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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_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>