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1. Two methods to construct the cluster of servers
(1). DNS based load balancing cluster
DNS load balancing is probably the simplest method to build network
service cluster. It uses Domain Name System to distribute requests to
different servers through resolving the domain name to different IP
addresses of servers. When a DNS request comes to the DNS server to
resolve the domain name, the DNS server gives out one of server IP
addresses based on scheduling strategies, such as in round-robin
manner, then subsequent requests from clients using the same local
caching name server are sent to the same server in the specified
time-to-live (TTL) of name resolving.
(2). Dispatcher based load balancing cluster
Dispatcher, also known as load balancer, can be used to distribute
load among servers in a cluster, so that the parallel services of
servers can appear as a virtual service on a single IP address, and
end users interact as if it were a single server without knowing all
servers in clusters. Compared to DNS based load balancing, dispatcher
can schedule requests at fine granularity, such as per connection, for
better load balancing among servers. Failure can be masked when one
server or more fail. Server management is becoming easy, and
administrator can take a server or more in and out of service at any
time, which won't interrupt services to end users.
2.  What is virtual server?
Virtual server is a highly scalable and highly available server
built on a cluster of real servers. The architecture of server cluster
is fully transparent to end users, and the users interact with the
cluster system as if it were only a single high-performance virtual
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