Sysplex distributor roles

With sysplex distributor, the LPARs in the sysplex are assigned roles. A TCP/IP stack in one LPAR is given the role of being a front-end host. It receives inbound connection requests and redirects them to a specific target back-end TCP/IP stack. Other LPARs run target TCP/IP stacks that ultimately function as the real endpoint of communication.

Note: In the context of sysplex distributor, multiple TCP/IP stacks in a single LPAR can be functionally ignored. Consequently, to simplify our discussion, the term "host" is used to denote an LPAR containing a single TCP/IP stack.

Sysplex distributor requires that a choice be made as to whether a host is to function as a distributor, a target, a backup distributor, or some combination.

Things are about to get a little bit more complicated. So let's define some simple terminology.

Distributing host: The designated contact (point of entry) for the sysplex. It is the normal owner of the IP address that clients out in the network use to connect to the sysplex.
Target host: A host within the sysplex to which a distributing host can redirect a connection request. The target host must be running an instance of the target application. For example, if a client wants to connect to FTP, then there must be an FTP server running on the target host to which the session is distributed.
Backup host: A host that is designated as a backup in the event that the distributing host should malfunction. The backup host takes over the IP address of the distributing host when required. There can be more than one backup host.
Combinations: A distributing host can also be a target host; some sessions are distributed to itself. A target host can also be a backup distributing host. The idea here is to not waste an LPAR just because it is assigned a certain role.

Sysplex distributor in action

With these options and flexibility, there are a considerable number of configurations possible. The layout in Figure 1 is just one simple possible scenario.

For example, the only application shown as being available on all target hosts is FTP. Such a sysplex might be functioning as a very busy FTP server farm, but it presents a single IP address to all incoming FTP clients. Many other applications could be added to these hosts as target applications.

Figure 1. Distributed DVIPA, sysplex distributor

In Figure 1, there are four LPARs being used for sysplex distributor. LPAR 1 is a distributing host, and LPAR 2 is its backup. All LPARs are configured as targets for connections. The target application, of course, is the z/OS FTP server.