Application Gateway (Layer 7) in Azure - Step-by-Step Execution

Process Flow - Application Gateway (Layer 7)

Client sends HTTP/HTTPS request

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Application Gateway receives request

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Inspect request at Layer 7 (HTTP)

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Apply routing rules based on URL/path/headers

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Forward request to appropriate backend pool

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Backend pool processes request and sends response

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Application Gateway forwards response to client

The Application Gateway receives client requests, inspects them at the web layer, applies routing rules, and forwards them to backend servers, then returns responses.

Execution Sample

Azure

az network application-gateway create \
  --name myAppGateway \
  --resource-group myResourceGroup \
  --location eastus \
  --sku Standard_v2 \
  --frontend-port 80 \
  --http-settings-cookie-based-affinity Enabled \
  --routing-rule-type PathBasedRouting

This command creates an Application Gateway that listens on port 80 and routes requests based on URL paths.

Process Table

Step	Action	Request Detail	Routing Decision	Backend Pool Selected	Response
1	Client sends HTTP GET	GET /images/logo.png	Match path /images/*	BackendPool1 (Image Servers)	Response from Image Server
2	Client sends HTTP GET	GET /api/data	Match path /api/*	BackendPool2 (API Servers)	Response from API Server
3	Client sends HTTP GET	GET /home	Default routing rule	BackendPool3 (Web Servers)	Response from Web Server
4	Client sends HTTPS POST	POST /api/upload	Match path /api/*	BackendPool2 (API Servers)	Response from API Server
5	Client sends HTTP GET	GET /unknownpath	Default routing rule	BackendPool3 (Web Servers)	Response from Web Server
6	Request with no matching rule	GET /no-match	Default routing rule	BackendPool3 (Web Servers)	Response from Web Server
7	End of requests	-	-	-	-

💡 All requests processed and routed according to path-based rules or default rule.

Status Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
Request Path	-	/images/logo.png	/api/data	/home	/api/upload	/unknownpath	/no-match
Routing Rule Matched	-	/images/*	/api/*	default	/api/*	default	default
Backend Pool	-	BackendPool1	BackendPool2	BackendPool3	BackendPool2	BackendPool3	BackendPool3
Response	-	Image Server	API Server	Web Server	API Server	Web Server	Web Server

Key Moments - 3 Insights

Why does a request to /unknownpath go to the default backend pool?

Because /unknownpath does not match any specific path-based routing rule, the Application Gateway uses the default routing rule as shown in execution_table row 5.

How does the Application Gateway decide which backend pool to send the request to?

It inspects the HTTP request path and matches it against configured routing rules. The matched rule determines the backend pool, as seen in execution_table rows 1-4.

What happens if no routing rule matches the request path?

The Application Gateway forwards the request to the default backend pool, ensuring the request is still handled, as shown in execution_table rows 5 and 6.

Visual Quiz - 3 Questions

Look at the execution table, which backend pool handles the request for GET /api/data?

A BackendPool1 (Image Servers)

B BackendPool2 (API Servers)

C BackendPool3 (Web Servers)

D No backend pool

💡 Hint

Check execution_table row 2 under 'Backend Pool Selected'

At which step does the Application Gateway use the default routing rule for the first time?

A Step 1

B Step 3

C Step 5

D Step 2

💡 Hint

Look at execution_table rows and find where 'Routing Decision' is 'Default routing rule'

If a new path-based rule for /blog/* is added, how would the routing change for GET /blog/post1?

A It would route to BackendPool1

B It would route to BackendPool2

C It would route to a new backend pool for blog

D It would still route to the default backend pool

💡 Hint

Adding a new path-based rule directs matching requests to its specific backend pool instead of default

Concept Snapshot

Application Gateway (Layer 7) routes web traffic based on HTTP details like URL paths.
It inspects requests, applies routing rules, and forwards to backend pools.
Supports path-based routing, cookie affinity, and SSL termination.
Default rule handles unmatched requests.
Configured via Azure CLI or portal with frontend, backend pools, and routing rules.

Full Transcript

An Azure Application Gateway works at Layer 7, the web layer, to route client HTTP/HTTPS requests. When a client sends a request, the gateway inspects the URL path and other HTTP details. It matches the request against routing rules configured to direct traffic to specific backend pools. For example, requests to /images/* go to image servers, /api/* to API servers, and others to a default web server pool. If no specific rule matches, the default backend pool handles the request. This ensures all requests are served appropriately. The gateway then forwards the backend response back to the client. This process allows flexible, secure, and efficient web traffic management.