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4.2 Three-Tier Web Service
The three-tier web service is a pattern built from three layers: the load balancer layer, the
web server layer, and the data service layer (see Figure 4.2 ). The web servers all rely on
a common backend data server, often an SQL database. Requests enter the system by go-
ing to the load balancer. The load balancer picks one of the machines in the middle layer
and relays the request to that web server. The web server processes the request, possibly
queryingthedatabasetoaiditindoingso.Thereplyisgeneratedandsentbackviatheload
balancer.
Figure 4.2: Three-tier web service architecture
A load balancer works by receiving requests and forwarding them to one of many rep-
licas—thatis,webserversthatareconfiguredsuchthattheycanallservicethesameURLs.
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