Application Architectures - The Practice of Cloud System Administration - page 80

4.5 Cloud-Scale Service
Cloud-scaleservicesaregloballydistributed.Theserviceinfrastructureusesoneofthepre-
viously discussed architectures, such as the four-tier web service, which is then replicated
in many places around the world. A global load balancer is used to direct traffic to the
nearest location ( Figure 4.4 ).
Figure 4.4: Cloud-scale web service architecture
Ittakes time forpackets totravel acrosstheinternet. Thefarther thedistance, thelonger
it takes. Anyone who has spent significant time in Australia accessing computers in the
United States will tell you that even though data can travel at the speed of light, that's just
not fast enough. The latency over such a great distance gives such terrible performance for
interactive applications that they are unusable.
We fix this problem by bringing the data and computation closer to the users. We build
multiple datacenters around the world, or we rent space in other people's datacenters, and
replicate our services in each of them.
4.5.1 Global Load Balancer
A global load balancer (GLB) is a DNS server that directs traffic to the nearest data-center.
Normally a DNS server returns the same answer to a query no matter where the query ori-
ginated. A GLB examines the source IP address of the query and returns a different result
depending on the geolocation of the source IP address.