Application Architectures - The Practice of Cloud System Administration - page 75

4.2.5 Scaling
The three-tier web service has many advantages over the single web server. It is more ex-
pandable; replicas can be added. If each replica can process 500 queries per second, they
can continue to be added until the total required capacity is achieved. By splitting the data-
base service onto its own platform, it can be grown independently.
This pattern is also very flexible. This leads to many variations:
• Replica Groups: The load balancer can serve many groups of replicas, not just
one. Each group serves a different web site. The number of replicas in each group
can be grown independently as that web site requires.
• Dual Load Balancers: There can be multiple load balancers, each a replica of the
other. If one fails, the other can take over. This topic is discussed further in Section
6.6.3 .
• Multiple Data Stores: There may be many different data stores. Each replica
group may have its own data store, or all of the data stores may be shared. The
data stores may each use the same or different technology. For example, there may
be one data store dedicated to shared state and another that stores a product catalog
or other information needed for the service.
4.3 Four-Tier Web Service
Afour-tierwebserviceisusedwhentherearemanyindividualapplicationswithacommon
frontend infrastructure ( Figure 4.3 ). In this pattern, web requests come in as usual to the
load balancer, which divides the traffic among the various frontends.