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<title>APPN Topology Information APIs</title>
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<h2>APPN Topology Information APIs</h2>
<p>The APPN<sup>(R)</sup> topology information APIs are:</p>
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<ul>
<li><A HREF="QNMDRGTI.htm">Deregister APPN Topology Information</A> (QNMDRGTI) causes the queue associated with the specified queue handle to be deregistered for APPN topology information.</li>
<li><A HREF="QNMRGTI.htm">Register APPN Topology Information</A> (QNMRGTI) causes the requested APPN topology information to be reported.</li>
</ul>
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<p>APPN topology information APIs allow an application to obtain information
about the current APPN topology, and to register and deregister for information
about ongoing updates to the topology. Current topology information is an
option provided by the Register for APPN Topology Information (QNMRGTI) API.
When this option is requested, the current APPN topology is reported to a user
space specified by the application running the API. This API also provides
topology update options that allow the application to register a queue to
receive information about specific types of APPN topology updates. Topology
updates are reported asynchronously to the specified queue as they occur in the
network.</p>
<p>A queue remains registered for topology updates until one of the following
occurs:</p>
<ul>
<li>The queue is deregistered by the application using the Deregister APPN
Topology Information (QNMDRGTI) API.</li>
<li>An error is encountered enqueuing topology updates, forcing automatic
deregistration of the queue by the system.</li>
<li>The application's job is ended causing registration to be cleaned up.</li>
<li>An IPL is performed causing registration to be cleaned up.</li>
</ul>
<p>One application in each job on the system may register one queue for
topology updates. Multiple queues may not be registered for topology updates
within the same job.</p>
<p>The specific types of topology updates that an application may register to
receive are:</p>
<ul>
<li>Local end node (*EN) updates</li>
<li>Local virtual node (*VN) updates</li>
<li>Local network node (*NN) updates</li>
<li>Network network node (*NN) updates</li>
<li>Network virtual node (*VN) updates</li>
</ul>
<p>The queue and user space objects specified on the APPN topology information
APIs must be managed entirely by the application; for example, the application
must create, delete and maintain the objects itself, using the APIs for those
objects. The application is responsible for any error handling should these
objects become damaged or deleted.</p>
<p>If an error occurs while reporting the current topology to the specified
user space, an error is returned through the API. If an error occurs while
enqueuing ongoing topology updates to a registered queue, the resulting error
messages are sent to the job log, followed by diagnostic message CPD91C9, and
the queue is automatically deregistered by the system. If automatic handling of
this diagnostic error message is necessary, an application could periodically
scan the job log for this message using the List Job Log (QMHLJOBL) API and
take appropriate action.</p>
<p>After the current topology has been requested using the QNMRGTI API, the
data returned in the user space may be retrieved using the Retrieve User Space
(QUSRTVUS) API. If a data queue is registered for topology updates using the
QNMRGTI API, topology update records may be retrieved out of a data queue using
the Receive Data Queue (QRCVDTAQ) API. If a user queue is registered (rather
than a data queue), the application must use the dequeue (DEQ) MI instruction
to retrieve queue records. The first 10 characters of each queue entry contains
the value *APPNtop so that the application can distinguish these records from
others on the queue. This allows a queue to be used for multiple purposes.</p>
<br>
<h3>Local and Network Topology Updates</h3>
<p>Topology updates can be separated into two classes:</p>
<ul>
<li>Network topology updates</li>
<li>Local topology updates</li>
</ul>
<p>Local topology updates can be reported on an end node or network node
system, but network topology updates can be reported only on a network node
system.</p>
<br>
<h3>APPM Network Topology Updates</h3>
<p>An APPN subnetwork consists of nodes having a common network ID and the
links connecting those nodes. APPN network topology identifies the following in
an APPN subnetwork:</p>
<ul>
<li>All network nodes and virtual nodes in the subnetwork</li>
<li>Transmission groups interconnecting network nodes and virtual nodes in the
subnetwork</li>
<li>Transmission groups from network nodes in the subnetwork to network nodes
in adjacent subnetworks</li>
</ul>
<p>APPN network nodes exchange network topology updates in a subnetwork through
topology database updates (TDUs). Therefore, only network nodes can report
network topology updates. See <em>System Network Architecture Formats</em> for
details about TDUs.</p>
<h3>APPN Local Topology Updates</h3>
<p>The local topology for an APPN node consists of the following:</p>
<ul>
<li>The local node</li>
<li>Adjacent nodes (network nodes, end nodes, or virtual nodes to which the
local node has a direct connection)</li>
<li>Transmission groups from the local node to adjacent nodes</li>
</ul>
<p>Both end nodes and network nodes can report local topology updates.</p>
<br>
<h3>Adjacent Subnetworks</h3>
<p>Network nodes in separate subnetworks may be connected by an intersubnetwork
transmission group; that is, a group of links between directly attached nodes
of 2 or more subnetworks appearing as a single logical link for routing
messages. In this case, the network node at each end point of the transmission
group is present only in the partner network node's local topology, not in its
network topology. For example, consider two network nodes with different
network IDs in separate subnetworks:</p>
<ul>
<li>A network node with the following control point name and network ID:
CPNAME=NN1, NETWORK ID=A</li>
<li>A network node with the following control point name and network ID:
CPNAME=NN2, NETWORK ID=B</li>
</ul>
<p>When NN1 and NN2 are connected by an intersubnetwork transmission group, NN2
is present only in the NN1 local topology; it is not present in the NN1 network
topology or other nodes in network A. This is because TDUs for NN2 are not
exchanged in network A.</p>
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