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Starting and Setting Up RTR
2.12 How RTR Selects Processing-states (Roles) for Nodes
relevant to a backend. It is where the backend goes to obtain and verify partition
configuration and facility information.
Routers are made known to the frontend systems through the list specified in
the /ROUTER=(list) qualifier to the CREATE FACILITY command. This list
specifically determines the preferred router. If the first router specified is not
available, the next one on the list is chosen. When the facility is created on the
frontend, the list of routers specified can be a subset of the routers contained
within the entire facility. This can be used to prevent a frontend from selecting
a router that is reserved for other frontend systems. Fail back of routers is
supported. This means that if the preferred router was not available, and it
became available later, the frontend would automatically fail back and connect to
its preferred router.
Router connectivity can also be controlled through the use of the /BALANCE
qualifier either on the CREATE FACILITY command or on the SET FACILITY
command. When the /BALANCE qualifier is used, the list of routers specified
in the router list is randomized, making the preferred router a random selection
within the list. Assume the following command is issued from a frontend:
RTR CREATE FACILITY test/FRONTEND=Z/ROUTER=(A,B,C)
The frontend attempts to select a router based on the priority list A, B, C, with
A being the preferred router. If the /BALANCE qualifier is added to the end
of this command then the preferred router is randomly selected from the three
nodes. This random list exists for the duration of the facility. After the facility
is stopped, a new random list is made when the facility is created again. The
exception to this is if a router does not have quorum (sufficient access to backend
systems) then that router will no longer accept connections from frontend systems
until it has again achieved quorum. The /BALANCE qualifier is only valid for
frontend systems.
2–22 Starting and Setting Up RTR