-spec
extra_sync_targets(fun((cluster_id()) -> [node()])) -> classy_hook:hook().
Register a callback that returns a list of extra CRDT sync targets for a cluster. This is needed primarily for migrations.
In absence of this hook, classy syncs the cluster state with all nodes that are known to have previously hosted member sites. This hook allows to expand the list of nodes.
WARNING: this hook has performance implications. Currently it triggers a synchronous RPC on the discovered nodes, and if some of them are on very slow network, then the CRDT process will get blocked. Hence, hook implementation should not return extra sync targets unless really needed.