Prerequisites

See: Understanding Application Gateway Components

Operation Overview

From Microsoft doc.

Operation Explanation

• Domain name i.e. foo.com resolves to the frontend (public) IP of application gateway
• Application gateway accepts incoming traffic on one or more listeners
• Listener checks for connection requests
• If WAF is used, application gateway checks the request headers and the body, if present, against WAF rules
• WAF, determines if the request is valid request or a security threat
• If the request is valid, request routing rule is evaluated
• Request routing rule determines which backend pool to route the request to
• After selecting backend pool, it sends the request to one of the healthy backend servers in the pool (The health of the server is determined by a health probe)
• For healthy servers, it opens a new TCP session with the backend server based on HTTP settings
• HTTP settings specify the protocol, port, and other routing-related settings that are required to establish a new session with the backend server
• When an application gateway sends the original request to the backend server, it honors any custom configuration made in the HTTP settings related to overriding the hostname, path, and protocol ()
• Application gateway inserts five additional headers to all requests before it forwards the requests to the backend