Improve autocluster

doc/classy.texi

@@ -106,6 +106,14 @@ The document was typeset with
     Site is fully connected and operational.
   @end enumerate
 
+  Unless the node is stopped abruptly
+  (@code{erlang:halt}, @code{kill}, power loss),
+  run levels always advance and retard in sequence.
+
+  Business applications can track the run level via @ref{classy:run_level/2} hook.
+  This hook can, for example,
+  activate or deactivate some business applications.
+
   @node Network Partition
   @section Network Partitions
 
@@ -123,14 +131,19 @@ The document was typeset with
   Peer discovery is done with a help of several modules implementing
   @ref{api/classy/classy_discovery_strategy, discovery_strategy} behavior.
 
-  Active strategy is selected by @ref{discovery_strategy} OTP application environment variable.
+  The strategy is configured via @ref{discovery_strategy} OTP application environment variable.
   This variable has a form of 2-tuple with the following elements:
 
   @enumerate
-    @item Strategy
-    @item Strategy-specific parameters
+    @item Discovery method, such as @code{static}
+    @item Method-specific parameters
   @end enumerate
 
+  New methods can be registered using @ref{classy_discovery_strategy:hook/2} function.
+
+  Additionally, there is a hook that allows to filter and prioritize discovered nodes:
+  @ref{classy:pre_autocluster/2}.
+
   Autocluster logic runs only on singleton sites.
   It is inactive on sites that already have peers (up or down).
 

src/classy_autocluster.erl

@@ -245,20 +245,12 @@ wakeup(After, S = #s{t = T0}) ->
 
 with_strategy(Fun) ->
   case classy_discovery_strategy:get() of
-    {manual, _} ->
-      ignore;
-    {singleton, _} ->
-      ignore;
+    {Module, Options} ->
+      Fun(Module, Options);
     undefined ->
-      ignore;
-    {Strategy, Options} ->
-      Fun(strategy_module(Strategy), Options)
+      ignore
   end.
 
--spec strategy_module(atom()) -> module().
-strategy_module(Strategy) ->
-  list_to_atom("classy_discovery_" ++ atom_to_list(Strategy)).
-
 -spec discovery_interval() -> pos_integer().
 discovery_interval() ->
   application:get_env(classy, discovery_interval, 5_000).

src/classy_builtin_hooks.erl

@@ -83,13 +83,16 @@ log_run_level(From, To) ->
        , to => To
        }).
 
-log_peer_connection_change(_Cluster, _Local, Remote, Node, true) ->
-  ?tp(notice, classy_peer_connected,
-      #{ remote => Remote
-       , node   => Node
-       });
-log_peer_connection_change(_Cluster, _Local, Remote, Node, false) ->
-  ?tp(notice, classy_peer_disconnected,
+log_peer_connection_change(_Cluster, Local, Remote, Node, ConnStatus) ->
+  Kind = case ConnStatus of
+           true -> classy_peer_connected;
+           false -> classy_peer_disconnected
+         end,
+  Level = case Remote of
+            Local -> debug;
+            _     -> notice
+          end,
+  ?tp(Level, Kind,
       #{ remote => Remote
        , node   => Node
        }).

src/classy_discovery_strategy.erl

@@ -19,6 +19,7 @@
 This module defines a behavior of discovery strategy.
 """.
 
+-export([hook/2]).
 -export([get/0, discover/2, lock/2, unlock/2, register/2, unregister/2]).
 
 -export_type([options/0, t/0]).
@@ -43,15 +44,40 @@ This module defines a behavior of discovery strategy.
 
 -callback unregister(options()) -> ok | ignore | {error, term()}.
 
+-optional_callbacks([lock/1, unlock/1, register/1, unregister/1]).
+
+-define(hook, ?MODULE).
+
 %%================================================================================
 %% API functions
 %%================================================================================
 
 -doc """
-Read value of @ref{discovery_strategy} environment variable (with default).
+Register a discovery strategy.
+
+Callbacks registered here match on the @ref{discovery_strategy} configuration
+and return callback module implementing @code{classy_discovery_strategy} behavior.
+
+The first hook that returns @code{@{ok, Module@}} wins
+and handles all the callbacks for the next discovery cycle.
+""".
+-spec hook(fun(({atom(), options()}) -> {ok, module()} | undefined), classy_hook:prio()) -> classy_hook:hook().
+hook(Fun, Prio) when is_function(Fun, 1), is_number(Prio) ->
+  classy_hook:insert(?hook, Fun, Prio).
+
+-doc """
+Read @ref{discovery_strategy} environment variable and decide which strategy to use.
 """.
+-spec get() -> t() | undefined.
 get() ->
-  application:get_env(classy, discovery_strategy, {manual, []}).
+  Conf = application:get_env(classy, discovery_strategy, {manual, []}),
+  case classy_hook:first_match(?hook, [Conf]) of
+    {ok, Module} ->
+      {_Method, Options} = Conf,
+      {Module, Options};
+    undefined ->
+      undefined
+  end.
 
 %%================================================================================
 %% Internal exports
@@ -65,22 +91,42 @@ discover(Mod, Options) ->
 -doc false.
 -spec lock(module(), options()) -> ok | ignore | {error, term()}.
 lock(Mod, Options) ->
-  safe_call(Mod, ?FUNCTION_NAME, Options).
+  case erlang:function_exported(Mod, ?FUNCTION_NAME, 1) of
+    true ->
+      safe_call(Mod, ?FUNCTION_NAME, Options);
+    false ->
+      ok
+  end.
 
 -doc false.
 -spec unlock(module(), options()) -> ok | ignore | {error, term()}.
 unlock(Mod, Options) ->
-  safe_call(Mod, ?FUNCTION_NAME, Options).
+  case erlang:function_exported(Mod, ?FUNCTION_NAME, 1) of
+    true ->
+      safe_call(Mod, ?FUNCTION_NAME, Options);
+    false ->
+      ok
+  end.
 
 -doc false.
 -spec register(module(), options()) -> ok | ignore | {error, term()}.
 register(Mod, Options) ->
-  safe_call(Mod, ?FUNCTION_NAME, Options).
+  case erlang:function_exported(Mod, ?FUNCTION_NAME, 1) of
+    true ->
+      safe_call(Mod, ?FUNCTION_NAME, Options);
+    false ->
+      ok
+  end.
 
 -doc false.
 -spec unregister(module(), options()) -> ok | ignore | {error, term()}.
 unregister(Mod, Options) ->
-  safe_call(Mod, ?FUNCTION_NAME, Options).
+  case erlang:function_exported(Mod, ?FUNCTION_NAME, 1) of
+    true ->
+      safe_call(Mod, ?FUNCTION_NAME, Options);
+    false ->
+      ok
+  end.
 
 %%================================================================================
 %% Internal functions

src/classy_hook.erl

@@ -65,6 +65,11 @@ init() ->
   classy:on_membership_change(fun classy_builtin_hooks:log_membership_change/4, 100),
   classy:run_level(fun classy_builtin_hooks:log_run_level/2, -100),
   classy:on_peer_connection_change(fun classy_builtin_hooks:log_peer_connection_change/5, 100),
+  %% Discovery strategies:
+  classy_discovery_static:hook(),
+  classy_discovery_dns:hook(),
+  classy_discovery_k8s:hook(),
+  classy_discovery_etcd:hook(),
   %% User initialization:
   case application:get_env(classy, setup_hooks) of
     {ok, {Mod, Func, Args}} ->
@@ -74,7 +79,6 @@ init() ->
       ok
   end.
 
--doc false.
 -spec insert(hookpoint(), fun(), prio()) -> hook().
 insert(Hookpoint, Hook, Prio) when is_atom(Hookpoint), is_integer(Prio), is_function(Hook) ->
   Key = {Hookpoint, -Prio, Hook},

src/classy_internal.hrl

@@ -54,6 +54,7 @@
 -define(classy_vote_part_perform_action, classy_vote_part_perform_action).
 -define(classy_vote_flow_start, classy_vote_flow_start).
 -define(classy_vote_part_established, classy_vote_part_established).
+-define(classy_vote_part_recv_outcome, classy_vote_part_recv_outcome).
 -define(classy_vote_alloc_id, classy_vote_alloc_id).
 -define(classy_vote_part_recv, classy_vote_part_recv).
 -define(classy_vote_part_send_vote, classy_vote_part_send_vote).