Identify appropriate use cases - Learn Tech From Zero

3.2 Azure Application Gateway

Before identifying use cases, you must understand what Azure Application Gateway is.

Azure Application Gateway is a Layer 7 (Application Layer) load balancer.
It helps distribute incoming web traffic based on HTTP/HTTPS requests, not just IPs or ports.
It can also secure, route, and optimize web applications.

Think of it as a smart traffic manager specifically for web applications in Azure.

Knowing these features helps determine when to use it:

URL-based routing
- Can direct requests to different backend servers based on the URL path.
- Example: /images goes to one server, /videos goes to another.
Multi-site hosting
- Host multiple websites on a single Application Gateway using different domains.
- Example: www.contoso.com and blog.contoso.com on same gateway.
SSL Termination
- Decrypts HTTPS traffic at the gateway instead of at the web server.
- Reduces load on backend servers.
Web Application Firewall (WAF)
- Protects web apps from common threats like SQL injection and cross-site scripting.
- Can enable OWASP rules to block attacks.
Session Affinity
- Keeps a user session on the same backend server.
- Useful for apps that store session state locally.
Autoscaling
- Automatically scales capacity based on traffic load.
- Ensures performance during high traffic periods.
Redirection & Rewriting
- Can rewrite URLs or redirect traffic, e.g., force HTTPS or redirect old URLs.

Azure Application Gateway is used in situations where web application traffic management, security, and optimization are needed.

Here’s a breakdown of appropriate use cases:

Scenario: You have multiple web servers hosting the same application.
Reason to use Application Gateway: It can distribute user traffic intelligently at Layer 7, based on URL paths or host headers.
Example: /api traffic goes to API servers, /frontend traffic goes to web servers.

Scenario: Your organization hosts multiple websites or apps in the same Azure environment.
Reason: Application Gateway allows multi-site hosting on a single gateway, reducing costs.
Example: shop.contoso.com and support.contoso.com on the same gateway.

Scenario: You need to protect your web apps from attacks like SQL injection, XSS, or DDoS.
Reason: Enable the Web Application Firewall (WAF) feature.
Example: A public-facing customer portal that must follow security compliance.

Scenario: Your backend servers are under heavy load from decrypting HTTPS traffic.
Reason: Application Gateway can handle SSL/TLS termination at the gateway, reducing backend load.
Example: A web API that receives thousands of HTTPS requests per second.

Scenario: Your application needs to keep a user connected to the same server for the session.
Reason: Application Gateway can use cookie-based session affinity.
Example: Online shopping carts where a user’s cart is stored on a specific server.

Scenario: You want specific paths of a website to go to different servers.
Reason: Application Gateway allows path-based routing and URL rewriting.
Example: /images served from an image server, /videos served from a video server.

Scenario: Your application experiences variable traffic (e.g., during product launches).
Reason: Application Gateway can scale automatically to handle spikes.
Example: Promotional campaigns with sudden high traffic on an e-commerce site.

Understanding limitations is also exam-relevant:

Non-HTTP/S Traffic: If your traffic is TCP/UDP (like SSH, FTP), use Azure Load Balancer instead.
Simple Load Balancing: If you just need basic round-robin load balancing without Layer 7 features, a standard Load Balancer is enough.
Internal-only Apps without Web Requirements: For non-web apps, Application Gateway is overkill.

For the AZ-700 exam:

Remember: Application Gateway = Web App traffic + Security + Routing.
Focus on Layer 7 routing, WAF, SSL offloading, session affinity, and multi-site hosting.
Compare with Azure Load Balancer for TCP/UDP scenarios to know when not to use it.

Feature / Scenario	Use Case Example
Layer 7 Load Balancing	Distribute web traffic intelligently based on URL
Multi-site Hosting	Host multiple websites on one gateway
Web Application Firewall (WAF)	Protect against SQL injection, XSS, and other attacks
SSL Termination	Reduce backend server load by handling HTTPS at gateway
Session Affinity	Keep users on same server for shopping carts or sessions
URL-based Routing	Direct `/images` to image server, `/videos` to video server
Auto-scaling	Handle traffic spikes automatically
Redirection & Rewriting	Force HTTPS or redirect old URLs

This covers everything you need to know for the exam about identifying appropriate use cases for Azure Application Gateway.