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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 :: term()} |
{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 :: term()} |
{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)