View Source PartitionSupervisor (Elixir v1.14.5)

A supervisor that starts multiple partitions of the same child.

Certain processes may become bottlenecks in large systems. If those processes can have their state trivially partitioned, in a way there is no dependency between them, then they can use the PartitionSupervisor to create multiple isolated and independent partitions.

Once the PartitionSupervisor starts, you can dispatch to its children using {:via, PartitionSupervisor, {name, key}}, where name is the name of the PartitionSupervisor and key is used for routing.


The DynamicSupervisor is a single process responsible for starting other processes. In some applications, the DynamicSupervisor may become a bottleneck. To address this, you can start multiple instances of the DynamicSupervisor through a PartitionSupervisor, and then pick a "random" instance to start the child on.

Instead of starting a single DynamicSupervisor:

children = [
  {DynamicSupervisor, name: MyApp.DynamicSupervisor}

Supervisor.start_link(children, strategy: :one_for_one)

and starting children on that dynamic supervisor directly:

DynamicSupervisor.start_child(MyApp.DynamicSupervisor, {Agent, fn -> %{} end})

You can do start the dynamic supervisors under a PartitionSupervisor:

children = [
   child_spec: DynamicSupervisor,
   name: MyApp.DynamicSupervisors}

Supervisor.start_link(children, strategy: :one_for_one)

and then:

  {:via, PartitionSupervisor, {MyApp.DynamicSupervisors, self()}},
  {Agent, fn -> %{} end}

In the code above, we start a partition supervisor that will by default start a dynamic supervisor for each core in your machine. Then, instead of calling the DynamicSupervisor by name, you call it through the partition supervisor using the {:via, PartitionSupervisor, {name, key}} format. We picked self() as the routing key, which means each process will be assigned one of the existing dynamic supervisors. See start_link/1 to see all options supported by the PartitionSupervisor.

Implementation notes

The PartitionSupervisor uses either an ETS table or a Registry to manage all of the partitions. Under the hood, the PartitionSupervisor generates a child spec for each partition and then acts as a regular supervisor. The ID of each child spec is the partition number.

For routing, two strategies are used. If key is an integer, it is routed using rem(abs(key), partitions) where partitions is the number of partitions. Otherwise it uses :erlang.phash2(key, partitions). The particular routing may change in the future, and therefore must not be relied on. If you want to retrieve a particular PID for a certain key, you can use GenServer.whereis({:via, PartitionSupervisor, {name, key}}).

Link to this section Summary


The name of the PartitionSupervisor.


Returns a map containing count values for the supervisor.

Returns the number of partitions for the partition supervisor.

Starts a partition supervisor with the given options.

Synchronously stops the given partition supervisor with the given reason.

Returns a list with information about all children.

Link to this section Types

@type name() :: atom() | {:via, module(), term()}

The name of the PartitionSupervisor.

Link to this section Functions

Link to this function


View Source (since 1.14.0)
@spec count_children(name()) :: %{
  specs: non_neg_integer(),
  active: non_neg_integer(),
  supervisors: non_neg_integer(),
  workers: non_neg_integer()

Returns a map containing count values for the supervisor.

The map contains the following keys:

  • :specs - the number of partitions (children processes)

  • :active - the count of all actively running child processes managed by this supervisor

  • :supervisors - the count of all supervisors whether or not the child process is still alive

  • :workers - the count of all workers, whether or not the child process is still alive

Link to this function


View Source (since 1.14.0)
@spec partitions(name()) :: pos_integer()

Returns the number of partitions for the partition supervisor.

Link to this function


View Source (since 1.14.0)
@spec start_link(keyword()) :: Supervisor.on_start()

Starts a partition supervisor with the given options.

This function is typically not invoked directly, instead it is invoked when using a PartitionSupervisor as a child of another supervisor:

children = [
  {PartitionSupervisor, child_spec: SomeChild, name: MyPartitionSupervisor}

If the supervisor is successfully spawned, this function returns {:ok, pid}, where pid is the PID of the supervisor. If the given name for the partition supervisor is already assigned to a process, the function returns {:error, {:already_started, pid}}, where pid is the PID of that process.

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.


  • :name - an atom or via tuple representing the name of the partition supervisor (see name/0).

  • :partitions - a positive integer with the number of partitions. Defaults to System.schedulers_online() (typically the number of cores).

  • :strategy - the restart strategy option, defaults to :one_for_one. You can learn more about strategies in the Supervisor module docs.

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

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

  • :with_arguments - a two-argument anonymous function that allows the partition to be given to the child starting function. See the :with_arguments section below.


Sometimes you want each partition to know their partition assigned number. This can be done with the :with_arguments option. This function receives the list of arguments of the child specification and the partition. It must return a new list of arguments that will be passed to the child specification of children.

For example, most processes are started by calling start_link(opts), where opts is a keyword list. You could inject the partition into the options given to the child:

with_arguments: fn [opts], partition ->
  [Keyword.put(opts, :partition, partition)]
Link to this function

stop(supervisor, reason \\ :normal, timeout \\ :infinity)

View Source (since 1.14.0)
@spec stop(name(), reason :: term(), timeout()) :: :ok

Synchronously stops the given partition 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.

Link to this function


View Source (since 1.14.0)
@spec which_children(name()) :: [
  {:undefined, pid() | :restarting, :worker | :supervisor,
   [module()] | :dynamic}

Returns a list with information about all children.

This function returns a list of tuples containing:

  • id - the partition number

  • child - the PID of the corresponding child process or the atom :restarting if the process is about to be restarted

  • type - :worker or :supervisor as defined in the child specification

  • modules - as defined in the child specification