General.Operation
Figure 4 shows the general interaction between clients and the replicated service. This Figure
represents three replicas placed on geographically-distant sites interconnected by a WAN,
which cooperate to serve client requests. Each replica is composed of a complete copy of the
Web application and a HA-module. This module is the software component that enhances
availability. The HA-module intercepts client requests and coordinates itself with the rest
of the HA-modules to ensure that all replicas are mutually consistent.
An important factor about this general operation is the mechanism that clients use to locate
replicas to request services. The most common solution is shown in the Figure itself and uses
the Internet DNS support for multiple IP addresses (Brisco, 1995). With this support, when
a client requests a name such as http://dot.com, the navigator sends a lookup request to the
DNS server. The server will then reply with just one IP among the configured IP addresses.
Once the navigator has the chosen IP address, it connects to that address to request the service.
Subsequent requests done by this client or by other clients will work the same way, but the
DNS server will return a different configured IP addresses in a round-robin fashion.
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