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NAME
supervisor - Generic supervisor behavior.
DESCRIPTION
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 include 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 terminates, 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() 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(), 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.
DATA TYPES
auto_shutdown() = never | any_significant | all_significant
child() = undefined | pid()
child_id() = term()
Not a pid().
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.
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.
modules() = [module()] | dynamic
restart() = permanent | transient | temporary
shutdown() = brutal_kill | timeout()
significant() = boolean()
startchild_err() =
already_present | {already_started, Child :: child()} | term()
startchild_ret() =
{ok, Child :: child()} |
{ok, Child :: child(), Info :: term()} |
{error, startchild_err()}
startlink_err() =
{already_started, pid()} | {shutdown, term()} | term()
startlink_ret() =
{ok, pid()} | ignore | {error, startlink_err()}
strategy() =
one_for_all | one_for_one | rest_for_one | simple_one_for_one
sup_flags() =
#{strategy => strategy(),
intensity => integer() >= 0,
period => integer() >= 1,
auto_shutdown => auto_shutdown()} |
{RestartStrategy :: strategy(),
Intensity :: integer() >= 0,
Period :: integer() >= 1}
The tuple format is kept for backward compatibility only. A map is preferred; see
more details above.
sup_ref() =
(Name :: atom()) |
{Name :: atom(), Node :: node()} |
{global, Name :: atom()} |
{via, Module :: module(), Name :: any()} |
pid()
worker() = worker | supervisor
EXPORTS
check_childspecs(ChildSpecs) -> Result
check_childspecs(ChildSpecs, AutoShutdown) -> Result
Types:
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 optional AutoShutdown argument is given and not undefined, also checks if
the child specifications are allowed for the given auto_shutdown option.
count_children(SupRef) -> PropListOfCounts
Types:
SupRef = sup_ref()
PropListOfCounts = [Count]
Count =
{specs, ChildSpecCount :: integer() >= 0} |
{active, ActiveProcessCount :: integer() >= 0} |
{supervisors, ChildSupervisorCount :: integer() >= 0} |
{workers, ChildWorkerCount :: integer() >= 0}
Returns a property list (see proplists) 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.
For a description of SupRef, see start_child/2.
delete_child(SupRef, Id) -> Result
Types:
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.
For a description of SupRef, see start_child/2.
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}.
get_childspec(SupRef, Id) -> Result
Types:
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.
For a description of SupRef, see start_child/2.
restart_child(SupRef, Id) -> Result
Types:
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.
For a description of SupRef, see start_child/2.
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.
start_child(SupRef, ChildSpec) -> startchild_ret()
Types:
SupRef = sup_ref()
ChildSpec = child_spec() | (List :: [term()])
startchild_ret() =
{ok, Child :: child()} |
{ok, Child :: child(), Info :: term()} |
{error, startchild_err()}
startchild_err() =
already_present | {already_started, Child :: child()} | term()
Dynamically adds a child specification to supervisor SupRef, which starts the
corresponding child process.
SupRef can be any of the following:
* The pid
* Name, if the supervisor is locally registered
* {Name,Node}, if the supervisor is locally registered at another node
* {global,Name}, if the supervisor is globally registered
* {via,Module,Name}, if the supervisor is registered through an alternative
process registry
ChildSpec must be a valid child specification (unless the supervisor is a
simple_one_for_one supervisor; see below). The child process is started by using
the start function as defined in the child specification.
For a simple_one_for_one supervisor, the child specification defined in
Module:init/1 is used, and ChildSpec must instead be an arbitrary list of terms
List. The child process is then started by appending List to the existing start
function arguments, that is, by calling apply(M, F, A++List), 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 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}.
For 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.
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.
start_link(Module, Args) -> startlink_ret()
start_link(SupName, Module, Args) -> startlink_ret()
Types:
SupName = sup_name()
Module = module()
Args = term()
startlink_ret() =
{ok, pid()} | ignore | {error, startlink_err()}
startlink_err() =
{already_started, pid()} | {shutdown, term()} | term()
sup_name() =
{local, Name :: atom()} |
{global, Name :: atom()} |
{via, Module :: module(), Name :: any()}
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.
If no name is provided, the supervisor is not registered.
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}}.
terminate_child(SupRef, Id) -> Result
Types:
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() 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(), 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}.
For a description of SupRef, see start_child/2.
which_children(SupRef) -> [{Id, Child, Type, Modules}]
Types:
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.
For a description of SupRef, see start_child/2.
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.
CALLBACK FUNCTIONS
The following function must be exported from a supervisor callback module.
EXPORTS
Module:init(Args) -> Result
Types:
Args = term()
Result = {ok,{SupFlags,[ChildSpec]}} | ignore
SupFlags = sup_flags()
ChildSpec = child_spec()
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.
SEE ALSO
gen_event(3erl), gen_statem(3erl), gen_server(3erl), sys(3erl)