Loosely coupled scale-out architectures
modern Web 2.0 and cloud computing data center scale-out system
architecture deployment has at the front a Web server tier and an
application server tier (sometimes merged together), and in the
back-end a reliable data tier, usually hosted by database servers,
which are typically slow and expensive elements. They often operate at
very low CPU utilization due to blocking on HDD accesses, lock
serialization effects and low HDD capacity utilization due to having to
minimize head movement to reduce access latencies.
the Web server tier and the back-end server tier are a content-caching
tier and specialized application services, which may perform generic
functions such as search, ad serving, photo store/retrieval,
authentication or specific functions for the enterprise. Completing a
response to a customer interaction involves accessing a Web server,
application servers, database servers and various other generic and
specialized applications and servers.
centers generally require that user responses complete in less than a
quarter second. A DRAM-caching tier, consisting of servers filled with
DRAM, usually completes this. Customer information, data retrieved from
slow databases and common user interaction results are cached in this
DRAM tier so they can be accessed very quickly.
the performance of a site can be improved dramatically through
extensive caching, many racks of caching servers are typically
deployed, with each holding a limited amount of DRAM, so the data must
be partitioned among the caching servers. IT staff need to carefully
lay out the data between the caching servers, which typically operate
at very low network and CPU utilization, as they are simply storing and
retrieving small amounts of data.
loosely coupled scale-out architectures are examined closely, it
becomes clear that the database and caching tiers suffer from very low
utilization, high power consumption and excessive programmatic and
administrative complexity-all contributing to high TCO.