Content-Based RouterMessaging Patterns » Message Routing

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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 CamelNEW

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

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