DNS resolution flow: Part 2 - Routing traffic to nearest web server

In Last article (DNS resolution flow: Part 1 - User accessing web server hosted at Azure), we discussed how DNS resolution is done when there is single server deployed to a region. Consider the web server hosting the web site grows popularity and users from different regions also accessing it with more latency than user from same region. And, the reason is simple, each access request has to travel across regions.

To address the performance issue, web site is now deployed to other regions as well (say US and Europe) and Azure traffic manager is used to route the traffic based upon the routing method.

URL of different services involved

Since there are two Web Server (Using Azure App Service) deployed at two different locations:

1. US Location : https://learningwebapp-1.azurewebsites.net
2. Europe Location : https://learningwebapp-2.azurewebsites.net
3. Traffic Manager: https://learningtm-1.trafficmanager.net

Traffic Manager Configuration

Traffic Manager Endpoints needs to be configured. Refer here to know more

* US Location : https://learningwebapp-1.azurewebsites.net

* Europe Location : https://learningwebapp-2.azurewebsites.net

Understand flow in detail

1. User typed web server URL (https://learningtm-1.trafficmanager.net) in its browser to access the content. Browser looks for the associated IP in its cache and use it if it finds.
2. If its not find in cache, then it takes help from recursive DNS by passing web server URL.
3. Recursive DNS returns the IP from its cache if it has else ask to Root Name server.
4. Root Name server extract the TLD (Top level domain), which is .net, from the web server URL and return the TLD (Top level domain) server IPs to recursive DNS.
5. Recursive DNS picks one of the TLD IPs to connect to. TLD server extracts domain from the URL, which is trafficmanager, and return the list of Authoritative Name server IPs (which is Traffic Manager Name servers) to recursive DNS.
6. Recursive DNS then picks up one of the IPs and establish connection with Traffic Manager Name server to get the IP of web server. Traffic Manager Name server picks the Web server based upon the Routing method setting.
7. During the process of finding the appropriate web server, it considers Recursive DNS IP address is considered instead of user's desktop/laptop.
8. Traffic Manager Name server looks for URL to Endpoints mapping in its database and return Web server URL (https://learningwebapp-1.azurewebsites.net) to recursive DNS.
9. Recursive DNS still does not have IP. Instead it has web server URL. It starts the process again from calling Root Server to getting the IPs of Name servers holding info of azurewebsites.net domain which is cloud app name server.
10. Recursive DNS then contacts the cloud app name server to get the IP.
11. Recursive DNS stores this mapping (Web server URL to IP) in its cache and return IP to browser.
12. Browser then uses this IP to connect to WebServer.

References

https://docs.microsoft.com/en-us/azure/traffic-manager/traffic-manager-how-it-works

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