An office building may have shared
bathrooms, copy rooms, and lobby.
The engineering for the building is typically common for the whole structure.
To get these shared benefits, tenants
may have a fixed office layout, as well as
some rules for usage. Normally, people
cannot sleep at work, cannot have pets
at work, and may even have a dress
code for the building.
A retail mall provides shared engineering, parking, security, and common space. An advertising budget
may benefit all the mall tenants. In
exchange, there are common hours,
limits on the allowable retail activities,
and constraints on the appearance of
Each of these building types man-
dates constraints on usage patterns
and offers concierge services in ex-
change. To enjoy the benefits, you need
to accept the constraints.
figure 1. Cloud computing, utility
computing, and software as a service.
SaaS Provider / Cloud user
What is Cloud Computing?
Cloud computing delivers applications
as services over an intranet or the Internet. A number of terms have emerged
to characterize the layers of cloud-computing solutions.
˲ SaaS. This refers to the user’s ability
to access an application across the Internet or intranet. SaaS has been around
for years now (although the term for it is
more recent). What is new is the ability
to project the application over the Web
without building a data center.
˲ PaaS. This is a nascent area in
which higher-level application services are supplied by the vendor with
two goals: first, a good PaaS can make
developing an application easier; second, a good PaaS can make it easier for
the cloud provider to share resources
efficiently and provide concierge services to the app. Today, the leading examples of PaaS are Salesforce’s Force.
com5 and Google’s App Engine. 2
˲ IaaS (infrastructure as a service).
Sometimes called utility computing,
figure 2. saas computing and storage.
SaaS Computing and Storage
user and System data
this is virtualized hardware and computing available over the Web. The user
of an IaaS can access virtual machines
(VMs) and storage to accompany them
Figure 1 shows the relationship
between the cloud and SaaS providers and users. (The figure was derived
from a technical report from the
University of California at Berkeley,
“Above the Clouds: A Berkeley View of
Cloud Computing.” 3) As observed in
“Above the Clouds,” cloud computing
has three new aspects: the illusion of
infinite computing resources on demand; the elimination of upfront commitment by cloud users; and the ability to pay for computing resources on a
Cloud computing allows the deployment of SaaS—and scaling on demand—without having to build or provision a data center.
Public and private clouds. Clouds
are about sharing. The question is
whether you share within a company or
go to a third-party provider and share
In a public cloud, a cloud-computing provider owns the data center. Other companies access their computing
and storage for a pay-as-you-go fee. This
has tremendous advantages of scale,
but it is more challenging to manage
the trust relationship. Trust ensures
that the computing resources are available when they are needed (this could
be called an SLA, or service-level agreement). In addition, there are issues of
privacy trust in which the subscribing
company needs to have confidence its
private data will not be accessed by prying eyes. Demonstrating privacy is easier if the company owns the data center.
A public cloud can project its shared
resources as VMs and low-level storage
requiring the application to build on
what appears to be a pool of physical
machines (even though they are really
virtual). This would be a public-cloud
IaaS. Amazon’s AWS (Amazon Web Service) 1 is a leading example of this.
Alternatively, in a public-cloud
PaaS, higher-level abstractions can be
presented to the applications that allow finer-grained multitenancy than a
VM. The shape and form of these abstractions are undergoing rapid evolution. Again, Force.com and App Engine
are emerging examples.