Cloudware Operations and Management - Guide to Cloud Computing for Business and Technology Managers - page 386

are stored at multiple nodes and replication techniques (see Section
3.4) are used to keep multiple instances consistent. Then, only the
number of replicas and their placement affect the degree of availabil-
ity. For instance, Amazon recommends to deploy to EC2 instances in
different availability zones to increase availability by having geo-
graphically distributed replicas, which is done for data in SimpleDB
automatically.
2. Consistency : Consistency as service guarantee depends on the kind
of service provision. In case of a hosted database like Amazon RDS
or Microsoft Azure SQL, full ACID guarantees are given, whereas
scalable distributed data stores such as Amazon SimpleDB guaran-
tee only levels of eventual consistency. Techniques for ensuring dif-
ferent levels of consistency are standard database techniques that
can be found in the concerned textbooks.
3. ( Query ) response time : The response time of a query can either be
defined in the form of deadline constraints, for example, response
time of a given query is ≤10 s, or not per query but as percentile con-
straints. The latter means, for example, that 90% of all requests need
to be processed within 10 s; otherwise, the provider will pay a pen-
alty charge. Though response time guarantees are not the domain
of standard SQL database systems, there exist some techniques in
real-time databases.
18.6.2 Pricing Models for Cloud Systems
One of the central ideas and key success factors of the cloud computing
paradigm is the pay-per-use price model. Ideally, customers would pay
only for the amount of the resources they have consumed. Looking at the
current market, services differ widely in their price models and range from
free or advertisement-based models over time or volume-based models
to subscription models. Based on a discussion of the different cost types
for cloud services, we present some fundamentals of pricing models in the
following.
18.6.2.1 Cost Types
For determining a pricing model, all direct and indirect costs of a provided
service have to be taken into account. The total costs typically comprise
capital expenditures (CapEx) and operational expenditures (OpEx). Capital
expenditures describes all costs for acquiring assets such as server and net-
work hardware, software licenses, but also facilities, power, and cooling
infrastructure. Operational expenditures (OpEx) includes all costs for run-
ning the service, for example, maintenance costs for servers, facilities, infra-
structure, payroll, but also legal and insurance fees. One of the economical