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<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_d0e171898_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>

<section id="GUID-F27C5A21-25B3-4179-AC7D-ABA1BA4DD41B"><title>See also</title><p><xref href="GUID-940F3F6E-BA9C-5E19-9AC5-D848B5E175FB.dita">Application Architecture

Overview</xref></p></section>

</conbody></concept>