Cloudware Basics - Guide to Cloud Computing for Business and Technology Managers - page 276

2. Configurable instances : In this level, all customers share the same ver-
sion of the program. However, customization is possible through
configuration and other options. Customization could include the
ability to put the customer's logo on the screen and tailoring of
workflows to the customer's processes. In this level, there are sig-
nificant manageability savings over the previous level, since there
is only one copy of the software that needs to be maintained and
administered. For instance, upgrades are seamless and simple.
3. Conigurable, multitenant efficient instances : Cloud systems at this level
in addition to sharing the same version of the program also have
only one instance of the program running that is shared among all
the customers. This leads to additional efficiency since there is only
one running instance of the program.
4. Scalable, configurable, multitenant efficient instances : In addition to
the attributes of the previous level, the software is also hosted on
a cluster of computers, allowing the capacity of the system to scale
almost limitlessly. Thus, the number of customers can scale from a
small number to a very large number, and the capacity used by each
customer can range from being small to very large. Performance
bottlenecks and capacity limitations that may have been present
in the earlier level are eliminated. For instance, in a cloud e-mail
service like Gmail or Yahoo Mail, multiple users share the same
physical e-mail server as well as the same e-mail server processes.
Additionally, the e-mails from different users are stored in the same
set of storage devices and perhaps the same set of files. This results in
management efficiencies; as a contrary example, if each user had to
have a dedicated set of disks for storing e-mail, the space allocation
for each user would have to be managed separately. However, the
drawback of shared storage devices is that security requirements are
greater; if the e-mail server has vulnerabilities and can be hacked, it
is possible for one user to access the e-mails of another.
13.6.3 Availability
Cloud services also need special techniques to reach high levels of avail-
ability. Mission-critical enterprise services generally have availability in the
99.999% range. This corresponds to a downtime of 5 min in an entire year!
Clearly, sophisticated techniques are needed to reach such high levels of reli-
ability. Even for non-mission-critical applications, downtime implies loss of
revenue. It is therefore extremely important to ensure high availability for
both mission-critical as well as non-mission-critical cloud services. There
are basically two approaches to ensuring availability. The first approach is
to ensure high availability for the underlying application upon which the
cloud service is built. This generally involves one of the three techniques: