Page 72 - Cloud Essentials

transmitted back as an XML or JSON document, or possibly even plain,

unformatted text. Although PaaS applications are intended to be used by
other applications anywhere on the Internet, they can also be accessed using
an ordinary browser.
For example, consider the following URL to a web service:

http://ws.geonames.org/postalCodeSearch?postalcode=90210&country=us

Here we are accessing a web service called postalCodeSearch on a web
server called ws.geonames.org and providing the postal code 90210 and the
country us as parameters. The web service results in a web page telling us
that 90210 is the postal code for Beverly Hills, California, and it also

provides the longitude and latitude coordinates.
PaaS is also the way rich Internet applications (RIAs) are made. An RIA
is a web application with many of the characteristics of desktop
applications. It is typically delivered by way of a site-specific browser, a
browser plug-in, independent sandboxes, or virtual machines. The web page
that is initially loaded in the browser displays some text and images, while

in the background it executes JavaScript that asynchronously makes
additional calls to web services using XML as the data format for queries
and responses. These additional calls bring in more text and graphics.
Together, these techniques are called Ajax (for asynchronous JavaScript
and XML). For example, Ajax is used in http://maps.google.com. Each time
you zoom or pan to a different location, the new map parts are brought in
asynchronously. In addition, a lot of the Twitter user interfaces are built this

way.

One drawback of PaaS is the potential for vendor lock-in. A PaaS

provider might provide a specific interface and tools to create web
services that are incompatible with other PaaS providers.

Infrastructure as a Service (IaaS)
Of all cloud services, Infrastructure as a Service most resembles the
components that can be found in a modern data center, even though IaaS

could actually be in different locations. IasS includes storage, firewalls,

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