5.1.4.3. Configure Application Gateway Listeners, Rules, and Backend Pools

💡 First Principle: The combination of listeners, rules, and backend pools forms the core logic of an Application Gateway, enabling it to intelligently receive, route, and direct web traffic based on Layer 7 attributes.

Scenario: You are managing a global web application. You need to route traffic to different backend services based on the URL path (e.g., /images to an image service, /api to an API backend). You also need to terminate SSL connections at the gateway to reduce backend server load.

What It Is: Application Gateway is a Layer 7 (HTTP/HTTPS) web traffic load balancer.

Core Components of Application Gateway:

• Listeners: Entry points for incoming requests. Each listener binds to a frontend IP, protocol, and port.
• Routing Rules: Connect listeners to backend pools. Rules define how requests are routed, using conditions like URL path or host header.
• Backend Pools: Collections of backend servers.
• HTTP Settings: Define how the gateway communicates with backend servers.

Configuration Flow:

1. Create listeners to accept client requests.
2. Define backend pools.
3. Set HTTP settings.
4. Create routing rules linking listeners to backend pools.

Visual: Application Gateway Components and Request Flow

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⚠️ Common Pitfall: Not configuring end-to-end SSL. While SSL offloading at the gateway is common, for highly sensitive data, traffic between the Application Gateway and the backend servers should also be encrypted.

Key Trade-Offs:

• Path-based vs. Host-based Routing: Path-based routing is ideal for separating different functions of a single application (e.g., /api, /images). Host-based routing is for hosting multiple distinct websites on the same gateway.

Reflection Question: How do Application Gateway listeners, routing rules, and backend pools (along with HTTP settings) collectively orchestrate web traffic, enabling intelligent Layer 7 routing and SSL termination for your web applications?