search

rabbitmq / ra

A Raft implementation for Erlang and Elixir that strives to be efficient and make it easier to use multiple Raft clusters in a single system.
Other
798 stars 93 forks source link

Allow machine implementations (machine and aux) to be notified of membership changes #408

Closed kjnilsson closed 1 month ago

kjnilsson commented 5 months ago

Currently there is no way for a machine to know what the current cluster membership is.

Add an option to call ra_machine:apply/4 with a new command type, similarly to how the noop command translates into a {machine_version, _, _} command that is dispatched against the machine. So when we apply a $cluster_change command we translate this into a {membership_change, OldMembers, NewMembers} command.

There has to be an opt-in mechanism to avoid this becoming a breaking change as current machine implementations may not handle unknown commands.

ra_machine:supports_feature(FeatureName :: atom())

where the feature name for this would be member_change_command

It may be necessary to inject a command at some point when a cluster is started with an initial member set else there would be no way to know the initial members unless they changed as there is no $cluster_change command for initial members.

kjnilsson commented 1 month ago

With the new handle_aux/5 API it is possible to detect member changes by checking the ra_aux:member_info/1 for each handle_aux(eval, event and comparing that to a stashed copy. Closing.