How deeply does Elixir in Action, Third Edition cover DynamicSupervisor.start_child/2?

Explains invocation, synchronous serialization, result handling, and the supervisor-bottleneck trade-off.

DynamicSupervisor.start_child/2

As explained in Elixir in Action, Third Edition

DynamicSupervisor.start_child/2 starts a new child process under a DynamicSupervisor and reports whether the start succeeded or conflicted with an already running child.

DynamicSupervisor.start_child/2 starts a child process on demand under a DynamicSupervisor when the required number of children is unknown in advance. The call sends a synchronous request to the supervisor, which starts the child and serializes concurrent start requests. section 9.2.2

Handle the returned result according to the child’s registration state:

Match {:ok, pid} when the child starts successfully. section 9.2.3
Match {:error, {:already_started, pid}} when registration finds an existing process, then reuse that pid. section 9.2.3
Recognize the efficiency boundary: repeated requests can briefly start and stop redundant children and make the supervisor a bottleneck. section 9.2.3

Listing 9.11 Finding a to-do server (dynamic_workers/lib/todo/cache.ex)

defmodule Todo.Cache do
  ...
 
  def server_process(todo_list_name) do
    case start_child(todo_list_name) do
      {:ok, pid} -> pid                          #1
      {:error, {:already_started, pid}} -> pid   #2
    end
  end
 
  defp start_child(todo_list_name) do
    DynamicSupervisor.start_child(
      __MODULE__,
      {Todo.Server, todo_list_name}
    )
  end
end

Use the Todo.Cache implementation to see successful and already-started results mapped to the process returned to callers. section 9.2.3

How Elixir in Action, Third Edition covers it

Cover of Elixir in Action, Third Edition

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Related concepts

DynamicSupervisorused by · DynamicSupervisor is an OTP and Elixir supervisor implementation that starts and supervises child processes dynamically at runtime.
child specificationuses · A child specification is an OTP configuration describing how a supervised child process or application component is started and managed.
memory bounduses · A memory-bound workload is one whose performance is limited primarily by the rate of accessing or transferring data rather than by computational throughput.

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