Welcome

Director is a production-ready supervisor and manager for Erlang/Elixir processes that focuses on speed, performance and flexibility. Don't worry about replacing Director with OTP/Supervisor Because a Director process is responsive for all API functions of OTP/Supervisor module and has its own useful API functions too. This is more flexible than OTP/supervisor. Since Director calls callback-function to dealing with process crash, By changing code you can change strategy! To seeing all advantages just read this readme file.

What is a supervisor? (for newbies)

According to the Erlang's manual:  

A supervisor is a process that supervises other processes called child processes. A child process can either be another supervisor or a worker process. Supervisors are used to build a hierarchical process structure called a supervision tree, a nice way to structure a fault-tolerant application.   In Erlang we tell supervisors to start other processes. Every child process has its own options called childspec.

Features

• Useful API functions:  
  • director:get_pid(DirectorRef, ChildId) gives pid of child if child is alive.  
  • director:get_pids(DirectorRef) gives pids of alive children.  
  • director:terminate_and_delete_child(DirectorRef, ChildId) terminates and deletes a child in one request.  
  • director:become_supervisor(DirectorRef, Pid, ChildSpec) makes Director supervisor of an alive process.  
  • director:get_restart_count(DirectorRef, ChildId) gives restart count of child (useful for debug).
  • All functions not listed here.
• Director is an Erlang behaviour and every callback-module of this behaviour should has following callback-functions (See "How to use?" section for detailed explanation):
  • init/1: For initialization. Here you can define children database type, debug mode and childspecs of some children that you want to start them in initialize state.
  • handle_start/4: Will be called after starting each child process.
  • handle_exit/5: Will be called after crashing a child process. This callback-function should tell Director how to deal with process crash. Restart child? Restart it after time interval? Delete child from children? Do nothing? Terminate yourself?
  • handle_terminate/5: Will be called when Director terminates a child process.
  • terminate/2: Will be called for termination of Director itself.
• Use different databases for keeping children. By default Director uses an Erlang list. It has three other modes for keeping children In a map or ETS or Mnesia table.
• A number of Director processes can use one ETS table on same node (sharing ETS table).
• A number of Director processes can use one Mnesia table on cluster of nodes (sharing Mnesia table).
• By sharing table Directors can start, restart, terminate, etc a number of children simultaneously.
• By using ETS or Mnesia you can read children info directly from table using API functions of director_table_ets and director_table_mnesia modules instead of getting them from Director process. For example director_table_ets:get_pids(Tab) or director_table_mnesia:count_children(Tab).
• You can define your own database for keeping children by implementing director_table behaviour. Also some test cases are ready for testing your module.
• Understandable debug output for every operation.

All features not listed here. For more info see Guide and examples. For contributing see CONTRIBUTING.md file.

How to use?

Since Director is an Erlang behaviour; So before explaining its workflow, I'll explain that "What a behaviour is?" for newbies.

In Erlang, a behaviour is a design pattern implemented in a module/library. It provides functionality in a fashion similar to inheritance in object-oriented programming. A number of functions called callback-functions must be defined for each behavior to work in a module called callback-module.

When you want to start a Director process, you should specify your callback-module which has Director's callback-functions defined and exported. In run-time, Director process calls those callback-functions in different states. I'll explain those callback-functions in below.

init/1

For starting a linked Director process, you should call one of the API functions:

director:start_link(Module::module(), InitArg::any()).
director:start_link(Module::module(), InitArg::any(), Opts::director:start_options()).
director:start_link(RegisterName::director:register_name(), Module::module(), InitArg::any()).
director:start_link(RegisterName::director:register_name(), Module::module(), InitArg::any(), Opts::director:start_options()).

For starting an stand-alone Director process, you should call one of the API functions:

director:start(Module::module(), InitArg::any()).
director:start(Module::module(), InitArg::any(), Opts::director:start_options()).
director:start(RegisterName::director:register_name(), Module::module(), InitArg::any()).
director:start(RegisterName::director:register_name(), Module::module(), InitArg::any(), Opts::director:start_options()).

After calling, Director process calls Module:init(InitArg), possible return values of init/1 are:

-type init_return() :: 'ok' % will be {ok, undefined, [], director:default_childspec(), []}
                     | {'ok', director:state()}
                     | {'ok', director:state(), [director:childspec()]|[]}
                     | {'ok', director:state(), [director:childspec()]|[], director:default_childspec()}
                     | {'ok', director:state(), [director:childspec()]|[], director:start_options()}
                     | {'ok', director:state(), [director:childspec()]|[], director:default_childspec(), director:start_options()}
                     | 'ignore'
                     | {'stop', Reason::any()}.

Childspec

A childspec is an Erlang map containing some mandatory and optional keys that belongs to one child process. I will explain these keys below.

• id: This key should be unique for child and can be any erlang term. We will understand usage of this key later. This key is mandatory.
• start: This key should be an mfa() (module, function and its arguments) and is mandatory. Director calls erlang:apply(Mod, Func, Args) using value of this key for starting child. Possible values of this key are:
  • mfa().
  • {module(), Func::atom()}. Will be {module(), Func::atom(), []}.
  • module(). Will be {module(), start_link, []}.
• state: This key is optional and if not defined, its default value will be atom undefined. This is state data of child process inside Director process. We will understand usage of this key later.
• type: A child process can be a worker or another supervisor. This key is optional and default value is atom worker and possible values are:
  • supervisor.
  • sup (Short for supervisor).
  • worker.
  • w (Short for worker).
  • s (Short for supervisor).
• terminate_timeout: When one Director process is terminating or when you call director:terminate_child/2 or director:terminate_and_delete_child/2 for a child, It terminates its alive children or that child using erlang:exit(ChildPid, Reason). Possible values are atom infinity and 0 and positive integers. When value is 0, Director calls erlang:exit(ChildPid, kill) otherwise when this value is X, it calls erlang:exit(ChildPid, shutdown) and waits X milli-second for termination. If child did not terminate after X, It calls erlang:exit(ChildPid, kill). This key is optional and if key type defined to worker its default value will be 5000 otherwise infinity.
• delete: When Director process is in termination state, It deletes all children from its database (from list, ETS, Mnesia, etc). When you have some Director processes and they are using one ETS, Mnesia, etc table for keeping their children (Using shared table), If one Director stops, You can restart its children from another Director which has access to that table for every child that has this key => false in its childspec. Default is true.
• modules: This key is optional and its default value will be 1st element of its start mfa. Possible values are:
  • [module()].
  • dynamic (Where name of callback module will determine in future. for example in gen_event process).
• append: What is usage of director:default_childspec() in above? If you want to have a number of children with similar childspec options, you can define a default childspec in return value of init/1 and all children with append => true in their childspec will combine with that default childspec. Default childspec is like normal childspec except that this has not id and append keys. Default default childspec is:

   #{terminate_timeout => 0, modules => [], type => worker, delete => true, state => undefined}

Start options

You can define start options in two places, in calling director:start/2-4 or director:start_link/2-4 and in return value of init/1. Start options is an Erlang proplist with following items:

• db: Should be another erlang proplist with following items:
  • table: Default is list. Can be one of list, ets or mnesia. If you defined new table x,name your module director_table_x and define value of table to x.
  • init_arg: If you are using list for keeping children, Its value not matters. for mnesia and ets must be table name. For new database callback module if this value not defined, atom undefined will be given to callback-function director_table_x:create/1 and if defined, argument {value, Value} will be given.
  • delete: Should be one of true or false. If defined to true. In termination of Director itself, It deletes the table. Default is true.
• debug: Standard OTP/sys debug options:

   -type dbg_opts() :: [dbg_opt()] | [].
   -type  dbg_opt() :: {'trace', 'true'}
                     | {'log',
                        {N :: non_neg_integer(),
                         [{Event :: system_event(),
                           FuncState :: _,
                           FormFunc :: format_fun()}]}}
                     | {'statistics', {file:date_time(),
                                       {'reductions', non_neg_integer()},
                                       MessagesIn :: non_neg_integer(),
                                       MessagesOut :: non_neg_integer()}}
                     | {'log_to_file', file:io_device()}
                     | {Func :: dbg_fun(), FuncState :: term()}.

  Default is nothing or []. For more info see OTP sys module.
• spawn_opt:: List of spawn options. For more info see OTP type proc_lib:spawn_option(). You cannot use this option in return value of init/1.
• timeout: How much time process needs for initialization? You cannot use this option in return value of init/1.

init/1 examples

-module(director_test).
...
-export([init/1]). % Do not forget to export it
...
-record(state, {}). %% State record for Director itself
-record(chstate, {}). %% State record for Director children

init(MyInitArg) ->
	Child_1 = #{id => child_1
	           ,start => {child_1_module, start_link, [MyInitArg]}
	           ,state => #chstate{}
	           ,terminate_timeout => 1000},
	Child_2 = #{id => child_2
	           ,start => {child_2_module, start_link, [MyInitArg, arg_2, arg_3]}
	           ,state => #chstate{}},
	{ok, #state{}, [Child_1, Child2], [{db, [{table, ets}, {init_arg, my_table}]}]}.
	%% In above if you want some children with similar childspecs, you can use default childspec:
%	Children = [#{id => Id, append => true} || _ <- lists:seq(1, 100)],
%	DefChildSpec = #{start => {foo, start, [arg_1, arg_2]}},
%	{ok, #state{}, Children, DefChildSpec}.
	%% You will have 100 childspecs with ids 1-100 and start {foo, start, [arg_1, arg_2]}
	
	%% If you want simple_one_for_one strategy of OTP/Supervisor:
%	Children = [#{id => Id, append => true, start => {foo, start, [arg_3]}} || _ <- lists:seq(1, 100)],
%	DefChildSpec = #{start => {foo, start, [arg_1, arg_2]}},
%	{ok, #state{}, Children, DefChildSpec}.
	%% You will have 100 childspecs with ids 1-100 and start {foo, start, [arg_1, arg_2, arg_3]}
...

handle_start/4

When Director starts a child process, It calls:

YourCallbackModule:handle_start(ChildId, ChildState, DirectorState, Metadata)

In above example when Director starts Child_1, It calls:

director_test:handle_start(child_1, #chstate{}, #state{}, #{restart_count := 0, pid := PidOfChild_1})

This callback-function should yield:

-type handle_start_return() :: {'ok', NewChildState::director:child_state(), NewDirectorState::director:state(), director:callback_return_options()}
                             | {'stop', director:child_state(), Reason::any(), director:callback_return_options()}.

Example of handle_start/4 which just tells Director to don't call error_logger about starting any child:

handle_start(_, ChState, State, _) ->
	{ok, ChState, State, [{log, false}]}.

handle_exit/5

When a child process crashes, Its Director will receive child's exit signal and calls:

YourCallbackModule:handle_exit(ChildId, ChildState, ReasonOfChildTermination, DirectorState, MetaData)

In above example when Child_1 exits with reason oops, Director calls:

director_test:handle_exit(child_1, #chstate{}, oops, #state{}, #{restart_count := 1})

This callback-function should yield:

-type handle_exit_return() :: {director:action(), director:child_state(), director:state(), director:callback_return_options()}.
-type  action() :: 'restart'
                 | {'restart', pos_integer()}
                 | 'delete'
                 | 'wait'
                 | 'stop'
                 | {'stop', Reason::any()}.

Example of handle_exit/5 which tells Director to restart child after 1000 milli-seconds:

handle_exit(_, ChState, _, State, _) ->
	{{restart, 1000}, ChState, State, []}.

If you define delete as action, Child will be removed from children table. If you define wait as action, Director does nothing and you have to call director:restart_child(DirectorProc, ChildId) for restarting child. If you define stop, Director will terminate itself with error reason of child crash.
What if you define restart or {restart, Int} and child does not restart? Director will restart child again, So calls handle_exit/5 again which its metadata argument has restart_count key plus one. For example in following code Director will restart child id foo for 5 times, then restarts it after 1000 milli-seconds for 6th time, and finally terminates itself with reason {max_restart, foo} for 7th time:

handle_exit(foo, ChState, _Reason, State, #{restart_count := RC}) when RC < 6 ->
	{restart, ChState, State, []};
handle_exit(foo, ChState, _Reason, State, #{restart_count := 6}) ->
	{{restart, 1000}, ChState, State, []};
handle_exit(foo, ChState, _Reason, State, _) ->
	{{stop, {max_restart, foo}}, ChState, State, []}.

handle_terminate/5

When you call director:terminate_child/2-3 or director:delete_and_terminate_child/2-3, Director terminates child process and calls:

YourCallbackModule:handle_terminate(ChildId, ChildState, ReasonOfChildTermination::shutdown|kill|term(), DirectorState, MetaData)

Also it calls handle_terminate/5 when it is in terminate state and is terminating its alive children.
For above example when you call director:terminate_child(DirectorProc, child_2), It calls:

director_test:handle_exit(child_2, #chstate{}, shutdown, #state{}, #{restart_count := 0})

This callback-function should yield:

-type handle_terminate_return() :: {'ok', director:child_state(), director:state(), director:callback_return_options()}.

For example following code tells Director don't call error_logger for termination of child id bar and call it for other children:

handle_terminate(bar, ChildState, _, DirectorState, _) ->
	{ok, ChildState, DirectorState, [{log, false}]};
handle_terminate(_, ChildState, _, DirectorState, _) ->
	{ok, ChildState, DirectorState, []}. % Default log value is true

terminate/2

When director is terminating itself, after terminating its alive children, It calls:

YourCallbackModule:terminate(ReasonOfTermination, DirectorState)

For above example if Director is terminating with reason normal, it calls:

director_test:terminate(normal, #state{})

This callback-function should yield:

-type terminate_return() :: {'ok', callback_return_options()}
                          | {'new_error', NewReason::any(), callback_return_options()}
                          | any().

For example following code tells Director to change crash reason oops to normal and do not call error_logger about terminating yourself:

terminate(oops, State) ->
	{new_error, normal, [{log, false}]}.

Anything other than {ok, Opts}and{new_error, _, Opts}causes **Director** to callerror_logger` and exit with its crash reason.

Build

Compile

~/director $ make
===> Verifying dependencies...
===> Compiling director

Use as dependency

Rebar3

Put this in deps in rebar.config:

{director, "18.4.29"}

Rebar

Put this in deps in rebar.config:

{director, ".*", {git, "https://github.com/Pouriya-Jahanbakhsh/director.git", {tag, "18.4.29"}}}

Mix

Put this in deps in mix.exs:

{:director, "~> 18.4.29"}

erlang.mk

dep_director = hex 18.4.29

API documentation

 /projects/director $ make doc
===> Verifying dependencies...
===> Fetching edown ({pkg,"edown","0.8.1"})
===> Compiling edown
===> Compiling director
===> Running edoc for director

 /projects/director $ ls doc/ | grep .md
director.md
director_table_ets.md
director_table_mnesia.md
README.md

Todo

• Add test for having something like OTP/Supervisor simple_one_for_one strategy.
• Add complete examples.
• Add documentation for writing new director_table behaviour based module.

License

BSD 3-Clause

Author

pouriya.jahanbakhsh@gmail.com

Hex version

18.4.29