supervisor(3erl)
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_fsm, 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.
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
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.
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.
intensity defaults to 1 and period defaults to 5.
The type definition of a child specification is as follows:
child_spec() = #{id => child_id(), % mandatory
start => mfargs(), % mandatory
restart => restart(), % 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.
* 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 is to
be set to infinity to give the subtree ample time to shut down. 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,
gen_statem, or gen_fsm, 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
child() = undefined | pid()
child_id() = term()
Not a pid().
child_spec() =
#{id := child_id(),
start := mfargs(),
restart => restart(),
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()
strategy() =
one_for_all | one_for_one | rest_for_one | simple_one_for_one
sup_flags() =
#{strategy => strategy(),
intensity => integer() >= 0,
period => integer() >= 1} |
{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
Types:
ChildSpecs = [child_spec()]
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}.
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
childrens 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_fsm(3erl), gen_statem(3erl), gen_server(3erl), sys(3erl)
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