What is DNSTag?
DNSTag is a reliable DNS service that allows websites to redirect visitors to different server clusters depending on their geographical location. This greatly reduces average loading time for your pages and improves customer experience. For example: US visitors go to US clusters, visitors from Japan go to servers in Japan.
Geographical Load Balancing with Redundancy
For a large website, multiple datacenters and locations should be utilized to provide high redundancy and low latency to visitors. All locations are actively serving visitors and there is no need to run a hot standby system that does not get used at all during normal operations. This saves operational costs and still maintains a high level of redundancy as all active sites have the same functionality and can take over traffic from another site. For example: if your website has 70% U.S. traffic, 20% European traffic and 10% Asian traffic, you can use 3 clusters to serve US, 2 clusters to serve EU and 2 clusters to serve Asia. If cost is a concern, you could share back-end servers between multiple load balancers. But we recommend to have two load balancers in case one goes down, the other one is still able to forward requests to the back-end servers. The reason for redundancy at multiple levels is that certain browsers and proxies might not honor the specified resolution time (TTL) for a DNS record.
Round Robin with Failover Check
In order to avoid and minimize outages, there must be at least two load balancers for each cluster/datacenter. Each load balancer directs requests to multiple back-end servers (web servers, application servers) at that location. The name servers will equally forward requests to all available load balancers with a round-robin algorithm. Additionally, each name server frequently (about every few minutes) performs a health check against these load balancers. If any load balancer is down or not functioning properly, it will be automatically taken out of rotation. When the load balancer comes back to normal (eg: after a network maintenance, after a DDoS attack), it is automatically brought back to service. To avoid false alarms or premature reactivations, there is a minimum threshold of failures/passes before the load balancer is deactivated or reactivated. Notifications are sent to available sysadmins on these activities.