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<!DOCTYPE concept
  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="GUID-F7CC81CE-E823-5196-944A-5F7524411A08" xml:lang="en"><title>Registration
and Discovery</title><prolog><metadata><keywords/></metadata></prolog><conbody>
<p>Server applications need to advertise what services they provide and clients
of server application services need to know what implementations of the services
exist. This happens through Apparc's application registration file system. </p>
<p>Application registration files have a table in which applications can list
the service UIDs that they implement. For each service UID listed, there is
an associated opaque (i.e. not interpreted by the framework) data field which
indicates how the service is implemented by that application. The opaque data
field is accessed through an <codeph>LLINK</codeph> into a resource elsewhere
in the application registration file group. The meaning of this opaque data
field is not known to the framework, it will vary according to the service.
For some services the opaque data field may be a name intended for user display,
for others it may be structured data that the service's client-side code can
interpret. </p>
<fig id="GUID-84072240-5DC6-5900-B767-FD13666A6639">
<image href="GUID-7F7A449F-CC19-570C-A6C1-BC470256AA8C_d0e172642_href.png" placement="inline"/>
</fig>
<p>Clients of a service can discover which applications implement a particular
service by using <xref href="GUID-5EEE8745-F483-33C0-A5B1-AEB5544DE2BA.dita"><apiname>RApaLsSession</apiname></xref>. This has a number of APIs
for retrieving an application list, filtered to only include applications
that implement a particular service. <codeph>RApaLsSession</codeph> can also
return other information about server applications, including how the services
are implemented as described by the opaque data field. If a service uses the
opaque data field to store localised strings to describe implementations,
these can be shown to the user. If a service uses structured data, a resource
reader can be created over the opaque data field to extract the data. </p>
<p>Also see the 'Service list' section in <xref href="GUID-A6116E8B-9C4A-5B9E-AA8A-BE031408AA2F.dita">Defining
application icons, captions and properties</xref>. </p>
<section><title>See also</title><p><xref href="GUID-940F3F6E-BA9C-5E19-9AC5-D848B5E175FB.dita">Application
Architecture Overview</xref></p></section>
</conbody></concept>