Java Reference
In-Depth Information
communicatewithbackendsoftwareandclientapplicationsacrosstheInternet,and
being inflexible.
Web services are a new form of middleware based on the Web and (typically)
XML. They overcome these and other traditional middleware problems by being
based on free and open standards, by their maintainability, by involving the Web,
andbybeingflexible.Forexample,unliketraditionalremoteprocedurecall(RPC)-
based middleware (see
http://en.wikipedia.org/wiki/Re-
mote_procedure_call
for a brief introduction to RPC), which depends upon
connections that are tightly coupled (and break easily when an application is mod-
ified, hence leading to maintenance headaches), RESTful web services (discussed
later)relyonlooselycoupledconnections,whichminimizetheeffectsofapplication
changes.Awebserviceinterface(oftenanXMLfile)offersanabstractionbetween
clientandserversoftware,sothatchangingoneofthesecomponentsdoesn'tautomat-
ically requirethattheothercomponent bechanged.Maintenance costsarereduced,
and reusability increases because the same interface makes it easier to reuse a web
service in other applications.
Anotherbenefitofwebservicesisthattheypreserveacompany'ssignificantinvest-
ment inlegacy software. Instead ofhaving torewrite this software (which wastyp-
icallywritteninvariouslanguages)fromscratchtomeetevolvingbusinessrequire-
ments(whichcanbeacostlyundertaking),thissoftwarecanbeexposedtoclientsvia
webservices,whichcanbemashedwithotherwebservicestoachievetheserequire-
ments in a cost-effective manner.
SOAP-Based Web Services
ASOAP-basedwebserviceisawidelyusedcategoryofwebservicebasedonSOAP,an
XMLlanguagefordefiningmessages(abstractfunctioninvocationsortheirresponses)
that can be understood by both ends of a network connection. An exchange of SOAP
messagesiscalledanoperation,whichcorrespondstoafunctioncallanditsresponse,
and which is depicted in
Figure 11-2
.
