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Figure 4.3: Four-tier web service architecture
The frontends handle interactions with the users, and communicate to the application serv-
ers for content. The application servers access shared data sources in the final layer.
Thedifferencebetweenthethree-tierandfour-tierdesignsisthattheapplicationandthe
web servers run on different machines. The benefits of using the latter design pattern are
that we decouple the customer-facing interaction, protocols, and security issues from the
applications. The downside is that it takes a certain amount of trust for application service
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