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How directory services locate resources in APPN

Networking on z/OS

Among the functions that directory services in an APPN network node provide is finding the location of a specified destination resource.

Finding the location of a specified destination resource is done using the following steps until one succeeds:
  • Search the local directory database for the location of the destination resource. If the location of the destination resource is known (or thought to be known) a directed search is sent to the suspected destination.
  • If no location information about the destination resource is found in the directory database (or if the directed search to the suspected destination fails because the resource has moved or not longer exists), perform a broadcast search of served end nodes.
  • If the destination resource still has not been located, this network node performs either a central directory server (CDS) search or a broadcast search of the native network.
  • If this network node is not a central directory server, then the topology database is used to determine if a central directory server exists in the network. If a central directory server exists, then a directed search is sent to the central directory server to determine the location of the resource. The central directory server is then completely responsible for locating the resource in the native network and any attached subarea networks. This is done by first performing a broadcast search of the native network and, if necessary, a serial (directed) search of all interchange nodes in the network to allow these ICNs to search their attached subarea network(s).
  • If this network node is a central directory server or if no central directory server exists in the network or if the directed search to a central directory server fails to reach the central directory server, then this network node is responsible for locating the resource in the native network and any attached subarea networks. This is done by first performing a broadcast search of the native network and, if necessary, a serial (directed) search of all interchange nodes in the network to allow these ICNs to search their attached subarea network(s).
  • If the destination resource still has not been located and this node is a border node, perform a serial search of adjacent APPN networks by sending directed searches to other border nodes.
  • Broadcast search
    A broadcast search is issued by a network node because a directed search using database information has failed or there is no database information for the requested resource. A broadcast search does not use database information about the location of a requested LU to propagate the search. Instead, a broadcast search is sent to every adjacent network node at the same time. Each of the adjacent network nodes then forwards the broadcast search to all other adjacent network nodes, and so forth. After propagating a broadcast search to all adjacent network nodes, each network node also searches all its client end nodes to determine if the target LU resides within the domain of the network node.
  • Directed search
    A directed search is sent by directory services to the node recorded as the owner of the requested LU to verify the information. A directed search can be sent to a network node server from an end node and to an end node from a network node server. A directed search can also be sent from a network node to a central directory server when the network node does not have information on the location of the destination logical unit or following a failed search.
  • How a CDS minimizes broadcast searches
    An example of another search illustrates the role of a central directory server.
Parent topic: APPN network topology
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