The supervisor:
-module(mod_guild_chapter_sup).
-include("guild_dungeon.hrl").
-behaviour(supervisor).
%% API
-export([start_link/0]).
%% Supervisor callbacks
-export([init/1]).
-define(SERVER, ?MODULE).
%%%===================================================================
%%% API functions
%%%===================================================================
%%--------------------------------------------------------------------
%% @doc
%% Starts the supervisor
%%
%% @spec start_link() -> {ok, Pid} | ignore | {error, Error}
%% @end
%%--------------------------------------------------------------------
start_link() ->
supervisor:start_link({local, ?SERVER}, ?MODULE, []).
%%%===================================================================
%%% Supervisor callbacks
%%%===================================================================
%%--------------------------------------------------------------------
%% @private
%% @doc
%% Whenever a supervisor is started using supervisor:start_link/[2,3],
%% this function is called by the new process to find out about
%% restart strategy, maximum restart frequency and child
%% specifications.
%%
%% @spec init(Args) -> {ok, {SupFlags, [ChildSpec]}} |
%% ignore |
%% {error, Reason}
%% @end
%%--------------------------------------------------------------------
init([]) ->
RestartStrategy = simple_one_for_one,
MaxRestarts = 1000,
MaxSecondsBetweenRestarts = 3600,
SupFlags = {RestartStrategy, MaxRestarts, MaxSecondsBetweenRestarts},
Restart = transient,
Shutdown = 60000,
Type = worker,
ModGuildChapter = {'guild_chapter', {'mod_guild_chapter', start_link, []},
Restart, Shutdown, Type, ['mod_guild_chapter']},
{ok, {SupFlags, [ModGuildChapter]}}.
The child:
-module(mod_guild_chapter).
-record(state, {}).
start_link(GuildId, ChapterId) ->
gen_server:start_link(?MODULE, [GuildId, ChapterId], []).
init([GuildId, ChapterId]) ->
case condition() of
true -> ignore;
false -> {ok, #state{})
end.
...omit other callbacks...
supervisor:which_children(mod_guild_chapter_sup):
[{undefined,<0.9635.0>,worker,[mod_guild_chapter]},
{undefined,<0.9539.0>,worker,[mod_guild_chapter]},
{undefined,<0.9475.0>,worker,[mod_guild_chapter]},
{undefined,<0.9493.0>,worker,[mod_guild_chapter]},
{undefined,<0.9654.0>,worker,[mod_guild_chapter]},
{undefined,undefined,worker,[mod_guild_chapter]},
{undefined,<0.9658.0>,worker,[mod_guild_chapter]},
{undefined,<0.9517.0>,worker,[mod_guild_chapter]},
{undefined,<0.9567.0>,worker,[mod_guild_chapter]}]
The exception when received shutdown:
2015-07-03 14:56:33 =CRASH REPORT====
crasher:
initial call: mod_guild_chapter:init/1
pid: <0.9475.0>
registered_name: []
exception exit: {{function_clause,[{supervisor,'-monitor_dynamic_children/2-fun-1-',[undefined,[100062,10003],{{set,3,16,16,8,80,48,{[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]},{{[],[],[],[],[],[],[],[<0.9658.0>],[],[],[<0.9517.0>],[],[<0.9567.0>],[],[],[]}}},{dict,0,16,16,8,80,48,{[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]},{{[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]}}}}],[{file,"supervisor.erl"},{line,992}]},{dict,fold_bucket,3,[{file,"dict.erl"},{line,441}]},{dict,fold_seg,4,[{file,"dict.erl"},{line,437}]},{dict,fold_segs,4,[{file,"dict.erl"},{line,433}]},{supervisor,terminate_dynamic_children,3,[{file,"supervisor.erl"},{line,959}]},{gen_server,terminate,6,[{file,"gen_server.erl"},{line,719}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,239}]}]},[{gen_server,terminate,6,[{file,"gen_server.erl"},{line,744}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,239}]}]}
ancestors: [mod_guild_chapter_sup,mod_guild_sup,yg_sup,<0.82.0>]
messages: []
links: []
dictionary: []
trap_exit: true
status: running
heap_size: 1598
stack_size: 27
reductions: 6586
neighbours:
As you can see, there is an undefined pid which should not be there.
There are two places in the document explaining this but have conflicts between them.
- The child specification will be kept by the supervisor if the child is not
temporaryeven if it returnsignore.
start defines the function call used to start the child process. It must be a module-function-arguments tuple {M,F,A} used as apply(M,F,A).
The start function must create and link to the child process, and must return {ok,Child} or {ok,Child,Info} where Child is the pid of the child process and Info an arbitrary term which is ignored by the supervisor.
The start function can also return ignore if the child process for some reason cannot be started, in which case the child specification will be kept by the supervisor (unless it is a temporary child) but the non-existing child process will be ignored.
simple_one_for_onepid will not added to the supervisor if returnsignorewhen start.
If the child process start function returns ignore, the child specification is added to the supervisor (unless the supervisor is a simple_one_for_one supervisor, see below), the pid is set to undefined and the function returns {ok,undefined}.
In the case of a simple_one_for_one supervisor, when a child process start function returns ignore the functions returns {ok,undefined} and no child is added to the supervisor.
I got confused by the document. I choose transient restart strategy because the child should restart when crashing. But how can I avoid this exception here?
