Selecting a Service Platform - The Practice of Cloud System Administration - page 51

PaaS providers charge for their services based on how much CPU, bandwidth, and stor-
age are used. This is similar to IaaS except the charges are higher to compensate for the
more extensive framework that is provided.
The downside of PaaS is that you are restricted to using what the vendor's platform
provides. The platform is generally programmable but not necessarily extensible. You do
not have direct access to the operating system. For instance, you may not be able to add
binaries or use popular libraries until the vendor makes them part of its service. Generally
processesruninasecure“jail” (similar toUNIX's“chroot” restricted environment), which
aimstopreventthemfrombreakingoutoftheservice'sframework.Forexample,onePaaS
offered the Python language but not the Python Imaging Library (PIL). It could not be in-
stalled by users because the framework does not permit Python libraries that include por-
tions written in compiled languages.
PaaS provides many high-level services including storage services, database services,
and many of the same services available in IaaS offerings. Some offer more more esoteric
services such as Google's Machine Learning service, which can be used to build a recom-
mendation engine. Additional services are announced periodically.
3.1.3 Software as a Service
SaaSiswhatweusedtocallawebsitebeforethemarketingdepartmentdecidedadding“as
a service” made it more appealing. SaaS is a web-accessible application. The application
is the service, and you interact with it as you would any web site. The provider handles all
the details of hardware, operating system, and platform.
Some common examples include Salesforce.com, which replaces locally run sales team
management software; Google Apps, which eliminates the need for locally run email and
calendaring software; and Basecamp, which replaces locally run project management soft-
ware. Nearly any business process that is common among many companies is offered as
SaaS: human resources (HR) functions such as hiring and performance management; ac-
counting functions such as payroll, expense tracking, and general ledgers; IT incident, re-
quest,andchangemanagementsystems;andmanyaspectsofmarketingandsalesmanage-
ment.
The major selling point of SaaS is that customers do not have to concern themselves
with software installation, upgrades, and operations. There is no client software to down-
load. The service is fully managed, upgraded, and maintained by the provider. Because the
service is accessed via the web, it can be used from any location.
As a SaaS provider, you need to design the service to obscure upgrades and other op-
erational details. Developers must avoid features that require client software or browser
plug-ins. In designing the service you need to recognize that since it can be accessed from