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 implemented using this module has a standard set of interface functions and includes functionality for tracing and error reporting.

Examples

In order to define a supervisor, we need to first define a child process that will be supervised. As an example, we will define a GenServer that represents a stack:

defmodule Stack do
  use GenServer

  def start_link(state) do
    GenServer.start_link(__MODULE__, state, name: __MODULE__)
  end

  ## Callbacks

  def init(stack) do
    {:ok, stack}
  end

  def handle_call(:pop, _from, [h | t]) do
    {:reply, h, t}
  end

  def handle_cast({:push, h}, t) do
    {:noreply, [h | t]}
  end
end

The stack is a small wrapper around lists. It allows us to put an element on the top of the stack, by prepending to the list, and to get the top of the stack by pattern matching.

We can now start a supervisor that will start and supervise our stack process as follows:

# Start the supervisor with the stack as a single child.
#
# The first element of the tuple is the module containing
# the child implementation, the second is the argument
# given to start_link, in this case a stack with `:hello`.
{:ok, pid} = Supervisor.start_link([
  {Stack, [:hello]}
], 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}

Notice that when starting the GenServer, we are registering it with name Stack, which allows us to call it directly and get what is on the stack:

GenServer.call(Stack, :pop)
#=> :hello

GenServer.cast(Stack, {:push, :world})
#=> :ok

GenServer.call(Stack, :pop)
#=> :world

However, there is a bug in our stack server. If we call :pop and the stack is empty, it is going to crash because no clause matches:

GenServer.call(Stack, :pop)
** (exit) exited in: GenServer.call(Stack, :pop, 5000)

Luckily, since the server is being supervised by a supervisor, the supervisor will automatically start a new one, with the initial stack of [:hello]:

GenServer.call(Stack, :pop)
#=> :hello

Supervisors support different strategies; in the example above, we have chosen :one_for_one. Furthermore, each supervisor can have many workers and supervisors as children, each of them with their specific configuration, shutdown values, and restart strategies.

The rest of this document will cover how child processes are started, how they can be specified, different supervision strategies and more.

The initialization process

In the previous section, we have started a supervisor with one child:

Supervisor.start_link([
  {Stack, [:hello]}
], strategy: :one_for_one)

The first argument given to start_link is a list of children. In the example above, we have passed a tuple, where the child is implemented by the Stack module and receives an initial argument of [:hello] on Stack.start_link/1.

Generally speaking, starting the child process happens in three steps:

1. First the supervisor calls Stack.child_spec([:hello]). This function must return a child specification which describes how the Stack process is supervised. When you use GenServer, a child_spec/1 is automatically defined for you but we will see when and how it can be configured. This function is called once when the supervisor starts (or in case of hot code upgrades).

2. The child specification tells the supervisor which function to invoke to start the child process. By default, it is the start_link/1 function, receiving the same argument and defined in the same module as the child_spec/1 function. This function is called every time a new child process is necessary. For example, when we crashed a Stack in the previous session, Stack.start_link([:hello]) was called once more to start a new stack.

3. Finally, Stack.start_link/1 starts a new process that runs Stack.init/1, which is responsible for setting a process that will react to messages.

In summary, when the Supervisor.start_link(children, opts) is called, it traverses the list of children and retrieves their child_spec/1. Then each child specification describes how each child is started, typically via the start_link/1 function. The supervisor invokes the start_link/1 when it initializes and whenever the child process needs to be restarted. The new process started by start_link/1 often executes the init callback as its first step. The init callback is where we initialize and configure the child process.

Child specification

The child specification describes how the supervisor should start and supervise a child process. We have learned that, when we invoked use GenServer, a Stack.child_spec/1 was automatically defined for us. Let’s invoke it and see what it returns:

Stack.child_spec([:hello])
#=> %{
  id: Stack,
  start: {Stack, :start_link, [[:hello]]},
  restart: :permanent,
  shutdown: 5000,
  type: :worker
}

The child specification contains 5 keys. The first two are required and the remaining ones are optional:

• :id - a value used to identify the child specification internally by the supervisor; defaults to the given module. In case of conflicting :id, the supervisor will refuse to initialize and require explicit IDs. This key is required.

• :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 atom that defines how a child process should be terminated (see the “Shutdown values” section below). This key is optional and defaults to 5000 if the type is :worker or :infinity if the type is :supervisor.

• :type - if the child process is a :worker or a :supervisor. This key is optional and defaults to :worker.

There is a sixth key, called :modules, which is rarely changed and it is set automatically based on the value in :start.

Most times, the behaviour module you are implementing will take care of setting up a proper child_spec/1 for you. For example, use Supervisor will define a child_spec/1 where the :type is set to :supervisor and the :shutdown is :infinity. Still, if you need to customize a certain behaviour, you can do so by defining your own child_spec/1 function or by passing options on use. For example, to specify a GenServer with a shutdown limit of 10 seconds (10_000 miliseconds), one might do:

use GenServer, shutdown: 10_000

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 terminated using Process.exit(child, :kill).

• any integer >= 0 - the amount of time in miliseconds that the supervisor will wait for 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 in miliseconds to terminate. If it doesn’t terminate within the specified time, the child process is unconditionally terminated by the supervisor via Process.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 recommend value is :infinity to give the supervisor and its children enough time to shutdown. This option can be used with regular workers but doing so is discouraged and requires extreme care. If not used carefully and the child process does not terminate, it means your application will never terminate 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.

• :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” section next.

Exit reasons

A supervisor restarts a child process depending on its :restart configuration. For example, when :restart is set :transient, the supervisr 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 exits

• any 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

Notice that supervisor that reached 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 is set to :permanent (the default).

Module-based supervisors

In the example above, a 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(arg) do
    Supervisor.start_link(__MODULE__, arg, name: __MODULE__)
  end

  def init(_arg) do
    Supervisor.init([
      {Stack, [:hello]}
    ], 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 children that are automatically initialized, we have defined a supervisor alongside its init/1 callback and manually initialized the children by calling Supervisor.init/2, passing the same arguments we would have given to start_link/2.

You may want to use a module-based supervisor if:

• You need to perform some particular action on supervisor initialization, like setting up an ETS table.

• You want to perform partial hot-code swapping of the tree. For example, if you add or remove children, the module-based supervision will add and remove the new children directly, while dynamic supervision requires the whole tree to be restarted in order to perform such swaps.

Note use Supervisor defines a child_spec/1 function, allowing the defined module to be put under a supervision tree. The generated child_spec/1 can be customized with the following options:

• :id - the child specification id, defauts to the current module
• :start - how to start the child process (defaults to calling __MODULE__.start_link/1)
• :restart - when the supervisor should be restarted, defaults to :permanent

start_link/2, init/2 and strategies

So far we have started the supervisor passing a single child as a tuple as well as a strategy called :one_for_one:

Supervisor.start_link([
  {Stack, [:hello]}
], strategy: :one_for_one)

Or:

Supervisor.init([
  {Stack, [:hello]}
], strategy: :one_for_one)

However, children can be specified in three different formats and supervisors support different options. Let’s formally define those.

The first argument given to start_link/2 is a list of children which may be either:

• a module - such as Stack. In this case, it is equivalent to passing {Stack, []} (which means Stack.child_spec/1 is invoked with an empty keywords list)
• a tuple with a module as first element and the start argument as second - such as {Stack, [:hello]}. When such format is used, the supervisor will retrieve the child specification from the given module.
• a map representing the child specification itself - such as the child specification map outlined in the previous section.

The second argument is a keyword list of options:

• :strategy - the restart strategy option. It can be either :one_for_one, :rest_for_one, :one_for_all, or :simple_one_for_one. See the “Strategies” section.

• :max_restarts - the maximum amount of restarts allowed in a time frame. Defaults to 3.

• :max_seconds - the time frame in which :max_restarts applies. Defaults to 5.

The :strategy option is required and by default a maximum of 3 restarts is allowed within 5 seconds.

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 “rest” of the child processes, i.e., the child processes after the terminated one in start order, are terminated. Then the terminated child process and the rest of the child processes are restarted.

• :simple_one_for_one - similar to :one_for_one but suits better when dynamically attaching children. This strategy requires the supervisor specification to contain only one child. Many functions in this module behave slightly differently when this strategy is used.

Simple one for one

The :simple_one_for_one supervisor is useful when you want to dynamically start and stop supervised children. As an example, let’s start multiple agents dynamically to keep state.

One important aspect in :simple_one_for_one supervisors is that we often want to pass the :start arguments later on, when starting the children dynamically, rather than when the child specification is defined. In such cases, we should not do

Supervisor.start_link [
  {Agent, fn -> 0 end}
]

as the example above would force all agents to have the same state. In such cases, we can use the child_spec/2 function to build and override the fields in a child specification:

# Override the :start field to have no args.
# The second argument has no effect thanks to it.
agent_spec =
  Supervisor.child_spec(Agent, start: {Agent, :start_link, []})

# We start a supervisor with a simple one for one strategy.
# The agent won't be started now but later on.
{:ok, sup_pid} =
  Supervisor.start_link([agent_spec], strategy: :simple_one_for_one)

# No child worker is active until start_child is called
Supervisor.count_children(sup_pid)
#=> %{active: 0, specs: 1, supervisors: 0, workers: 0}

The simple one for one strategy can define only one child which works as a template for when we call start_child/2.

With the supervisor started, let’s dynamically start agents:

{:ok, agent1} = Supervisor.start_child(sup_pid, [fn -> 0 end])
Agent.update(agent1, & &1 + 1)
Agent.get(agent1, & &1) #=> 1

{:ok, agent2} = Supervisor.start_child(sup_pid, [fn -> %{} end])
Agent.get(agent2, & &1) #=> %{}

Supervisor.count_children(sup_pid)
#=> %{active: 2, specs: 1, supervisors: 0, workers: 2}

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.