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Global Server Load Balancing

The most important reason for the use of GSLB is to provide high availability by hosting websites more than one geographic location. With GSLB technology, incoming requests are redirected to servers in different locations on the internet for load balancing. In addition, when service on a site was interrupted by a broken internet connection, power failure, local load balance device(SLB) failure or DoS attack (Denial of Service), GSLB device becomes aware of  the situation, redirects incoming requests to another site and prevent interruption of the service. GSLB makes necessary routing processes by responding to DNS requests.

DNS Resolution
DNS resolution process in a GSLB infrastructure takes place in the following order.
Suppose that there is a site in Ankara which has virtual IP(VIP) address1.1.1.1 and there is a site in Istanbul which has VIP address 2.2.2.2. Our GSLB device will act as the authoritative DNS server and decide where to redirect incoming requests.
1. Stub resolver (application running on the client computer) makes a DNS request from which is defined on itself. Requests made from the clients are called recursive queries. Client computer will not try any other request because DNS server is responsible for this task.
2. DNS server makes queries on the root DNS servers on behalf of the client and reaches GSLB device acting as Authoritative DNS server.
3. GSLB device contacts with local SLB devices in each site and receives information about local SLB's status like number of connections, response time etc.
4. Local SLB devices can make dynamic performance measurements like Round trip time RTT, topgraphical footrace, BGP hop count if desired.
5. GSLB device decides to route requests to which site using information obtained from the third and fourth steps. In this example response can be 1.1.1.1 or 2.2.2.2. This response is sent to DNS server which is defined on the client computer.
6. DNS server sends IP information to the stub resolver (application running on the client). So the client begins communicating most available site on the internet.

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