Is this why in an HA setup I would see a rack controller spewing:
2017-09-21 15:27:29 provisioningserver.rpc.clusterservice: [info] Rack controller 'sfyeaw' registered (via infra2:pid=5575).
2017-09-21 15:27:29 provisioningserver.rpc.clusterservice: [info] Rack controller 'sfyeaw' registered (via infra2:pid=5612).
2017-09-21 15:27:30 provisioningserver.rpc.clusterservice: [info] Rack controller 'sfyeaw' registered (via infra2:pid=5612).
2017-09-21 15:27:30 provisioningserver.rpc.clusterservice: [info] Rack controller 'sfyeaw' registered (via infra2:pid=5328).
2017-09-21 15:27:31 provisioningserver.rpc.clusterservice: [info] Rack controller 'sfyeaw' registered (via infra2:pid=5328).
2017-09-21 15:27:31 provisioningserver.rpc.clusterservice: [info] Rack controller 'sfyeaw' registered (via infra2:pid=5328).
2017-09-21 15:27:31 provisioningserver.rpc.clusterservice: [info] Rack controller 'sfyeaw' registered (via infra2:pid=5328).
2017-09-21 15:27:31 provisioningserver.rpc.clusterservice: [info] Rack controller 'sfyeaw' registered (via infra2:pid=5328).
2017-09-21 15:27:31 provisioningserver.rpc.clusterservice: [info] Rack controller 'sfyeaw' registered (via infra2:pid=5328).
2017-09-21 15:27:31 provisioningserver.rpc.clusterservice: [info] Rack controller 'sfyeaw' registered (via infra2:pid=5328).
2017-09-21 15:27:31 provisioningserver.rpc.clusterservice: [info] Rack controller 'sfyeaw' registered (via infra2:pid=5612).
2017-09-21 15:27:32 provisioningserver.rpc.clusterservice: [info] Rack controller 'sfyeaw' registered (via infra2:pid=5328).
2017-09-21 15:27:32 provisioningserver.rpc.clusterservice: [info] Rack controller 'sfyeaw' registered (via infra2:pid=5328).
2017-09-21 15:27:32 provisioningserver.rpc.clusterservice: [info] Rack controller 'sfyeaw' registered (via infra2:pid=5575).
2017-09-21 15:27:33 provisioningserver.rpc.clusterservice: [info] Rack controller 'sfyeaw' registered (via infra2:pid=5612).
2017-09-21 15:27:33 provisioningserver.rpc.clusterservice: [info] Rack controller 'sfyeaw' registered (via infra2:pid=5328).
2017-09-21 15:27:33 provisioningserver.rpc.clusterservice: [info] Rack controller 'sfyeaw' registered (via infra2:pid=5612).
2017-09-21 15:27:33 provisioningserver.rpc.clusterservice: [info] Rack controller 'sfyeaw' registered (via infra2:pid=5328).
2017-09-21 15:27:34 provisioningserver.rpc.clusterservice: [info] Rack controller 'sfyeaw' registered (via infra2:pid=5328).
2017-09-21 15:27:34 provisioningserver.rpc.clusterservice: [info] Rack controller 'sfyeaw' registered (via infra2:pid=5328).
2017-09-21 15:27:34 provisioningserver.rpc.clusterservice: [info] Rack controller 'sfyeaw' registered (via infra2:pid=5328).



And the corresponding region:
2017-09-21 15:27:34 maasserver.rpc.regionservice: [info] Process [5328] - registered rack controller 'sfyeaw'.
2017-09-21 15:27:34 maasserver.rpc.regionservice: [info] Process [5328] - registered rack controller 'sfyeaw'.
2017-09-21 15:27:34 maasserver.rpc.regionservice: [info] Process [5328] - registered rack controller 'sfyeaw'.
2017-09-21 15:27:34 maasserver.rpc.regionservice: [info] Process [5328] - registered rack controller 'sfyeaw'.
2017-09-21 15:27:34 maasserver.rpc.regionservice: [info] Process [5328] - registered rack controller 'sfyeaw'.
2017-09-21 15:27:34 maasserver.rpc.regionservice: [info] Process [5328] - registered rack controller 'sfyeaw'.
2017-09-21 15:27:34 maasserver.rpc.regionservice: [info] Process [5328] - registered rack controller 'sfyeaw'.
2017-09-21 15:27:34 maasserver.rpc.regionservice: [info] Process [5328] - registered rack controller 'sfyeaw'.
2017-09-21 15:27:34 maasserver.rpc.regionservice: [info] Process [5328] - registered rack controller 'sfyeaw'.
2017-09-21 15:27:34 maasserver.region_controller: [info] Successfully configured DNS.
2017-09-21 15:27:34 maasserver.rpc.regionservice: [info] Process [5328] - registered rack controller 'sfyeaw'.
2017-09-21 15:27:34 maasserver.rpc.regionservice: [info] Process [5575] - registered rack controller 'sfyeaw'.
2017-09-21 15:27:34 maasserver.rpc.regionservice: [info] Process [5328] - registered rack controller 'sfyeaw'.
2017-09-21 15:27:35 maasserver.rpc.regionservice: [info] Process [5328] - registered rack controller 'sfyeaw'.
2017-09-21 15:27:35 maasserver.rpc.regionservice: [info] Process [5328] - registered rack controller 'sfyeaw'.
2017-09-21 15:27:35 maasserver.region_controller: [info] Successfully configured DNS.
2017-09-21 15:27:35 maasserver.rpc.regionservice: [info] Process [5575] - registered rack controller 'sfyeaw'.
2017-09-21 15:27:36 maasserver.region_controller: [info] Successfully configured DNS.
2017-09-21 15:27:36 maasserver.rpc.regionservice: [info] Process [5612] - registered rack controller 'sfyeaw'.
2017-09-21 15:27:37 maasserver.region_controller: [info] Successfully configured DNS.


I believe this may be related to https://bugs.launchpad.net/maas/2.2/+bug/1707971 where we are attempting to register too many times causing contention for the rack api endpoint when servicing api calls to maascli/python-libmaas clients. Might that be the case?