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A tiny resource-pool implementation for Elixir

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Online Documentation.

is a tiny resource-pool implementation.

Pools in the Erlang VM, and therefore in Elixir, are generally process-based: they manage a group of processes. The downside of said pools is that when they have to manage resources, such as sockets or ports, the additional process leads to overhead.

In such pools, you usually end-up with two scenarios:

  • You invoke the pool manager, which returns the pooled process, which performs the operation on the socket or port for you, returning you the reply. This approach is non-optimal because all of the data sent and returned by the resource needs to be copied between processes

  • You invoke the pool manager, which returns the pooled process, which gives you access to the resource. Then you can act directly on the resource, avoiding the data copying, but you need to keep the state of the resource in sync with the process

NimblePool allows you to implement the second scenario without the addition of processes, which leads to a simpler and more efficient implementation. You should consider using NimblePool whenever you have to manage sockets, ports, or NIF resources and you want the client to perform one-off operations on them. For example, NimblePool is a good solution to manage HTTP 1 connections, ports that need to communicate with long-running programs, etc.

The downside of NimblePool is that, because all resources are under a single process, any resource management operation will happen on this single process, which is more likely to become a bottleneck. This can be addressed, however, by starting one NimblePool per scheduler and by doing scheduler-based dispatches.

NimblePool may not be a good option to manage processes. Also avoid using NimblePool to manage resources that support multiplexing, such as HTTP 2 connections (in fact, pools are not a good option to manage resources with multiplexing in general).

Types of callbacks

NimblePool has two types of callbacks. Worker callbacks and pool callbacks. The worker callbacks configure the behaviour of each worker, such as initialization, checkin and checkout. The pool callbacks configure general pool behaviour, such as initialization, and potentially queueing and dequeuing in the future.


To use

, you must define a module that implements the pool worker logic, outlined in the

Port-based example

The example below keeps ports on the pool and check them out on every command. Please read the docs for

before using the approach below, especially in regards to zombie ports.
defmodule PortPool do
  @behaviour NimblePool

@doc ~S""" Executes a given command against a port kept by the pool.

First we start the port:

  iex> child = {NimblePool, worker: {PortPool, :cat}, name: PortPool}
  iex> Supervisor.start_link([child], strategy: :one_for_one)

Now we can run commands against the pool of ports:

  iex> PortPool.command(PortPool, "hello\n")
  iex> PortPool.command(PortPool, "world\n")

""" def command(pool, command, opts \ []) do pool_timeout = Keyword.get(opts, :pool_timeout, 5000) receive_timeout = Keyword.get(opts, :receive_timeout, 15000)

NimblePool.checkout!(pool, :checkout, fn {pid, _}, port ->
  send(port, {self(), {:command, command}})

  receive do
    {^port, {:data, data}} ->
      try do
        Port.connect(port, pid)
        {data, :ok}
        _ -> {data, :close}
    receive_timeout ->
end, pool_timeout)


@impl NimblePool def init_worker(:cat = pool_state) do path = System.find_executable("cat") port ={:spawn_executable, path}, [:binary, args: ["-"]]) {:ok, port, pool_state} end

@impl NimblePool

Transfer the port to the caller

def handle_checkout(:checkout, {pid, _}, port, pool_state) do Port.connect(port, pid) {:ok, port, port, pool_state} end

@impl NimblePool

We got it back

def handle_checkin(:ok, _from, port, pool_state) do {:ok, port, pool_state} end

def handle_checkin(:close, _from, _port, pool_state) do {:remove, :closed, pool_state} end

@impl NimblePool

On terminate, effectively close it

def terminate_worker(_reason, port, pool_state) do Port.close(port) {:ok, pool_state} end end

HTTP1-based example

The pool below uses Mint for HTTP1 connections. It establishes connections eagerly. A better approach may be to establish connections lazily on checkout, as done by Finch, which is built on top of Mint+NimbleOptions.

defmodule HTTP1Pool do
  @behaviour NimblePool

@doc ~S""" Executes a given command against a connection kept by the pool.

First we start the connection:

  child = {NimblePool, worker: {HTTP1Pool, {:http, "", 80}}, name: HTTP1Pool}
  Supervisor.start_link([child], strategy: :one_for_one)

Then we can access it:

  iex> HTTP1Pool.get(HTTP1Pool, "/foo")
  {:ok, %{...}}

""" def get(pool, path, opts \ []) do pool_timeout = Keyword.get(opts, :pool_timeout, 5000) receive_timeout = Keyword.get(opts, :receive_timeout, 15000)

NimblePool.checkout!(pool, :checkout, fn _from, conn ->
  {kind, conn, result_or_error} =
    with {:ok, conn, ref} 
    response = Map.put(response, kind, value)
    receive_response(entries, conn, ref, response, timeout)

  {:data, ^ref, data} ->
    response = Map.update(response, :data, data, &(&1 <> data))
    receive_response(entries, conn, ref, response, timeout)

  {:done, ^ref} ->
    {:ok, conn, response}

  {:error, ^ref, error} ->
    {:error, conn, error}


@impl NimblePool def init_worker({scheme, host, port} = pool_state) do parent = self()

async = fn ->
  # TODO: Add back-off
  {:ok, conn} = Mint.HTTP1.connect(scheme, host, port, [])
  {:ok, conn} = Mint.HTTP1.controlling_process(conn, parent)

{:async, async, pool_state}


@impl NimblePool

Transfer the conn to the caller.

If we lost the connection, then we remove it to try again.

def handle_checkout(:checkout, {pid, _}, conn, pool_state) do with {:ok, conn} {:remove, :closed, pool_state} end end

@impl NimblePool

We got it back.

def handle_checkin(state, _from, _old_conn, pool_state) do with {:ok, conn} {:remove, :closed, pool_state} end end

@impl NimblePool

If it is closed, drop it.

def handle_info(message, conn) do case, message) do {:ok, _, _} -> {:ok, conn} {:error, _, _, _} -> {:remove, :closed} :unknown -> {:ok, conn} end end

@impl NimblePool

On terminate, effectively close it.

This will succeed even if it was already closed or if we don't own it.

def terminate(_reason, conn, pool_state) do Mint.HTTP1.close(conn) {:ok, pool_state} end end



to your list of dependencies in
def deps do
  [{:nimble_pool, "~> 0.2"}]


All nimble libraries by Dashbit:

  • NimbleCSV - simple and fast CSV parsing
  • NimbleOptions - tiny library for validating and documenting high-level options
  • NimbleParsec - simple and fast parser combinators
  • NimblePool - tiny resource-pool implementation
  • NimblePublisher - a minimal filesystem-based publishing engine with Markdown support and code highlighting
  • NimbleTOTP - tiny library for generating time-based one time passwords (TOTP)


Copyright 2019 Plataformatec \ Copyright 2020 Dashbit

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

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