View Source supervisor behaviour (stdlib v6.0)

Generic supervisor behavior.

This behavior module provides a supervisor, a process that supervises other processes called child processes. A child process can either be another supervisor or a worker process. Worker processes are normally implemented using one of the gen_event, gen_server, or gen_statem behaviors. A supervisor implemented using this module has a standard set of interface functions and includes functionality for tracing and error reporting. Supervisors are used to build a hierarchical process structure called a supervision tree, a nice way to structure a fault-tolerant application. For more information, see Supervisor Behaviour in OTP Design Principles.

A supervisor expects the definition of which child processes to supervise to be specified in a callback module exporting a predefined set of functions.

Unless otherwise stated, all functions in this module fail if the specified supervisor does not exist or if bad arguments are specified.

Supervision Principles

The supervisor is responsible for starting, stopping, and monitoring its child processes. The basic idea of a supervisor is that it must keep its child processes alive by restarting them when necessary.

The children of a supervisor are defined as a list of child specifications. When the supervisor is started, the child processes are started in order from left to right according to this list. When the supervisor is going to terminate, it first terminates its child processes in reversed start order, from right to left.

Supervisor flags

The supervisor properties are defined by the supervisor flags. The type definition for the supervisor flags is as follows:

sup_flags() = #{strategy => strategy(),           % optional
                intensity => non_neg_integer(),   % optional
                period => pos_integer(),          % optional
                auto_shutdown => auto_shutdown()} % optional

Restart Strategies

A supervisor can have one of the following restart strategies specified with the strategy key in the above map:

  • one_for_one - If one child process terminates and is to be restarted, only that child process is affected. This is the default restart strategy.

  • one_for_all - If one child process terminates and is to be restarted, all other child processes are terminated and then all child processes are restarted.

  • rest_for_one - If one child process terminates and is to be restarted, the 'rest' of the child processes (that is, the child processes after the terminated child process in the start order) are terminated. Then the terminated child process and all child processes after it are restarted.

  • simple_one_for_one - A simplified one_for_one supervisor, where all child processes are dynamically added instances of the same process type, that is, running the same code.

    Functions delete_child/2 and restart_child/2 are invalid for simple_one_for_one supervisors and return {error,simple_one_for_one} if the specified supervisor uses this restart strategy.

    Function terminate_child/2 can be used for children under simple_one_for_one supervisors by specifying the child's pid/0 as the second argument. If instead the child specification identifier is used, terminate_child/2 return {error,simple_one_for_one}.

    As a simple_one_for_one supervisor can have many children, it shuts them all down asynchronously. This means that the children do their cleanup in parallel, and therefore the order in which they are stopped is not defined.

Restart intensity and period

To prevent a supervisor from getting into an infinite loop of child process terminations and restarts, a maximum restart intensity is defined using two integer values specified with keys intensity and period in the above map. Assuming the values MaxR for intensity and MaxT for period, then, if more than MaxR restarts occur within MaxT seconds, the supervisor terminates all child processes and then itself. The termination reason for the supervisor itself in that case will be shutdown. intensity defaults to 1 and period defaults to 5.

Automatic Shutdown

A supervisor can be configured to automatically shut itself down with exit reason shutdown when significant children terminate with the auto_shutdown key in the above map:

  • never - Automic shutdown is disabled. This is the default setting.

    With auto_shutdown set to never, child specs with the significant flag set to true are considered invalid and will be rejected.

  • any_significant - The supervisor will shut itself down when any significant child terminates, that is, when a transient significant child terminates normally or when a temporary significant child terminates normally or abnormally.

  • all_significant - The supervisor will shut itself down when all significant children have terminated, that is, when the last active significant child terminates. The same rules as for any_significant apply.

For more information, see the section Automatic Shutdown in Supervisor Behavior in OTP Design Principles.

Warning

The automatic shutdown feature appeared in OTP 24.0, but applications using this feature will also compile and run with older OTP versions.

However, such applications, when compiled with an OTP version that predates the appearance of the automatic shutdown feature, will leak processes because the automatic shutdowns they rely on will not happen.

It is up to implementors to take proper precautions if they expect that their applications may be compiled with older OTP versions.

Child specification

The type definition of a child specification is as follows:

child_spec() = #{id => child_id(),             % mandatory
                 start => mfargs(),            % mandatory
                 restart => restart(),         % optional
                 significant => significant(), % optional
                 shutdown => shutdown(),       % optional
                 type => worker(),             % optional
                 modules => modules()}         % optional

The old tuple format is kept for backwards compatibility, see child_spec/0, but the map is preferred.

  • id is used to identify the child specification internally by the supervisor.

    The id key is mandatory.

    Notice that this identifier on occations has been called "name". As far as possible, the terms "identifier" or "id" are now used but to keep backward compatibility, some occurences of "name" can still be found, for example in error messages.

  • 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 any term that 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 is kept by the supervisor (unless it is a temporary child) but the non-existing child process is ignored.

    If something goes wrong, the function can also return an error tuple {error,Error}.

    Notice that the start_link functions of the different behavior modules fulfill the above requirements.

    The start key is mandatory.

  • restart defines when a terminated child process must be restarted. A permanent child process is always restarted. A temporary child process is never restarted (even when the supervisor's restart strategy is rest_for_one or one_for_all and a sibling's death causes the temporary process to be terminated). A transient child process is restarted only if it terminates abnormally, that is, with another exit reason than normal, shutdown, or {shutdown,Term}.

    The restart key is optional. If it is not specified, it defaults to permanent.

  • significant defines if a child is considered significant for automatic self-shutdown of the supervisor.

    Setting this option to true when the restart type is permanent is invalid. Also, it is considered invalid to start children with this option set to true in a supervisor when the auto_shutdown supervisor flag is set to never.

    The significant key is optional. If it is not specified, it defaults to false.

  • shutdown defines how a child process must be terminated. brutal_kill means that the child process is unconditionally terminated using exit(Child,kill). An integer time-out value means that the supervisor tells the child process to terminate by calling exit(Child,shutdown) and then wait for an exit signal with reason shutdown back from the child process. If no exit signal is received within the specified number of milliseconds, the child process is unconditionally terminated using exit(Child,kill).

    If the child process is another supervisor, the shutdown time must be set to infinity to give the subtree ample time to shut down.

    Warning

    Setting the shutdown time to anything other than infinity for a child of type supervisor can cause a race condition where the child in question unlinks its own children, but fails to terminate them before it is killed.

    It is also allowed to set it to infinity, if the child process is a worker.

    Warning

    Be careful when setting the shutdown time to infinity when the child process is a worker. Because, in this situation, the termination of the supervision tree depends on the child process, it must be implemented in a safe way and its cleanup procedure must always return.

    Notice that all child processes implemented using the standard OTP behavior modules automatically adhere to the shutdown protocol.

    The shutdown key is optional. If it is not specified, it defaults to 5000 if the child is of type worker and it defaults to infinity if the child is of type supervisor.

  • type specifies if the child process is a supervisor or a worker.

    The type key is optional. If it is not specified, it defaults to worker.

  • modules is used by the release handler during code replacement to determine which processes are using a certain module. As a rule of thumb, if the child process is a supervisor, gen_server or, gen_statem, this is to be a list with one element [Module], where Module is the callback module. If the child process is an event manager (gen_event) with a dynamic set of callback modules, value dynamic must be used. For more information about release handling, see Release Handling in OTP Design Principles.

    The modules key is optional. If it is not specified, it defaults to [M], where M comes from the child's start {M,F,A}.

  • Internally, the supervisor also keeps track of the pid Child of the child process, or undefined if no pid exists.

See Also

gen_event, gen_statem, gen_server, sys

Summary

Types

Not a pid/0.

The tuple format is kept for backward compatibility only. A map is preferred; see more details above.

Value undefined for A (the argument list) is only to be used internally in supervisor. If the restart type of the child is temporary, the process is never to be restarted and therefore there is no need to store the real argument list. Value undefined is then stored instead.

The tuple format is kept for backward compatibility only. A map is preferred; see more details above.

Name specification to use when starting a supervisor. See function start_link/2,3 and the type sup_ref/0 below.

Supervisor specification to use when addressing a supervisor. See count_children/1, delete_child/2, get_childspec/2, restart_child/2, start_child/2, terminate_child/2, which_children/1 and the type sup_name/0 above.

Callbacks

Whenever a supervisor is started using start_link/2,3, this function is called by the new process to find out about restart strategy, maximum restart intensity, and child specifications.

Functions

Takes a list of child specification as argument and returns ok if all of them are syntactically correct, otherwise {error,Error}.

Returns a property list containing the counts for each of the following elements of the supervisor's child specifications and managed processes

Tells supervisor SupRef to delete the child specification identified by Id. The corresponding child process must not be running. Use terminate_child/2 to terminate it.

Returns the child specification map for the child identified by Id under supervisor SupRef. The returned map contains all keys, both mandatory and optional.

Tells supervisor SupRef to restart a child process corresponding to the child specification identified by Id. The child specification must exist, and the corresponding child process must not be running.

Dynamically adds a child specification to supervisor SupRef, which starts the corresponding child process.

Creates a nameless supervisor process as part of a supervision tree.

Creates a supervisor process as part of a supervision tree.

Tells supervisor SupRef to terminate the specified child.

Returns a newly created list with information about all child specifications and child processes belonging to supervisor SupRef.

Types

Link to this type

auto_shutdown()

View Source (not exported)
-type auto_shutdown() :: never | any_significant | all_significant.
Link to this type

child()

View Source (not exported)
-type child() :: undefined | pid().
Link to this type

child_id()

View Source (not exported)
-type child_id() :: term().

Not a pid/0.

Link to this type

child_rec()

View Source (not exported)
-type child_rec() ::
    #child{pid :: child() | {restarting, pid() | undefined} | [pid()],
           id :: child_id(),
           mfargs :: mfargs(),
           restart_type :: restart(),
           significant :: significant(),
           shutdown :: shutdown(),
           child_type :: worker(),
           modules :: modules()}.
-type child_spec() ::
    #{id := child_id(),
      start := mfargs(),
      restart => restart(),
      significant => significant(),
      shutdown => shutdown(),
      type => worker(),
      modules => modules()} |
    {Id :: child_id(),
     StartFunc :: mfargs(),
     Restart :: restart(),
     Shutdown :: shutdown(),
     Type :: worker(),
     Modules :: modules()}.

The tuple format is kept for backward compatibility only. A map is preferred; see more details above.

Link to this type

children()

View Source (not exported)
-type children() :: {Ids :: [child_id()], Db :: #{child_id() => child_rec()}}.
Link to this type

mfargs()

View Source (not exported)
-type mfargs() :: {M :: module(), F :: atom(), A :: [term()] | undefined}.

Value undefined for A (the argument list) is only to be used internally in supervisor. If the restart type of the child is temporary, the process is never to be restarted and therefore there is no need to store the real argument list. Value undefined is then stored instead.

Link to this type

modules()

View Source (not exported)
-type modules() :: [module()] | dynamic.
Link to this type

restart()

View Source (not exported)
-type restart() :: permanent | transient | temporary.
Link to this type

shutdown()

View Source (not exported)
-type shutdown() :: brutal_kill | timeout().
Link to this type

significant()

View Source (not exported)
-type significant() :: boolean().
-type startchild_err() :: already_present | {already_started, Child :: child()} | term().
-type startchild_ret() ::
    {ok, Child :: child()} | {ok, Child :: child(), Info :: term()} | {error, startchild_err()}.
-type strategy() :: one_for_all | one_for_one | rest_for_one | simple_one_for_one.
-type sup_flags() ::
    #{strategy => strategy(),
      intensity => non_neg_integer(),
      period => pos_integer(),
      auto_shutdown => auto_shutdown()} |
    {RestartStrategy :: strategy(), Intensity :: non_neg_integer(), Period :: pos_integer()}.

The tuple format is kept for backward compatibility only. A map is preferred; see more details above.

-type sup_name() ::
    {local, Name :: atom()} | {global, Name :: term()} | {via, Module :: module(), Name :: any()}.

Name specification to use when starting a supervisor. See function start_link/2,3 and the type sup_ref/0 below.

  • {local,LocalName} - Register the supervisor locally as LocalName using register/2.

  • {global,GlobalName} - Register the supervisor process id globally as GlobalName using global:register_name/2.

  • {via,RegMod,ViaName} - Register the supervisor process with the registry represented by RegMod. The RegMod callback is to export the functions register_name/2, unregister_name/1, whereis_name/1, and send/2, which are to behave like the corresponding functions in global. Thus, {via,global,GlobalName} is a valid reference equivalent to {global,GlobalName}.

-type sup_ref() ::
    (Name :: atom()) |
    {Name :: atom(), Node :: node()} |
    {global, Name :: term()} |
    {via, Module :: module(), Name :: any()} |
    pid().

Supervisor specification to use when addressing a supervisor. See count_children/1, delete_child/2, get_childspec/2, restart_child/2, start_child/2, terminate_child/2, which_children/1 and the type sup_name/0 above.

It can be:

  • pid/0 - The supervisor's process identifier.

  • LocalName - The supervisor is locally registered as LocalName with register/2.

  • {Name,Node} - The supervisor is locally registered on another node.

  • {global,GlobalName} - The supervisor is globally registered in global.

  • {via,RegMod,ViaName} - The supervisor is registered in an alternative process registry. The registry callback module RegMod is to export functions register_name/2, unregister_name/1, whereis_name/1, and send/2, which are to behave like the corresponding functions in global. Thus, {via,global,GlobalName} is the same as {global,GlobalName}.

Link to this type

worker()

View Source (not exported)
-type worker() :: worker | supervisor.

Callbacks

-callback init(Args :: term()) -> {ok, {SupFlags :: sup_flags(), [ChildSpec :: child_spec()]}} | ignore.

Whenever a supervisor is started using start_link/2,3, this function is called by the new process to find out about restart strategy, maximum restart intensity, and child specifications.

Args is the Args argument provided to the start function.

SupFlags is the supervisor flags defining the restart strategy and maximum restart intensity for the supervisor. [ChildSpec] is a list of valid child specifications defining which child processes the supervisor must start and monitor. See the discussion in section Supervision Principles earlier.

Notice that when the restart strategy is simple_one_for_one, the list of child specifications must be a list with one child specification only. (The child specification identifier is ignored.) No child process is then started during the initialization phase, but all children are assumed to be started dynamically using start_child/2.

The function can also return ignore.

Notice that this function can also be called as a part of a code upgrade procedure. Therefore, the function is not to have any side effects. For more information about code upgrade of supervisors, see section Changing a Supervisor in OTP Design Principles.

Functions

Link to this function

check_childspecs(ChildSpecs)

View Source (since OTP 24.0)
-spec check_childspecs(ChildSpecs) -> Result
                    when ChildSpecs :: [child_spec()], Result :: ok | {error, Error :: term()}.

Equivalent to check_childspecs(ChildSpecs, undefined).

Link to this function

check_childspecs(ChildSpecs, AutoShutdown)

View Source (since OTP 24.0)
-spec check_childspecs(ChildSpecs, AutoShutdown) -> Result
                    when
                        ChildSpecs :: [child_spec()],
                        AutoShutdown :: undefined | auto_shutdown(),
                        Result :: ok | {error, Error :: term()}.

Takes a list of child specification as argument and returns ok if all of them are syntactically correct, otherwise {error,Error}.

If the AutoShutdown argument is not undefined, also checks if the child specifications are allowed for the given auto_shutdown option.

Link to this function

count_children(SupRef)

View Source (since OTP R13B04)
-spec count_children(SupRef) -> PropListOfCounts
                  when
                      SupRef :: sup_ref(),
                      PropListOfCounts :: [Count],
                      Count ::
                          {specs, ChildSpecCount :: non_neg_integer()} |
                          {active, ActiveProcessCount :: non_neg_integer()} |
                          {supervisors, ChildSupervisorCount :: non_neg_integer()} |
                          {workers, ChildWorkerCount :: non_neg_integer()}.

Returns a property list containing the counts for each of the following elements of the supervisor's child specifications and managed processes:

  • specs - The total count of children, dead or alive.
  • active - The count of all actively running child processes managed by this supervisor. For a simple_one_for_one supervisors, no check is done to ensure that each child process is still alive, although the result provided here is likely to be very accurate unless the supervisor is heavily overloaded.
  • supervisors - The count of all children marked as child_type = supervisor in the specification list, regardless if the child process is still alive.
  • workers - The count of all children marked as child_type = worker in the specification list, regardless if the child process is still alive.
Link to this function

delete_child(SupRef, Id)

View Source
-spec delete_child(SupRef, Id) -> Result
                when
                    SupRef :: sup_ref(),
                    Id :: child_id(),
                    Result :: ok | {error, Error},
                    Error :: running | restarting | not_found | simple_one_for_one.

Tells supervisor SupRef to delete the child specification identified by Id. The corresponding child process must not be running. Use terminate_child/2 to terminate it.

If successful, the function returns ok. If the child specification identified by Id exists but the corresponding child process is running or is about to be restarted, the function returns {error,running} or {error,restarting}, respectively. If the child specification identified by Id does not exist, the function returns {error,not_found}.

Link to this function

get_childspec(SupRef, Id)

View Source (since OTP 18.0)
-spec get_childspec(SupRef, Id) -> Result
                 when
                     SupRef :: sup_ref(),
                     Id :: pid() | child_id(),
                     Result :: {ok, child_spec()} | {error, Error},
                     Error :: not_found.

Returns the child specification map for the child identified by Id under supervisor SupRef. The returned map contains all keys, both mandatory and optional.

Link to this function

restart_child(SupRef, Id)

View Source
-spec restart_child(SupRef, Id) -> Result
                 when
                     SupRef :: sup_ref(),
                     Id :: child_id(),
                     Result ::
                         {ok, Child :: child()} |
                         {ok, Child :: child(), Info :: term()} |
                         {error, Error},
                     Error :: running | restarting | not_found | simple_one_for_one | term().

Tells supervisor SupRef to restart a child process corresponding to the child specification identified by Id. The child specification must exist, and the corresponding child process must not be running.

Notice that for temporary children, the child specification is automatically deleted when the child terminates; thus, it is not possible to restart such children.

If the child specification identified by Id does not exist, the function returns {error,not_found}. If the child specification exists but the corresponding process is already running, the function returns {error,running}.

If the child process start function returns {ok,Child} or {ok,Child,Info}, the pid is added to the supervisor and the function returns the same value.

If the child process start function returns ignore, the pid remains set to undefined and the function returns {ok,undefined}.

If the child process start function returns an error tuple or an erroneous value, or if it fails, the function returns {error,Error}, where Error is a term containing information about the error.

Link to this function

start_child(Supervisor, ChildSpecOrExtraArgs)

View Source
-spec start_child(SupRef, ChildSpec) -> startchild_ret()
               when SupRef :: sup_ref(), ChildSpec :: child_spec();
           (SupRef, ExtraArgs) -> startchild_ret() when SupRef :: sup_ref(), ExtraArgs :: [term()].

Dynamically adds a child specification to supervisor SupRef, which starts the corresponding child process.

For one_for_one, one_for_all and rest_for_one supervisors, the second argument must be a valid child specification ChildSpec. The child process is started by using the start function as defined in the child specification.

For simple_one_for_one supervisors, the child specification defined in Module:init/1 is used, and the second argument must instead be an arbitrary list of terms ExtraArgs. The child process is then started by appending ExtraArgs to the existing start function arguments, that is, by calling apply(M, F, A++ExtraArgs), where {M,F,A} is the start function defined in the child specification.

  • If there already exists a child specification with the specified identifier, ChildSpec is discarded, and the function returns {error,already_present} or {error,{already_started,Child}}, depending on if the corresponding child process is running or not.
  • If the child process start function returns {ok,Child} or {ok,Child,Info}, the child specification and pid are added to the supervisor and the function returns the same value.
  • If the child process start function returns ignore, the child specification ChildSpec is added to the supervisor if it is an one_for_one, one_for_all or rest_for_one supervisor, and the pid is set to undefined. For simple_one_for_one supervisors, no child is added to the supervisor. The function returns {ok,undefined}.

If the child process start function returns an error tuple or an erroneous value, or if it fails, the child specification is discarded, and the function returns {error,Error}, where Error is a term containing information about the error and child specification.

Link to this function

start_link(Module, Args)

View Source
-spec start_link(Module, Args) -> startlink_ret() when Module :: module(), Args :: term().

Creates a nameless supervisor process as part of a supervision tree.

Equivalent to start_link/3 except that the supervisor process is not registered.

Link to this function

start_link(SupName, Module, Args)

View Source
-spec start_link(SupName, Module, Args) -> startlink_ret()
              when SupName :: sup_name(), Module :: module(), Args :: term().

Creates a supervisor process as part of a supervision tree.

For example, the function ensures that the supervisor is linked to the calling process (its supervisor).

The created supervisor process calls Module:init/1 to find out about restart strategy, maximum restart intensity, and child processes. To ensure a synchronized startup procedure, start_link/2,3 does not return until Module:init/1 has returned and all child processes have been started.

  • If SupName={local,Name}, the supervisor is registered locally as Name using register/2.
  • If SupName={global,Name}, the supervisor is registered globally as Name using global:register_name/2.
  • If SupName={via,Module,Name}, the supervisor is registered as Name using the registry represented by Module. The Module callback must export the functions register_name/2, unregister_name/1, and send/2, which must behave like the corresponding functions in global. Thus, {via,global,Name} is a valid reference.

Module is the name of the callback module.

Args is any term that is passed as the argument to Module:init/1.

  • If the supervisor and its child processes are successfully created (that is, if all child process start functions return {ok,Child}, {ok,Child,Info}, or ignore), the function returns {ok,Pid}, where Pid is the pid of the supervisor.
  • If there already exists a process with the specified SupName, the function returns {error,{already_started,Pid}}, where Pid is the pid of that process.
  • If Module:init/1 returns ignore, this function returns ignore as well, and the supervisor terminates with reason normal.
  • If Module:init/1 fails or returns an incorrect value, this function returns {error,Term}, where Term is a term with information about the error, and the supervisor terminates with reason Term.
  • If any child process start function fails or returns an error tuple or an erroneous value, the supervisor first terminates all already started child processes with reason shutdown and then terminate itself and returns {error, {shutdown, Reason}}.
Link to this function

terminate_child(SupRef, Id)

View Source
-spec terminate_child(SupRef, Id) -> Result
                   when
                       SupRef :: sup_ref(),
                       Id :: pid() | child_id(),
                       Result :: ok | {error, Error},
                       Error :: not_found | simple_one_for_one.

Tells supervisor SupRef to terminate the specified child.

If the supervisor is not simple_one_for_one, Id must be the child specification identifier. The process, if any, is terminated and, unless it is a temporary child, the child specification is kept by the supervisor. The child process can later be restarted by the supervisor. The child process can also be restarted explicitly by calling restart_child/2. Use delete_child/2 to remove the child specification.

If the child is temporary, the child specification is deleted as soon as the process terminates. This means that delete_child/2 has no meaning and restart_child/2 cannot be used for these children.

If the supervisor is simple_one_for_one, Id must be the pid/0 of the child process. If the specified process is alive, but is not a child of the specified supervisor, the function returns {error,not_found}. If the child specification identifier is specified instead of a pid/0, the function returns {error,simple_one_for_one}.

If successful, the function returns ok. If there is no child specification with the specified Id, the function returns {error,not_found}.

-spec which_children(SupRef) -> [{Id, Child, Type, Modules}]
                  when
                      SupRef :: sup_ref(),
                      Id :: child_id() | undefined,
                      Child :: child() | restarting,
                      Type :: worker(),
                      Modules :: modules().

Returns a newly created list with information about all child specifications and child processes belonging to supervisor SupRef.

Notice that calling this function when supervising many children under low memory conditions can cause an out of memory exception.

The following information is given for each child specification/process:

  • Id - As defined in the child specification or undefined for a simple_one_for_one supervisor.
  • Child - The pid of the corresponding child process, the atom restarting if the process is about to be restarted, or undefined if there is no such process.
  • Type - As defined in the child specification.
  • Modules - As defined in the child specification.