View Source Supervisor behaviour (Elixir v1.14.1)
A behaviour module for implementing supervisors.
A supervisor is a process which supervises other processes, which we refer to as child processes. Supervisors are used to build a hierarchical process structure called a supervision tree. Supervision trees provide fault-tolerance and encapsulate how our applications start and shutdown.
A supervisor may be started directly with a list of child specifications via
start_link/2
or you may define a module-based supervisor that implements
the required callbacks. The sections below use start_link/2
to start
supervisors in most examples, but it also includes a specific section
on module-based ones.
Examples
In order to start a supervisor, we need to first define a child process
that will be supervised. As an example, we will define a GenServer
,
a generic server, that keeps a counter. Other processes can then send
messages to this process to read the counter and bump its value.
Note: in practice you would not define a counter as a GenServer. Instead, if you need a counter, you would pass it around as inputs and outputs to the functions that need it. The reason we picked a counter in this example is due to its simplicity, as it allows us to focus on how supervisors work.
defmodule Counter do
use GenServer
def start_link(arg) when is_integer(arg) do
GenServer.start_link(__MODULE__, arg, name: __MODULE__)
end
## Callbacks
@impl true
def init(counter) do
{:ok, counter}
end
@impl true
def handle_call(:get, _from, counter) do
{:reply, counter, counter}
end
def handle_call({:bump, value}, _from, counter) do
{:reply, counter, counter + value}
end
end
The Counter
receives an argument on start_link
. This argument
is passed to the init/1
callback which becomes the initial value
of the counter. Our counter handles two operations (known as calls):
:get
, to get the current counter value, and :bump
, that bumps
the counter by the given value
and returns the old counter.
We can now start a supervisor that will start and supervise our counter process. The first step is to define a list of child specifications that control how each child behaves. Each child specification is a map, as shown below:
children = [
# The Counter is a child started via Counter.start_link(0)
%{
id: Counter,
start: {Counter, :start_link, [0]}
}
]
# Now we start the supervisor with the children and a strategy
{:ok, pid} = Supervisor.start_link(children, strategy: :one_for_one)
# After started, we can query the supervisor for information
Supervisor.count_children(pid)
#=> %{active: 1, specs: 1, supervisors: 0, workers: 1}
Note that when starting the GenServer, we are registering it
with name Counter
via the name: __MODULE__
option. This allows
us to call it directly and get its value:
GenServer.call(Counter, :get)
#=> 0
GenServer.cast(Counter, {:bump, 3})
#=> 0
GenServer.call(Counter, :get)
#=> 3
However, there is a bug in our counter server. If we call :bump
with
a non-numeric value, it is going to crash:
GenServer.call(Counter, {:bump, "oops"})
** (exit) exited in: GenServer.call(Counter, {:bump, "oops"}, 5000)
Luckily, since the server is being supervised by a supervisor, the
supervisor will automatically start a new one, reset back to its initial
value of 0
:
GenServer.call(Counter, :get)
#=> 0
Supervisors support different strategies; in the example above, we
have chosen :one_for_one
. Furthermore, each supervisor can have many
workers and/or supervisors as children, with each one having its own
configuration (as outlined in the "Child specification" section).
The rest of this document will cover how child processes are specified, how they can be started and stopped, different supervision strategies and more.
Child specification
The child specification describes how the supervisor starts, shuts down, and restarts child processes.
The child specification is a map containing up to 6 elements. The first two keys in the following list are required, and the remaining ones are optional:
:id
- any term used to identify the child specification internally by the supervisor; defaults to the given module. This key is required. For supervisors, in the case of conflicting:id
values, the supervisor will refuse to initialize and require explicit IDs. This is not the case for dynamic supervisors though.:start
- a tuple with the module-function-args to be invoked to start the child process. This key is required.:restart
- an atom that defines when a terminated child process should be restarted (see the "Restart values" section below). This key is optional and defaults to:permanent
.:shutdown
- an integer or atom that defines how a child process should be terminated (see the "Shutdown values" section below). This key is optional and defaults to5_000
if the type is:worker
or:infinity
if the type is:supervisor
.:type
- specifies that the child process is a:worker
or a:supervisor
. This key is optional and defaults to:worker
.:modules
- a list of modules used by hot code upgrade mechanisms to determine which processes are using certain modules. It is typically set to the callback module of behaviours likeGenServer
,Supervisor
, and such. It is set automatically based on the:start
value and it is rarely changed in practice.
Let's understand what the :shutdown
and :restart
options control.
Shutdown values (:shutdown)
The following shutdown values are supported in the :shutdown
option:
:brutal_kill
- the child process is unconditionally and immediately terminated usingProcess.exit(child, :kill)
.any integer >= 0 - the amount of time in milliseconds that the supervisor will wait for its children to terminate after emitting a
Process.exit(child, :shutdown)
signal. If the child process is not trapping exits, the initial:shutdown
signal will terminate the child process immediately. If the child process is trapping exits, it has the given amount of time to terminate. If it doesn't terminate within the specified time, the child process is unconditionally terminated by the supervisor viaProcess.exit(child, :kill)
.:infinity
- works as an integer except the supervisor will wait indefinitely for the child to terminate. If the child process is a supervisor, the recommended value is:infinity
to give the supervisor and its children enough time to shut down. This option can be used with regular workers but doing so is discouraged and requires extreme care. If not used carefully, the child process will never terminate, preventing your application from terminating as well.
Restart values (:restart)
The :restart
option controls what the supervisor should consider to
be a successful termination or not. If the termination is successful,
the supervisor won't restart the child. If the child process crashed,
the supervisor will start a new one.
The following restart values are supported in the :restart
option:
:permanent
- the child process is always restarted.:temporary
- the child process is never restarted, regardless of the supervision strategy: any termination (even abnormal) is considered successful.:transient
- the child process is restarted only if it terminates abnormally, i.e., with an exit reason other than:normal
,:shutdown
, or{:shutdown, term}
.
For a more complete understanding of the exit reasons and their impact, see the "Exit reasons and restarts" section.
child_spec/1
function
When starting a supervisor, we may pass a list of child specifications. Those specifications are maps that tell how the supervisor should start, stop and restart each of its children:
%{
id: Counter,
start: {Counter, :start_link, [0]}
}
The map above defines a child with :id
of Counter
that is started
by calling Counter.start_link(0)
.
However, defining the child specification for each child as a map can be
quite error prone, as we may change the Counter
implementation and forget
to update its specification. That's why Elixir allows you to pass a tuple with
the module name and the start_link
argument instead of the specification:
children = [
{Counter, 0}
]
The supervisor will then invoke Counter.child_spec(0)
to retrieve a child
specification. Now the Counter
module is responsible for building its own
specification, for example, we could write:
def child_spec(arg) do
%{
id: Counter,
start: {Counter, :start_link, [arg]}
}
end
Luckily for us, use GenServer
already defines a Counter.child_spec/1
exactly like above, so you don't need to write the definition above yourself.
If you want to customize the automatically generated child_spec/1
function,
you can pass the options directly to use GenServer
:
use GenServer, restart: :transient
Finally, note it is also possible to simply pass the Counter
module as
a child:
children = [
Counter
]
When only the module name is given, it is equivalent to {Counter, []}
,
which in our case would be invalid, which is why we always pass the initial
counter explicitly.
By replacing the child specification with {Counter, 0}
, we keep it
encapsulated in the Counter
module. We could now share our
Counter
implementation with other developers and they can add it directly
to their supervision tree without worrying about the low-level details of
the counter.
Overall, a child specification can be one of the following:
a map representing the child specification itself - as outlined in the "Child specification" section
a tuple with a module as first element and the start argument as second - such as
{Counter, 0}
. In this case,Counter.child_spec(0)
is called to retrieve the child specificationa module - such as
Counter
. In this case,Counter.child_spec([])
would be called, which is invalid for the counter, but it is useful in many other cases, especially when you want to pass a list of options to the child process
If you need to convert a {module, arg}
tuple or a module child specification to a
child specification or modify a child specification itself,
you can use the Supervisor.child_spec/2
function.
For example, to run the counter with a different :id
and a :shutdown
value of
10 seconds (10_000 milliseconds):
children = [
Supervisor.child_spec({Counter, 0}, id: MyCounter, shutdown: 10_000)
]
Supervisor strategies and options
So far we have started the supervisor passing a single child as a tuple
as well as a strategy called :one_for_one
:
children = [
{Counter, 0}
]
Supervisor.start_link(children, strategy: :one_for_one)
The first argument given to start_link/2
is a list of child
specifications as defined in the "child_spec/1" section above.
The second argument is a keyword list of options:
:strategy
- the supervision strategy option. It can be either:one_for_one
,:rest_for_one
or:one_for_all
. Required. See the "Strategies" section.:max_restarts
- the maximum number of restarts allowed in a time frame. Defaults to3
.:max_seconds
- the time frame in which:max_restarts
applies. Defaults to5
.:name
- a name to register the supervisor process. Supported values are explained in the "Name registration" section in the documentation forGenServer
. Optional.
Strategies
Supervisors support different supervision strategies (through the
:strategy
option, as seen above):
:one_for_one
- if a child process terminates, only that process is restarted.:one_for_all
- if a child process terminates, all other child processes are terminated and then all child processes (including the terminated one) are restarted.:rest_for_one
- if a child process terminates, the terminated child process and the rest of the children started after it, are terminated and restarted.
In the above, process termination refers to unsuccessful termination, which
is determined by the :restart
option.
To efficiently supervise children started dynamically, see DynamicSupervisor
.
Name registration
A supervisor is bound to the same name registration rules as a GenServer
.
Read more about these rules in the documentation for GenServer
.
Module-based supervisors
In the example so far, the supervisor was started by passing the supervision
structure to start_link/2
. However, supervisors can also be created by
explicitly defining a supervision module:
defmodule MyApp.Supervisor do
# Automatically defines child_spec/1
use Supervisor
def start_link(init_arg) do
Supervisor.start_link(__MODULE__, init_arg, name: __MODULE__)
end
@impl true
def init(_init_arg) do
children = [
{Counter, 0}
]
Supervisor.init(children, strategy: :one_for_one)
end
end
The difference between the two approaches is that a module-based
supervisor gives you more direct control over how the supervisor
is initialized. Instead of calling Supervisor.start_link/2
with
a list of child specifications that are automatically initialized, we manually
initialize the children by calling Supervisor.init/2
inside its
init/1
callback. Supervisor.init/2
accepts the same :strategy
,
:max_restarts
, and :max_seconds
options as start_link/2
.
use Supervisor
also defines a child_spec/1
function which allows
us to run MyApp.Supervisor
as a child of another supervisor or
at the top of your supervision tree as:
children = [
MyApp.Supervisor
]
Supervisor.start_link(children, strategy: :one_for_one)
A general guideline is to use the supervisor without a callback
module only at the top of your supervision tree, generally in the
Application.start/2
callback. We recommend using module-based
supervisors for any other supervisor in your application, so they
can run as a child of another supervisor in the tree. The child_spec/1
generated automatically by Supervisor
can be customized with the
following options:
:id
- the child specification identifier, defaults to the current module:restart
- when the supervisor should be restarted, defaults to:permanent
The @doc
annotation immediately preceding use Supervisor
will be
attached to the generated child_spec/1
function.
Start and shutdown
When the supervisor starts, it traverses all child specifications and
then starts each child in the order they are defined. This is done by
calling the function defined under the :start
key in the child
specification and typically defaults to start_link/1
.
The start_link/1
(or a custom) is then called for each child process.
The start_link/1
function must return {:ok, pid}
where pid
is the
process identifier of a new process that is linked to the supervisor.
The child process usually starts its work by executing the init/1
callback. Generally speaking, the init
callback is where we initialize
and configure the child process.
The shutdown process happens in reverse order.
When a supervisor shuts down, it terminates all children in the opposite
order they are listed. The termination happens by sending a shutdown exit
signal, via Process.exit(child_pid, :shutdown)
, to the child process and
then awaiting for a time interval for the child process to terminate. This
interval defaults to 5000 milliseconds. If the child process does not
terminate in this interval, the supervisor abruptly terminates the child
with reason :kill
. The shutdown time can be configured in the child
specification which is fully detailed in the next section.
If the child process is not trapping exits, it will shutdown immediately
when it receives the first exit signal. If the child process is trapping
exits, then the terminate
callback is invoked, and the child process
must terminate in a reasonable time interval before being abruptly
terminated by the supervisor.
In other words, if it is important that a process cleans after itself
when your application or the supervision tree is shutting down, then
this process must trap exits and its child specification should specify
the proper :shutdown
value, ensuring it terminates within a reasonable
interval.
Exit reasons and restarts
A supervisor restarts a child process depending on its :restart
configuration.
For example, when :restart
is set to :transient
, the supervisor does not
restart the child in case it exits with reason :normal
, :shutdown
or
{:shutdown, term}
.
So one may ask: which exit reason should I choose when exiting? There are three options:
:normal
- in such cases, the exit won't be logged, there is no restart in transient mode, and linked processes do not exit:shutdown
or{:shutdown, term}
- in such cases, the exit won't be logged, there is no restart in transient mode, and linked processes exit with the same reason unless they're trapping exitsany other term - in such cases, the exit will be logged, there are restarts in transient mode, and linked processes exit with the same reason unless they're trapping exits
Note that the supervisor that reaches maximum restart intensity will exit with
:shutdown
reason. In this case the supervisor will only be restarted if its
child specification was defined with the :restart
option set to :permanent
(the default).
Link to this section Summary
Types
A child process.
The supervisor child specification.
Options given to start_link/2
and init/2
The supervisor name
Return values of start_link
functions
Return values of start_child
functions
Option values used by the start*
functions
Supported restart options
Supported shutdown options
Supported strategies
The supervisor flags returned on init
The supervisor reference
Supervisor type.
Callbacks
Callback invoked to start the supervisor and during hot code upgrades.
Functions
Builds and overrides a child specification.
Returns a map containing count values for the given supervisor.
Deletes the child specification identified by child_id
.
Receives a list of child specifications to initialize and a set of options
.
Restarts a child process identified by child_id
.
Adds a child specification to supervisor
and starts that child.
Starts a supervisor with the given children.
Starts a module-based supervisor process with the given module
and init_arg
.
Synchronously stops the given supervisor with the given reason
.
Terminates the given child identified by child_id
.
Returns a list with information about all children of the given supervisor.
Link to this section Types
@type child() :: pid() | :undefined
A child process.
It can be a PID when the child process was started, or :undefined
when
the child was created by a dynamic supervisor.
@type child_spec() :: %{ :id => atom() | term(), :start => {module(), function_name :: atom(), args :: [term()]}, optional(:restart) => restart(), optional(:shutdown) => shutdown(), optional(:type) => type(), optional(:modules) => [module()] | :dynamic }
The supervisor child specification.
It defines how the supervisor should start, stop and restart each of its children.
@type init_option() :: {:strategy, strategy()} | {:max_restarts, non_neg_integer()} | {:max_seconds, pos_integer()}
Options given to start_link/2
and init/2
The supervisor name
@type on_start() :: {:ok, pid()} | :ignore | {:error, {:already_started, pid()} | {:shutdown, term()} | term()}
Return values of start_link
functions
@type on_start_child() :: {:ok, child()} | {:ok, child(), info :: term()} | {:error, {:already_started, child()} | :already_present | term()}
Return values of start_child
functions
@type option() :: {:name, name()}
Option values used by the start*
functions
@type restart() :: :permanent | :transient | :temporary
Supported restart options
@type shutdown() :: pos_integer() | :infinity | :brutal_kill
Supported shutdown options
@type strategy() :: :one_for_one | :one_for_all | :rest_for_one
Supported strategies
@type sup_flags() :: %{ strategy: strategy(), intensity: non_neg_integer(), period: pos_integer() }
The supervisor flags returned on init
The supervisor reference
@type type() :: :worker | :supervisor
Supervisor type.
Whether the supervisor is a worker or a supervisor.
Link to this section Callbacks
@callback init(init_arg :: term()) :: {:ok, {sup_flags(), [child_spec() | (old_erlang_child_spec :: :supervisor.child_spec())]}} | :ignore
Callback invoked to start the supervisor and during hot code upgrades.
Developers typically invoke Supervisor.init/2
at the end of their
init callback to return the proper supervision flags.
Link to this section Functions
@spec child_spec( child_spec() | {module(), arg :: term()} | module(), keyword() ) :: child_spec()
Builds and overrides a child specification.
Similar to start_link/2
and init/2
, it expects a module, {module, arg}
,
or a child specification.
If a two-element tuple in the shape of {module, arg}
is given,
the child specification is retrieved by calling module.child_spec(arg)
.
If a module is given, the child specification is retrieved by calling
module.child_spec([])
.
After the child specification is retrieved, the fields on overrides
are directly applied on the child spec. If overrides
has keys that
do not map to any child specification field, an error is raised.
See the "Child specification" section in the module documentation for all of the available keys for overriding.
Examples
This function is often used to set an :id
option when
the same module needs to be started multiple times in the
supervision tree:
Supervisor.child_spec({Agent, fn -> :ok end}, id: {Agent, 1})
#=> %{id: {Agent, 1},
#=> start: {Agent, :start_link, [fn -> :ok end]}}
@spec count_children(supervisor()) :: %{ specs: non_neg_integer(), active: non_neg_integer(), supervisors: non_neg_integer(), workers: non_neg_integer() }
Returns a map containing count values for the given supervisor.
The map contains the following keys:
:specs
- the total count of children, dead or alive:active
- the count of all actively running child processes managed by this supervisor:supervisors
- the count of all supervisors whether or not these child supervisors are still alive:workers
- the count of all workers, whether or not these child workers are still alive
@spec delete_child(supervisor(), term()) :: :ok | {:error, error} when error: :not_found | :running | :restarting
Deletes the child specification identified by child_id
.
The corresponding child process must not be running; use terminate_child/2
to terminate it if it's running.
If successful, this function returns :ok
. This function may return an error
with an appropriate error tuple if the child_id
is not found, or if the
current process is running or being restarted.
@spec init( [ child_spec() | {module(), term()} | module() | (old_erlang_child_spec :: :supervisor.child_spec()) ], [init_option()] ) :: {:ok, {sup_flags(), [child_spec() | (old_erlang_child_spec :: :supervisor.child_spec())]}}
Receives a list of child specifications to initialize and a set of options
.
This is typically invoked at the end of the init/1
callback of
module-based supervisors. See the sections "Supervisor strategies and options" and
"Module-based supervisors" in the module documentation for more information.
This function returns a tuple containing the supervisor flags and child specifications.
Examples
def init(_init_arg) do
children = [
{Counter, 0}
]
Supervisor.init(children, strategy: :one_for_one)
end
Options
:strategy
- the supervision strategy option. It can be either:one_for_one
,:rest_for_one
, or:one_for_all
:max_restarts
- the maximum number of restarts allowed in a time frame. Defaults to3
.:max_seconds
- the time frame in seconds in which:max_restarts
applies. Defaults to5
.
The :strategy
option is required and by default a maximum of 3 restarts
is allowed within 5 seconds. Check the Supervisor
module for a detailed
description of the available strategies.
@spec restart_child(supervisor(), term()) :: {:ok, child()} | {:ok, child(), term()} | {:error, error} when error: :not_found | :running | :restarting | term()
Restarts a child process identified by child_id
.
The child specification must exist and the corresponding child process must not be running.
Note that for temporary children, the child specification is automatically deleted when the child terminates, and thus it is not possible to restart such children.
If the child process start function returns {:ok, child}
or {:ok, child, info}
,
the PID is added to the supervisor and this function returns the same value.
If the child process start function returns :ignore
, the PID remains set to
:undefined
and this function returns {:ok, :undefined}
.
This function may return an error with an appropriate error tuple if the
child_id
is not found, or if the current process is running or being
restarted.
If the child process start function returns an error tuple or an erroneous value,
or if it fails, this function returns {:error, error}
.
@spec start_child( supervisor(), child_spec() | {module(), term()} | module() | (old_erlang_child_spec :: :supervisor.child_spec()) ) :: on_start_child()
Adds a child specification to supervisor
and starts that child.
child_spec
should be a valid child specification. The child process will
be started as defined in the child specification.
If a child specification with the specified ID already exists, child_spec
is
discarded and this function returns an error with :already_started
or
:already_present
if the corresponding child process is running or not,
respectively.
If the child process start function returns {:ok, child}
or {:ok, child, info}
, then child specification and PID are added to the supervisor and
this function returns the same value.
If the child process start function returns :ignore
, the child specification
is added to the supervisor, the PID is set to :undefined
and this 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 this function
returns {:error, error}
where error
is a term containing information about
the error and child specification.
@spec start_link( [ child_spec() | {module(), term()} | module() | (old_erlang_child_spec :: :supervisor.child_spec()) ], [option() | init_option()] ) :: {:ok, pid()} | {:error, {:already_started, pid()} | {:shutdown, term()} | term()}
@spec start_link(module(), term()) :: on_start()
Starts a supervisor with the given children.
children
is a list of the following forms:
a module, where
module.child_spec([])
will be invoked to retrieve its child specificationa two-element tuple in the shape of
{module, arg}
, wheremodule.child_spec(arg)
will be invoked to retrieve its child specification
A strategy is required to be provided through the :strategy
option. See
"Supervisor strategies and options" for examples and other options.
The options can also be used to register a supervisor name.
The supported values are described under the "Name registration"
section in the GenServer
module docs.
If the supervisor and all child processes are successfully spawned
(if the start function of each child process returns {:ok, child}
,
{:ok, child, info}
, or :ignore
), this function returns
{:ok, pid}
, where pid
is the PID of the supervisor. If the supervisor
is given a name and a process with the specified name already exists,
the function returns {:error, {:already_started, pid}}
, where pid
is the PID of that process.
If the start function of any of the child processes fails or returns an error
tuple or an erroneous value, the supervisor first terminates with reason
:shutdown
all the child processes that have already been started, and then
terminates itself and returns {:error, {:shutdown, reason}}
.
Note that a supervisor started with this function is linked to the parent
process and exits not only on crashes but also if the parent process exits
with :normal
reason.
Starts a module-based supervisor process with the given module
and init_arg
.
To start the supervisor, the init/1
callback will be invoked in the given
module
, with init_arg
as its argument. The init/1
callback must return a
supervisor specification which can be created with the help of the init/2
function.
If the init/1
callback returns :ignore
, this function returns
:ignore
as well and the supervisor terminates with reason :normal
.
If it 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
.
The :name
option can also be given in order to register a supervisor
name, the supported values are described in the "Name registration"
section in the GenServer
module docs.
@spec stop(supervisor(), reason :: term(), timeout()) :: :ok
Synchronously stops the given supervisor with the given reason
.
It returns :ok
if the supervisor terminates with the given
reason. If it terminates with another reason, the call exits.
This function keeps OTP semantics regarding error reporting.
If the reason is any other than :normal
, :shutdown
or
{:shutdown, _}
, an error report is logged.
@spec terminate_child(supervisor(), term()) :: :ok | {:error, :not_found}
Terminates the given child identified by child_id
.
The process is terminated, if there's one. The child specification is kept unless the child is temporary.
A non-temporary child process may 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 successful, this function returns :ok
. If there is no child
specification for the given child ID, this function returns
{:error, :not_found}
.
@spec which_children(supervisor()) :: [ {term() | :undefined, child() | :restarting, :worker | :supervisor, [module()] | :dynamic} ]
Returns a list with information about all children of the given supervisor.
Note that calling this function when supervising a large number of children under low memory conditions can cause an out of memory exception.
This function returns a list of {id, child, type, modules}
tuples, where:
id
- as defined in the child specificationchild
- the PID of the corresponding child process,:restarting
if the process is about to be restarted, or:undefined
if there is no such processtype
-:worker
or:supervisor
, as specified by the child specificationmodules
- as specified by the child specification