Sommerville-Software-Engineering-10ed

532 Chapter 18 ■ Service-oriented software engineering
Restful API
Service
requestor 1
Figure 18.7 RESTful
and SOAP-based APIs
Resource
R
SOAP-based
API
Service
requestor 2
these services may be included. The requesting service may then process the
requests in its own way. Therefore, a request for weather information given a place
name that is not unique may return the URLs of all of the places that match the request.
For example:
http://weather-info-example.net/temperatures/edinburgh-scotland
http://weather-info-example.net/temperatures/edinburgh-australia
http://weather-info-example.net/temperatures/edinburgh-maryland
A fundamental design principle for RESTful services is that they should be stateless.
That is, in an interaction session, the resource itself should not include any state
information, such as the time of the last request. Instead, all necessary state information
should be returned to the requestor. If state information is required in later
requests, it should be returned to the server by the requestor.
RESTful services have become more widely used over the past few years because
of the widespread use of mobile devices. These devices have limited processing
capabilities, so the lower overhead of RESTful services allows better system performance.
They are also easy to use with existing websites—implementing a RESTful
API for a website is usually fairly straightforward.
However, there are problems with the RESTful approach:
1. When a service has a complex interface and is not a simple resource, it can be
difficult to design a set of RESTful services to represent this interface.
2. There are no standards for RESTful interface description, so service users must
rely on informal documentation to understand the interface.
3. When you use RESTful services, you have to implement your own infrastructure
for monitoring and managing the quality of service and the service reliability.
SOAP-based services have additional infrastructure support standards such
as WS-Reliability and WS-Transactions.
Pautasso et al. (Pautasso, Zimmermann, and Leymann 2008) discuss when
RESTful and SOAP-based should be used. However, it is often possible to provide
both SOAP-based and RESTful interfaces to the same service or resource (Figure
18.7). This dual approach is now common for cloud services from providers such as
Microsoft, Google, and Amazon. Service clients can then choose the service access
method that is best suited to their applications.