Sommerville-Software-Engineering-10ed

18.3 ■ Service engineering 535
that occurs when a single support service is provided? Can this fit into the
RESTful model, or should it be redesigned as an entity-oriented service.
3. Is the service independent? That is, to what extent does it rely on the availability
of other services?
4. Does the service have to maintain state? If state information is required, this
must either be maintained in a database or passed as a parameter to the service.
Using a database affects service reusability as there is a dependency between the
service and the required database. In general, services where the state is passed
to the service are easier to reuse, as no database binding is required.
5. Might this service be used by external clients? For example, an entity-oriented
service associated with a catalog could be made available to both internal and
external users.
6. Are different users of the service likely to have different non-functional requirements?
If they do, then more than one version of a service should perhaps be
implemented.
The answers to these questions help you select and refine abstractions that can be
implemented as services. However, there is no formulaic way of deciding which are
the best services. You need to use your experience and business knowledge to decide
on what are the most appropriate services.
The output of the service selection process is a set of identified services and associated
requirements for these services. The functional service requirements should
define what the service should do. The non-functional requirements should define
the security, performance, and availability requirements of the service.
To help you understand the process of service candidate identification and
implementation, consider the following example:
A company, which sells computer equipment, has arranged special prices for
approved configurations for some large customers. To facilitate automated
ordering, the company wishes to produce a catalog service that will allow
customers to select the equipment that they need. Unlike a consumer catalog,
orders are not placed directly through a catalog interface. Instead, goods are
ordered through the web-based procurement system of each company that
accesses the catalog as a web service. The reason for this is that large companies
usually have their own budgeting and approval procedures for orders that
must be followed when an order is placed.
The catalog service is an example of an entity-oriented service, where the underlying
resource is the catalog. The functional catalog service requirements are as follows:
1. A specific version of the catalog shall be provided for each user company. This
shall include the approved configurations and equipment that may be ordered by