After this change, the load balancer can make progress with active
migrations. If the algorithm is called with active tablet migrations
in tablet metadata, those are treated by load balancer as if they were
already completed. This allows the algorithm to incrementally make
decision which when executed with active migrations will produce the
desired result.
Overload of shards is limited by the fact that the algorithm tracks
streaming concurrency on both source and target shards of active
migrations and takes concurrency limit into account when producing new
migrations.
The coordinator executes the load balancer on edges of tablet state
machine stransitions. This allows new migrations to be started as soon
as tablets finish streaming.
The load balancer is also continuously invoked as long as it produces
a non-empty plan. This is in order to saturate the cluster with
streaming. A single make_plan() call is still not saturating, due
to the way algorithm is implemented.
Identifies tablet in the scope of the whole cluster. Not to be
confused with tablet replicas, which all share global_tablet_id.
Will be needed by load balancer and tablet migration algorithm to
identify tablets globally.
It's needed to implement tablet migration. It stores the current step
of tablet migration state machine. The state machine will be advanced
by the topology change coordinator.
See the "Tablet migration" section of topology-over-raft.md
We add the has_pending_ranges function to erm. The
implementation for vnode is similar to that of token_metadata.
For tablets, we add new code that checks if the given endpoint
is contained in tablet_map::_transitions.
