Content-Based Routing

In a large corporation's mail room, someone opens each piece of mail, reads the subject, and routes it to the right department. Tax documents go to finance. Resumes go to HR. Customer complaints go to support. The routing decision depends entirely on the content of the message.

Content-based routing works the same way in messaging. Instead of every message going to the same queue or consumer, a router inspects each message's content and forwards it to the appropriate destination.

An e-commerce platform receives order events. Domestic orders go to the domestic fulfillment queue. International orders go to the international fulfillment queue. High-value orders (over $1,000) go to a fraud review queue. The routing logic inspects the order's destination country and total amount to decide where each message goes.

This is one of the foundational Enterprise Integration Patterns (EIP), described by Gregor Hohpe and Bobby Woolf in their 2003 book. Nearly every message-driven system uses some form of content-based routing, whether they call it that or not. AWS EventBridge is a managed content-based router at its core: events arrive, rules evaluate them, and matching events route to targets. Azure Service Bus topic subscriptions with SQL filters do the same thing. Every enterprise messaging system has this pattern built into the broker or implemented as a service.

A content-based router sits between the source and multiple destinations. It inspects message attributes or payload and applies routing rules:

function routeOrder(message: OrderEvent) {
 if (message.country !== 'US') {
 return 'queue:international-fulfillment';
 }
 if (message.totalAmount > 1000) {
 return 'queue:fraud-review';
 }
 if (message.isPriority) {
 return 'queue:express-fulfillment';
 }
 return 'queue:standard-fulfillment';
}

Implementation approaches across brokers:

RabbitMQ topic exchange example:

Producer publishes to exchange "orders" with routing key "orders.US.standard"
Subscription "domestic" binds with pattern "orders.US.*"     -> matches
Subscription "intl" binds with pattern "orders.EU.*"         -> no match
Subscription "express" binds with pattern "orders.*.express" -> no match

The broker evaluates routing patterns without custom code. This is the most efficient form of content-based routing because the broker is already parsing the message metadata. No additional service, no additional hop, no additional latency.

AWS EventBridge rules example:

{
 "source": ["orders.service"],
 "detail-type": ["OrderCreated"],
 "detail": {
   "country": ["US"],
   "amount": [{"numeric": [">", 1000]}]
 }
}

EventBridge evaluates this rule against every event. Matching events route to the configured target (Lambda, SQS, Step Functions). No custom router service needed.