supervisor
Module
supervisor
Module Summary
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.
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 simplifiedone_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
andrestart_child/2
are invalid forsimple_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 undersimple_one_for_one
supervisors by specifying the child'spid()
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. The termination reason for the supervisor itself in that case will be shutdown
. 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 asapply(M,F,A)
.The start function must create and link to the child process, and must return
{ok,Child}
or{ok,Child,Info}
, whereChild
is the pid of the child process andInfo
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. Apermanent
child process is always restarted. Atemporary
child process is never restarted (even when the supervisor's restart strategy isrest_for_one
orone_for_all
and a sibling's death causes the temporary process to be terminated). Atransient
child process is restarted only if it terminates abnormally, that is, with another exit reason thannormal
,shutdown
, or{shutdown,Term}
.The
restart
key is optional. If it is not specified, it defaults topermanent
. -
shutdown
defines how a child process must be terminated.brutal_kill
means that the child process is unconditionally terminated usingexit(Child,kill)
. An integer time-out value means that the supervisor tells the child process to terminate by callingexit(Child,shutdown)
and then wait for an exit signal with reasonshutdown
back from the child process. If no exit signal is received within the specified number of milliseconds, the child process is unconditionally terminated usingexit(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 toinfinity
, if the child process is a worker.WarningBe 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 to5000
if the child is of typeworker
and it defaults toinfinity
if the child is of typesupervisor
. -
type
specifies if the child process is a supervisor or a worker.The
type
key is optional. If it is not specified, it defaults toworker
. -
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 asupervisor
,gen_server
or,gen_statem
, this is to be a list with one element[Module]
, whereModule
is the callback module. If the child process is an event manager (gen_event
) with a dynamic set of callback modules, valuedynamic
must be used. For more information about release handling, seeRelease Handling
in OTP Design Principles.The
modules
key is optional. If it is not specified, it defaults to[M]
, whereM
comes from the child's start{M,F,A}
. -
Internally, the supervisor also keeps track of the pid
Child
of the child process, orundefined
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
Types
Takes a list of child specification as argument and returns ok
if all of them are syntactically correct, otherwise {error,Error}
.
Types
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 asimple_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 aschild_type = supervisor
in the specification list, regardless if the child process is still alive. -
workers
- The count of all children marked aschild_type = worker
in the specification list, regardless if the child process is still alive.
For a description of SupRef
, see start_child/2
.
Types
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}
.
Types
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
.
Types
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.
Types
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 asimple_one_for_one
supervisor, see below), the pid is set toundefined
, 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.
Types
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 asName
usingregister/2
. -
If
SupName={global,Name}
, the supervisor is registered globally asName
usingglobal:register_name/2
. -
If
SupName={via,Module,Name}
, the supervisor is registered asName
using the registry represented byModule
. TheModule
callback must export the functionsregister_name/2
,unregister_name/1
, andsend/2
, which must behave like the corresponding functions inglobal
. 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}
, orignore
), the function returns{ok,Pid}
, wherePid
is the pid of the supervisor. -
If there already exists a process with the specified
SupName
, the function returns{error,{already_started,Pid}}
, wherePid
is the pid of that process. -
If
Module:init/1
returnsignore
, this function returnsignore
as well, and the supervisor terminates with reasonnormal
. -
If
Module:init/1
fails or returns an incorrect value, this function returns{error,Term}
, whereTerm
is a term with information about the error, and the supervisor terminates with reasonTerm
. -
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}}
.
Types
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
.
Types
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 orundefined
for asimple_one_for_one
supervisor. -
Child
- The pid of the corresponding child process, the atomrestarting
if the process is about to be restarted, orundefined
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
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(3)
, gen_statem(3)
, gen_server(3)
, sys(3)
© 2010–2017 Ericsson AB
Licensed under the Apache License, Version 2.0.