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provider could use one of the
XML
programmatic
API
s. Elements of this mes-
sage can be processed sequentially, so we could probably get by with the Sim-
ple
API
for
XML
(
SAX
). If instead the provider needed to construct the entire
tree to process the elements of the message out of sequence, the provider
would probably need a more robust
API
, such as the
DOM
API
.
We also have some flexibility in managing changes in the interface. We can
add processing steps at the client or server to translate between different
XML
formats, or even convert to a format that is not
XML
at all by using Extensible
Stylesheet Language Transformations (
XSLT
). We could even use
XSLT
to
translate this message format to a valid communication block for an older
interface.
XML
, combined with the incredible flexibility and tools at the client
and server sides, lets us decouple important interfaces.
7.4.4
Solution 2: Delay binding with web services
XML
allows us to decouple interfaces by adding an abstraction layer and struc-
tural clues to the message. We also have a technology that addresses the prob-
lem of premature binding: web services let us describe our interface as a
service that clients can register and bind. The interesting part of this process
for us is
delayed binding
.
Figure 7.8 shows how this works. The provider describes a service in an
XML
-based markup language called Web Services Description Language
(
WSDL
), and publishes the interface in a distributed registry. The requester
can search the registry for a service and then bind to the service. The standard
for the open registration of resources is called Universal Discovery Description
and Integration (
UDDI
), which specifies a common
XML
description of an
Provider
Find
Registry
Requester
Provider
Figure 7.8 Web services loosen coupling between the provider and the requester by delaying the
binding process. Providers publish their service in a distributed registry. Requesters can then
search the registry for an appropriate service. When one is found, the service is bound between the
requester and provider.
