Global Server Load Balancing (GSLB) is used to share the load between multiple locations (datacenters) or to make sure that users will always be connected to the datacenter that is closest, improving the overall user experience.

Global Server Load Balancing Domain: The Global Server Load Balancing Domain is a publicly resolvable domain or zone for which the Global Server Load Balancing setup is responsible. You can set up a NetScaler appliance to be the Authoritative Server for this domain or to proxy the information to an internal DNS server.

Global Server Load Balancing Site: The Global Server Load Balancing Site is the top-level entity for the Global Server Load Balancing communications. The information used when configuring the site is used for linking LOCAL sites to REMOTE sites and sharing monitoring data by using the Metric Exchange Protocol (MEP). The IP address used must be owned by the NetScaler appliance, such as subnet IP (SNIP), and must use TCP port 3011.

Global Server Load Balancing vServer: The Global Server Load Balancing VServer is used as the decision intermediary for directing client requests to the Load Balancing VServers of one of the GSLB site. The Global Server Load Balancing VServer is bound to a Global Server Load Balancing service.

Global Server Load Balancing Service: The Global Server Load Balancing Service is basically a monitoring link to the Load Balancing VServer. The Global Server Load Balancing Service monitors the link to the Load Balancing VServer that is created on the NetScaler appliance. The state of the local Global Server Load Balancing Service depends on the corresponding local virtual server state.

The Global Server Load Balancing is setup by adding a site identifier at each site. The site identifier includes a site name, a type, and an IP address that is owned by the NetScaler appliance and is used for the GSLB communications.

Each site, has a site type of LOCAL, which defines the site to be a local to the appliance. Additionally, each site, has two or more sites of type REMOTE, which defines the other sites as remote to the local appliance. On each site, a Global Server Load Balancing VServer is created with the same name. This VServer identifies the web site of the organization globally and does not have any IP address associated to it.

The GSLB services are pointing to the Load Balancing vServers configured on each GSLB site by specifying the IP address, protocol, and port number of the respective VServer. These services are bound to each GSLB VServer.

When configuring GSLB the NetScaler will serve as an authoritative nameserver for the address, or domain you would like to control between multiple locations. The NetScaler also serve as a proxy for the authoritative nameservers on the internal network. The authoritative DNS service (ADNS) resolves the DNS queries made and hands it over to the GSLB vServer. The NetScalers will function as authoritative nameservers within a so-called GSLB domain

The GSLB selection process (Load Balancing) can be based on multiple metrics like Round Robin, Least Connection, Least Bandwidth, SourceIPhash, random and a few more. Users can also be bound to a specific location / data center if needed using a method called Static Proximity. When using RTT, Round Trip Time it will calculate the distance between the local NetScaler and the DNS server from where the initial request originated. The RTT is then used to determine the site closest to the user.

After the authoritative nameserver has been contacted, the GSLB vServer will select a GSLB service representing either a load balance, content switching or Gateway vServer to reply with the VIP. Next, the client device from where the DNS request originated will contact the VIP address and will eventually end up on one of the load balanced web servers or the NetScaler Gateway login page.