4.1.16 fallback_get_cluster(Hook) ¶

-spec fallback_get_cluster(fun((node()) -> {ok,
                                            cluster_id()} |
                                           undefined)) -> classy_hook:hook().

Fallback: derive cluster ID of a remote node that doesn’t run classy.

WARNING: Fallback mechanism is meant to support rolling cluster upgrade. This functionality is inherently dangerous, but it won’t activate unless the user registers all fallback hooks.

The following limitations apply:

1. The hook should return a value that cannot change, even after the node is upgraded.
2. All peer nodes of this one should return the same cluster ID.
3. At most one fallback hook should be registered for this hookpoint. Registering more than one hook is dangerous, since transient errors in the hook may lead to inconsistent results between the calls.