Design Patterns for Scaling - The Practice of Cloud System Administration - page 99

5.3.1 x : Horizontal Duplication
Horizontal duplication increasesthroughputbyreplicatingtheservice.Itisalsoknownas
horizontal scaling or scaling out .
This kind of replication has been discussed in past chapters. For example, the technique
of using many replicas of a web server behind a load balancer is an example of horizontal
scaling.
A group of shared resources is called a resource pool. When adding resources to a pool,
it is necessary for each replica to be able to handle the same transactions, resulting in the
same or equivalent results.
The x -axis does not scale well with increases in data or with complex transactions that
require special handling. If each transaction can be completed independently on all rep-
licas, then the performance improvement can be proportional to the number of replicas.
There is no loss of efficiency at scale.
Recommended Books on Scalability
The Art of Scalability: Scalable Web Architecture, Processes, and Organizations
for the Modern Enterprise by Abbott and Fisher ( 2009 ) is an extensive catalog of
techniques and discussion of scalability of people, processes, and technologies.
Scalability Rules: 50 Principles for Scaling Web Sites ,alsobyAbbottandFish-
er ( 2011 ) , is a slimmer volume, focused on technical strategy and techniques.
When the transactions require replicas to communicate, the scaling is less efficient. For
example, transactions that write new data that must be communicated to all replicas may
require all replicas to hold off on any future transactions related to the update until all rep-
licas have received the change. This is related to the CAP Principle (discussed in Section
1.5 ) .
Techniques that involve x -axis scaling include the following:
• Adding more machines or replicas
• Adding more disk spindles
• Adding more network connections
5.3.2 y : Functional or Service Splits
A functional or service split meansscalingasystembysplitting outeachindividual func-
tion so that it can be allocated additional resources.