A declarative, static supervisor. "Static" isn't an Erlang/OTP supervisor type here: it means the child set is fixed, returned once by init/1, unlike portunus_registry, whose children are added and removed at runtime.

Only one of each child spec returned by init/1 can exist in the cluster at any given time. The elected owner runs it under a local Erlang/OTP supervisor, and portunus drives the cross-node leadership (ownership) transfer.

It is a thin layer built on top of portunus_registry: start_link/3,4 reads the desired children from init/1 once and registers each one, and returns the underlying portunus_registry pid. The fixed child set fixes only what runs, not where it runs, so which_children/1 reports what this node runs and transfer/3 hands a child to a chosen node, the same deliberate rebalancing that dynamic children get. Both take the returned handle, which is a portunus_registry, so its other operations act on it too. The registry owns the local supervisor, so the whole tree shares one lifetime and nothing is leaked or orphaned. Children are namespaced by group (the callback module by default, or #{group => G}), so several declarative supervisors share one cluster without colliding. The callback mirrors supervisor:

-callback init(Args :: term()) ->
    {ok, {supervisor:sup_flags(),
          [portunus_delayed_restart:child_spec_in()]}} |
    ignore.

Child specs are validated at registration: an invalid one aborts start_link with {error, {invalid_child_spec, _}}.