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Content-Based RouterContent-Based Router

Messaging Patterns

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Assume that we are building an order processing system. When an incoming order is received, we first validate the order and then verify that the ordered item is available in the warehouse. This function is performed by the inventory system. This sequence of processing steps is a perfect candidate for the Pipes and Filters style. We create two filters, one for the validation step and one for the inventory system, and route the incoming messages through both filters. However, in many enterprise integration scenarios more than one inventory system exists with each system being able to handle only specific items.

How do we handle a situation where the implementation of a single logical function (e.g., inventory check) is spread across multiple physical systems?

Use a Content-Based Router to route each message to the correct recipient based on message content.

The Content-Based Router examines the message content and routes the message onto a different channel based on data contained in the message. The routing can be based on a number of criteria such as existence of fields, specific field values etc. When implementing a Content-Based Router, special caution should be taken to make the routing function easy to maintain as the router can become a point of frequent maintenance. In more sophisticated integration scenarios, the Content-Based Router can take on the form of a configurable rules engine that computes the destination channel based on a set of configurable rules.

...

Example: Apache Camel (NEW)

Many open source ESBs like Apache Camel have direct support for many Enterprise Integration Patterns, wither in a Java-based DSL or as configuration. For example, setting up a Content-Based Router in Camel is quite easy and idiomatic by using a RouteBuilder:

RouteBuilder builder = new RouteBuilder() {
  public void configure() {
    errorHandler(deadLetterChannel("mock:error"));

    from("direct:in")
      .choice()
        .when(header("type").isEqualTo("widget"))
          .to("direct:widget")
        .when(header("type").isEqualTo("gadget"))
          .to("direct:gadget")
        .otherwise()
          .to("direct:other");
  }
};

Related patterns: Dynamic Router, Message Filter, Invalid Message Channel, Message Router, Pipes and Filters, Publish-Subscribe Channel, Routing Slip, 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