As fleet uses etcd for cluster wide coordination, making it scale requires minimizing the load it puts on etcd. This is true for reads, writes, and watches.
Currently when fleet schedules a job all fleetds are woken up (via a watch)
and then do a recursive GET on the Unit file in etcd to figure out if it should
schedule a job. This is a very expensive operation.
To make fleet scale further in the future we should consider rearchitecting
fleet to provide per-machine schedules (so that only those fleetds are woken
up that actually have work for them). This is akin to the Thundering herd
problem, but in a
distributed fashion. Once such a change is in we can also drop the periodic
wakeups (agent TTLs) that cause fleet wide wake ups on a regular clock.
The above proposed change is a large architectural change. However, scaling fleet to a couple of thousand machine is already possible with a few quick wins:
-
Removing watches from fleet: By removing the watches from fleet we stop the entire cluster from walking up whenever a new job is to be scheduled. The downside of this change is that fleet's responsiveness is lower. Proposal: coreos#1264
-
Disallowing (some) nodes to partake in the fleet leadership election. Again this is an expensive operation. The fewer nodes that are engaged in this election, the better. Possible downside is that if there isn't a leader at all, the cluster is inoperable. However the (usually) 5 machines running etcd are also a single point of failure. See the
--disable-engineflag. -
Making some defaults exported and allow them to be overridden. For instance fleet's tokenLimit controls how many Units are listed per "page". See the
--token-limitflag.