elector

%%%-----------------------------------------------------------------------------
%% @doc Defines the base behaviour for election strategies.
%% @end
%%%-----------------------------------------------------------------------------
-module(elector_strategy_behaviour).

%%------------------------------------------------------------------------------
%% Callbacks
%%------------------------------------------------------------------------------
-callback elect(CandidateNodes :: candidate_nodes()) -> Leader :: leader().

%%------------------------------------------------------------------------------
%% Exported API
%%------------------------------------------------------------------------------
-export([elect/0, candidate_nodes/0, is_local_candidate/0]).

%%------------------------------------------------------------------------------
%% Type definitions
%%------------------------------------------------------------------------------
-type leader() :: node().
-type candidate_nodes() :: [node()].

%%------------------------------------------------------------------------------
%% Exported functions
%%------------------------------------------------------------------------------
%% @doc Starts the election process by triggering the strategy
%% modules elect() function.
%% The election implementation should only contain the logic for
%% selecting the leader node and returning the leader node name.
%% The elect/0 function is triggered on all nodes automatically
%% by the elector application. This means the strategy implementation
%% does not have to worry about starting the election on all nodes.
%% @end
-spec elect() -> Leader :: leader().
elect() ->
    CandidateNodes = candidate_nodes(),
    erlang:apply(
        elector_config_handler:strategy_module(), elect, [CandidateNodes]
    ).

%% @doc Returns the candidate nodes for the next election.
%%
%% Queries each connected node live (in parallel) for its current candidate
%% status. The async candidate cache races with `candidate_status'
%% propagation during cluster join — a node electing against an unpopulated
%% cache self-elects and the wrong leader can stick — so we ask the source
%% of truth on every election instead.
%% @end
-spec candidate_nodes() -> CandidateNodes :: candidate_nodes().
candidate_nodes() ->
    Nodes = [node() | nodes()],
    Results = erpc:multicall(Nodes, ?MODULE, is_local_candidate, [], 500),
    [Node || {Node, {ok, true}} <- lists:zip(Nodes, Results)].

%% @doc Returns whether the local node is currently a candidate. Exported so
%% remote nodes can query it via erpc from `candidate_nodes/0'.
%% @end
-spec is_local_candidate() -> boolean().
is_local_candidate() ->
    case erlang:whereis(elector_candidate) of
        undefined -> false;
        Pid ->
            try gen_server:call(Pid, is_candidate_node, 300) of
                {ok, true} -> true;
                _ -> false
            catch
                _:_ -> false
            end
    end.