Content-Based Router Pattern — Route by Message Content

In this tutorial, you'll learn how the Content-Based Router pattern examines message content and routes it to the appropriate destination.

What You'll Learn

how the Content-Based Router pattern examines message content and routes it to the appropriate destination.

Why It Matters

Hard-coded routing if-else chains don't scale. Content-Based Router centralizes routing by content.

Real-World Use

Apache Camel content-based router, Spring Integration header router, and Kafka Streams branches.

The Content-Based Router Pattern

The Content-Based Router pattern addresses a specific recurring design problem by providing a reusable solution structure. Understanding when and how to apply it is essential for writing maintainable, scalable code.

Key Concepts

• Message Routing: Content-Based Router directs messages from producers to consumers.
• Transformation: Converts message formats between systems.
• Decoupling: Producers and consumers have no direct knowledge of each other.
• Reliability: Ensures delivery even when components fail.

Structure

The following diagram shows the structure of this pattern:

flowchart LR
    Producer -- Message --> ContentBasedRouter
    ContentBasedRouter -- Route --> ConsumerA
    ContentBasedRouter -- Route --> ConsumerB

Implementation

from typing import List, Dict
from dataclasses import dataclass

@dataclass
class Message:
    key: str
    payload: str

class ContentBasedRouter:
    def __init__(self):
        self._subscribers: Dict[str, List] = {}

    def subscribe(self, key: str, handler):
        self._subscribers.setdefault(key, []).append(handler)

    def publish(self, msg: Message):
        handlers = self._subscribers.get(msg.key, [])
        for h in handlers:
            h(msg)

def log_handler(msg: Message):
    print(f"LOG: {msg.key} -> {msg.payload}")

def alert_handler(msg: Message):
    print(f"ALERT: {msg.key} -> {msg.payload.upper()}")

bus = ContentBasedRouter()
bus.subscribe("order.created", log_handler)
bus.subscribe("order.created", alert_handler)
bus.subscribe("order.shipped", log_handler)

bus.publish(Message("order.created", "Order #1234"))
print("---")
bus.publish(Message("order.shipped", "Order #5678"))

Expected output:

LOG: order.created -> Order #1234
ALERT: order.created -> ORDER #1234
---
LOG: order.shipped -> Order #5678

Key Participants

• Producer: Component that sends messages.
• Consumer: Component that receives messages.
• Content-Based Router: Routes and transforms messages.
• Channel: Medium through which messages flow.

Real-World Examples

• DodaTech uses this pattern internally for consistent cross-cutting concerns.
• Major frameworks and libraries implement this pattern as a core architectural element.
• Production systems at scale depend on this pattern for reliability.

Related Patterns

• Message Router

• Message Filter

• Routing Slip

• Design Patterns — the complete patterns catalog.

Pros and Cons

Common Mistakes

1. **Over-engineering: Applying Content-Based Router where a simpler solution suffices, adding unnecessary complexity.

2. **Wrong granularity: Implementing Content-Based Router at the wrong level of abstraction.

3. **Thread Safety ignored: Using Content-Based Router in concurrent context without proper synchronization.

4. **Tight coupling: Violating the pattern intent by creating hidden dependencies.

5. **Premature optimization: Introducing Content-Based Router before there is evidence it is needed.

Practice Questions

1. What problem does the Content-Based Router pattern solve? Describe a real-world scenario where using it improves code quality.

2. How does Content-Based Router differ from alternative approaches? What are the trade-offs?

3. What testing Strategy would you use for code that implements Content-Based Router?

4. How would you refactor legacy code to introduce Content-Based Router?

5. When should you NOT use Content-Based Router? Describe scenarios where it adds unnecessary complexity.

Challenge

Implement a complete Content-Based Router example in Python with unit tests. Include error handling, edge cases (empty data, null values, concurrent access), and a performance comparison against a simpler alternative. Document your design decisions.

Real-World Task

Find a section of code in your current project that could benefit from the Content-Based Router pattern. Refactor it, write tests, and measure the improvement in testability, coupling, and cohesion.

Security Tip: When implementing Content-Based Router, ensure proper input validation, avoid exposing internal state, and follow Least Privilege. At DodaTech, all implementations undergo security review.

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