control its transactions with the messaging system?|
|Messaging Gateway||How do you encapsulate access to the messaging system from the rest of the application?|
|Service Activator||How can an application design a service to be invoked both via various messaging technologies and via non-messaging techniques?|
The system management patterns, shown in Table 3.7, “System Management”, describe how to monitor, test, and administer a messaging system.
Table 3.7. System Management
|Wire Tap||How do you inspect messages that travel on a point-to-point channel?|