From 01134b6677a76d4eee11056d19371ecb8f565c57 Mon Sep 17 00:00:00 2001
From: KinutaDev <development.tanuki2024@gmail.com>
Date: Mon, 17 Aug 2026 13:53:27 +0900
Subject: [PATCH 1/5] feat: add DBConnection.SingleConnection for AtomVM

- Add a one-connection pool selected via :pool
- Add tests for checkout, FIFO queue, metrics, and disconnect_all
---
 lib/db_connection.ex                          |   2 +
 lib/db_connection/single_connection.ex        | 238 +++++++++++++++++
 test/db_connection/single_connection_test.exs | 250 ++++++++++++++++++
 test/test_helper.exs                          |   4 +
 4 files changed, 494 insertions(+)
 create mode 100644 lib/db_connection/single_connection.ex
 create mode 100644 test/db_connection/single_connection_test.exs

diff --git a/lib/db_connection.ex b/lib/db_connection.ex
index 58718943..629958bf 100644
--- a/lib/db_connection.ex
+++ b/lib/db_connection.ex
@@ -76,6 +76,8 @@ defmodule DBConnection do
   DBConnection connections support using different pools via the `:pool` option
   passed to `start_link/2`. The default pool is `DBConnection.ConnectionPool`.
   Another supported pool that is commonly used for tests is `DBConnection.Ownership`.
+  `DBConnection.SingleConnection` is a one-connection pool without CoDel
+  queueing or `DynamicSupervisor`, intended for constrained runtimes.
 
   For now, using *custom* pools is not supported since the API for pools is not
   public.
diff --git a/lib/db_connection/single_connection.ex b/lib/db_connection/single_connection.ex
new file mode 100644
index 00000000..204c6025
--- /dev/null
+++ b/lib/db_connection/single_connection.ex
@@ -0,0 +1,238 @@
+defmodule DBConnection.SingleConnection do
+  @moduledoc """
+  A single-connection pool for constrained runtimes.
+
+  Unlike `DBConnection.ConnectionPool`, this pool keeps exactly one connection,
+  does not use CoDel queueing, idle polling, or `DynamicSupervisor`.
+
+  Pass this module as the `:pool` option to `DBConnection.start_link/2`.
+  The default pool remains `DBConnection.ConnectionPool`. `:pool_size` is
+  ignored except that values below 1 raise, same as the default pool.
+  """
+
+  use GenServer
+  alias DBConnection.Holder
+  alias DBConnection.Util
+
+  @behaviour DBConnection.Pool
+
+  @doc false
+  def start_link({mod, opts}) do
+    GenServer.start_link(__MODULE__, {mod, opts}, start_opts(opts))
+  end
+
+  @doc false
+  @impl DBConnection.Pool
+  def checkout(pool, callers, opts) do
+    Holder.checkout(pool, callers, opts)
+  end
+
+  @doc false
+  @impl DBConnection.Pool
+  def disconnect_all(pool, interval, _opts) do
+    GenServer.call(pool, {:disconnect_all, interval}, :infinity)
+  end
+
+  @doc false
+  @impl DBConnection.Pool
+  def get_connection_metrics(pool) do
+    GenServer.call(pool, :get_connection_metrics, :infinity)
+  end
+
+  @impl GenServer
+  def init({mod, opts}) do
+    Process.flag(:trap_exit, true)
+    DBConnection.register_as_pool(mod)
+
+    size = Keyword.get(opts, :pool_size, 1)
+
+    if size < 1 do
+      raise ArgumentError, "pool size must be greater or equal to 1, got #{size}"
+    end
+
+    tag = make_ref()
+
+    state = %{
+      mod: mod,
+      opts: opts,
+      tag: tag,
+      holder: nil,
+      waiters: :queue.new(),
+      ts: {nil, max_lifetime(opts)},
+      conn: nil,
+      restarts: [],
+      max_restarts: Keyword.get(opts, :max_restarts, 3),
+      max_seconds: Keyword.get(opts, :max_seconds, 5)
+    }
+
+    {:ok, pid} = start_connection(state)
+    {:ok, %{state | conn: pid}}
+  end
+
+  @impl GenServer
+  def handle_call(:get_connection_metrics, _from, state) do
+    metrics = %{
+      source: {:pool, self()},
+      ready_conn_count: if(state.holder, do: 1, else: 0),
+      checkout_queue_length: :queue.len(state.waiters)
+    }
+
+    {:reply, [metrics], state}
+  end
+
+  def handle_call({:disconnect_all, interval}, _from, state) do
+    {_, max_lifetime} = state.ts
+    ts = {{System.monotonic_time(), interval}, max_lifetime}
+    {:reply, :ok, %{state | ts: ts}}
+  end
+
+  @impl GenServer
+  def handle_info({:db_connection, from, {:checkout, _callers, _now, queue?}}, state) do
+    {:noreply, checkout_request(from, queue?, state)}
+  end
+
+  def handle_info({:"ETS-TRANSFER", holder, _pid, {msg, tag, extra}}, %{tag: tag} = state) do
+    case msg do
+      :checkin ->
+        owner = self()
+
+        case :ets.info(holder, :owner) do
+          ^owner ->
+            {interval, max_lifetime} = state.ts
+
+            if Holder.maybe_disconnect(holder, interval, max_lifetime) do
+              {:noreply, %{state | holder: nil}}
+            else
+              handle_checkin(holder, extra, state)
+            end
+
+          :undefined ->
+            {:noreply, state}
+        end
+
+      :disconnect ->
+        Holder.handle_disconnect(holder, extra)
+        {:noreply, %{state | holder: nil}}
+
+      :stop ->
+        Holder.handle_stop(holder, extra)
+        {:noreply, %{state | holder: nil}}
+    end
+  end
+
+  def handle_info({:"ETS-TRANSFER", holder, pid, tag}, %{tag: tag} = state) do
+    message = "client #{Util.inspect_pid(pid)} exited"
+    err = DBConnection.ConnectionError.exception(message: message, severity: :info)
+    Holder.handle_disconnect(holder, err)
+    {:noreply, %{state | holder: nil}}
+  end
+
+  def handle_info({:timeout, deadline, {tag, holder, pid, len}}, %{tag: tag} = state) do
+    if Holder.handle_deadline(holder, deadline) do
+      message =
+        "client #{Util.inspect_pid(pid)} timed out because " <>
+          "it queued and checked out the connection for longer than #{len}ms"
+
+      exc =
+        case Process.info(pid, :current_stacktrace) do
+          {:current_stacktrace, stacktrace} ->
+            message <>
+              "\n\n#{Util.inspect_pid(pid)} was at location:\n\n" <>
+              Exception.format_stacktrace(stacktrace)
+
+          _ ->
+            message
+        end
+        |> DBConnection.ConnectionError.exception()
+
+      Holder.handle_disconnect(holder, exc)
+    end
+
+    {:noreply, state}
+  end
+
+  def handle_info({:EXIT, pid, reason}, %{conn: pid} = state) do
+    restart_connection(reason, state)
+  end
+
+  def handle_info({:EXIT, _pid, reason}, state) do
+    {:stop, reason, state}
+  end
+
+  @impl GenServer
+  def terminate(_reason, %{conn: pid}) when is_pid(pid) do
+    Process.exit(pid, :shutdown)
+    :ok
+  end
+
+  def terminate(_reason, _state), do: :ok
+
+  defp checkout_request(from, queue?, %{holder: {holder, checked_in_at}} = state) do
+    if Holder.handle_checkout(holder, from, state.tag, checked_in_at) do
+      %{state | holder: nil}
+    else
+      checkout_request(from, queue?, %{state | holder: nil})
+    end
+  end
+
+  defp checkout_request(from, true, state) do
+    %{state | waiters: :queue.in(from, state.waiters)}
+  end
+
+  defp checkout_request(from, false, state) do
+    message = "connection not available and queuing is disabled"
+    Holder.reply_error(from, DBConnection.ConnectionError.exception(message))
+    state
+  end
+
+  defp handle_checkin(holder, now_in_native, state) do
+    case :queue.out(state.waiters) do
+      {:empty, waiters} ->
+        {:noreply, %{state | holder: {holder, now_in_native}, waiters: waiters}}
+
+      {{:value, from}, waiters} ->
+        state = %{state | waiters: waiters}
+
+        if Holder.handle_checkout(holder, from, state.tag, now_in_native) do
+          {:noreply, %{state | holder: nil}}
+        else
+          handle_checkin(holder, now_in_native, state)
+        end
+    end
+  end
+
+  defp restart_connection(reason, state) do
+    now = System.monotonic_time(:second)
+    cutoff = now - state.max_seconds
+    restarts = Enum.filter(state.restarts, &(&1 >= cutoff))
+
+    if length(restarts) >= state.max_restarts do
+      {:stop, reason, state}
+    else
+      {:ok, pid} = start_connection(state)
+      {:noreply, %{state | conn: pid, holder: nil, restarts: [now | restarts]}}
+    end
+  end
+
+  defp start_connection(%{mod: mod, opts: opts, tag: tag}) do
+    DBConnection.Connection.start_link(mod, Keyword.put(opts, :pool_index, 1), self(), tag)
+  end
+
+  defp max_lifetime(opts) do
+    case Keyword.fetch(opts, :max_lifetime) do
+      {:ok, %Range{first: first, last: last, step: 1}} when first >= 0 and last >= first ->
+        {System.convert_time_unit(first, :millisecond, :native), last - first}
+
+      {:ok, invalid} ->
+        raise ArgumentError,
+              "invalid value for :max_lifetime, expected a non-negative step-1 range, got: #{inspect(invalid)}"
+
+      :error ->
+        nil
+    end
+  end
+
+  defp start_opts(opts) do
+    Keyword.take(opts, [:name, :spawn_opt])
+  end
+end
diff --git a/test/db_connection/single_connection_test.exs b/test/db_connection/single_connection_test.exs
new file mode 100644
index 00000000..35c2a1b0
--- /dev/null
+++ b/test/db_connection/single_connection_test.exs
@@ -0,0 +1,250 @@
+defmodule DBConnection.SingleConnectionTest do
+  use ExUnit.Case, async: true
+
+  alias TestSingleConnection, as: P
+  alias TestAgent, as: A
+  alias TestQuery, as: Q
+  alias TestResult, as: R
+
+  test "start_link workflow with unregistered name" do
+    stack = [{:ok, :state}]
+    {:ok, agent} = A.start_link(stack)
+
+    opts = [agent: agent, pool_size: 1]
+    {:ok, conn} = P.start_link(opts)
+
+    {:links, links} = Process.info(self(), :links)
+    assert conn in links
+
+    _ = :sys.get_state(conn)
+    assert [{:connect, [connect_opts]}] = A.record(agent)
+    assert Keyword.get(connect_opts, :pool_index) == 1
+    assert Keyword.get(connect_opts, :agent) == agent
+  end
+
+  test "start_link workflow with registered name", %{test: name} do
+    stack = [{:ok, :state}]
+    {:ok, agent} = A.start_link(stack)
+
+    opts = [agent: agent, name: name, pool_size: 1]
+    {:ok, conn} = P.start_link(opts)
+
+    assert Process.info(conn, :registered_name) == {:registered_name, name}
+
+    _ = :sys.get_state(conn)
+    assert [{:connect, [connect_opts]}] = A.record(agent)
+    assert Keyword.get(connect_opts, :pool_index) == 1
+  end
+
+  test "connection_module/1 returns the connection module" do
+    {:ok, agent} = A.start_link([{:ok, :state}, {:idle, :state}, {:idle, :state}])
+    {:ok, pool} = P.start_link(agent: agent)
+
+    assert {:ok, TestConnection} = DBConnection.connection_module(pool)
+
+    P.run(pool, fn conn ->
+      assert {:ok, TestConnection} = DBConnection.connection_module(conn)
+    end)
+  end
+
+  test "checkout and checkin via run" do
+    stack = [{:ok, :state}] ++ List.duplicate({:idle, :state}, 8)
+    {:ok, agent} = A.start_link(stack)
+    {:ok, pool} = P.start_link(agent: agent)
+
+    assert P.run(pool, fn _conn -> :result end) == :result
+    assert P.run(pool, fn _conn -> :again end) == :again
+  end
+
+  test "execute after checkout" do
+    stack = [
+      {:ok, :state},
+      {:ok, %Q{}, %R{}, :state},
+      {:ok, %Q{}, %R{}, :state}
+    ]
+
+    {:ok, agent} = A.start_link(stack)
+    {:ok, pool} = P.start_link(agent: agent)
+
+    assert P.execute(pool, %Q{}, [:param1]) == {:ok, %Q{}, %R{}}
+    assert P.execute(pool, %Q{}, [:param2]) == {:ok, %Q{}, %R{}}
+
+    assert [
+             connect: [_],
+             handle_execute: [%Q{}, [:param1], _, :state],
+             handle_execute: [%Q{}, [:param2], _, :state]
+           ] = A.record(agent)
+  end
+
+  test "queue: false raises when the connection is busy" do
+    stack = [{:ok, :state}, {:idle, :state}, {:idle, :state}]
+    {:ok, agent} = A.start_link(stack)
+    {:ok, pool} = P.start_link(agent: agent)
+
+    P.run(pool, fn _ ->
+      {queue_time, _} =
+        :timer.tc(fn ->
+          opts = [queue: false]
+
+          assert_raise DBConnection.ConnectionError,
+                       ~r"connection not available and queuing is disabled",
+                       fn -> P.run(pool, fn _ -> flunk("got connection") end, opts) end
+        end)
+
+      assert queue_time <= 1_000_000, "request was queued"
+    end)
+  end
+
+  test "queues waiters and serves them in FIFO order" do
+    stack = [{:ok, :state}] ++ List.duplicate({:idle, :state}, 20)
+    {:ok, agent} = A.start_link(stack)
+    {:ok, pool} = P.start_link(agent: agent)
+
+    parent = self()
+
+    holder =
+      spawn_link(fn ->
+        Process.put(:agent, agent)
+
+        P.run(pool, fn _ ->
+          send(parent, {:held, self()})
+
+          receive do
+            {:release, ^parent} -> :ok
+          end
+        end)
+      end)
+
+    assert_receive {:held, ^holder}
+
+    first =
+      Task.async(fn ->
+        Process.put(:agent, agent)
+        P.run(pool, fn _ -> send(parent, {:ran, 1}) end)
+        :ok
+      end)
+
+    TestHelpers.poll(fn ->
+      assert [%{checkout_queue_length: 1, ready_conn_count: 0}] =
+               P.get_connection_metrics(pool)
+    end)
+
+    second =
+      Task.async(fn ->
+        Process.put(:agent, agent)
+        P.run(pool, fn _ -> send(parent, {:ran, 2}) end)
+        :ok
+      end)
+
+    TestHelpers.poll(fn ->
+      assert [%{checkout_queue_length: 2, ready_conn_count: 0}] =
+               P.get_connection_metrics(pool)
+    end)
+
+    send(holder, {:release, self()})
+    assert_receive {:ran, 1}
+    assert_receive {:ran, 2}
+
+    assert :ok = Task.await(first)
+    assert :ok = Task.await(second)
+  end
+
+  test "reports connection metrics" do
+    stack = [
+      {:ok, :state},
+      fn _, _, _, _ ->
+        receive do
+          :continue -> {:ok, %Q{}, %R{}, :state}
+        end
+      end
+    ]
+
+    {:ok, agent} = A.start_link(stack)
+    {:ok, pool} = P.start_link(agent: agent)
+
+    TestHelpers.poll(fn ->
+      assert [%{source: {:pool, ^pool}, checkout_queue_length: 0, ready_conn_count: 1}] =
+               P.get_connection_metrics(pool)
+    end)
+
+    query =
+      spawn_link(fn ->
+        Process.put(:agent, agent)
+        assert P.execute(pool, %Q{}, [:client])
+      end)
+
+    TestHelpers.poll(fn ->
+      assert [%{source: {:pool, ^pool}, checkout_queue_length: 0, ready_conn_count: 0}] =
+               P.get_connection_metrics(pool)
+    end)
+
+    send(query, :continue)
+
+    TestHelpers.poll(fn ->
+      assert [%{source: {:pool, ^pool}, checkout_queue_length: 0, ready_conn_count: 1}] =
+               P.get_connection_metrics(pool)
+    end)
+  end
+
+  test "disconnect_all disconnects on checkin" do
+    stack = [
+      {:ok, :state},
+      {:ok, %Q{}, %R{}, :new_state1},
+      {:ok, %Q{}, %R{}, :new_state2},
+      :ok,
+      {:ok, :final_state},
+      {:ok, %Q{}, %R{}, :final_state1},
+      {:ok, %Q{}, %R{}, :final_state2}
+    ]
+
+    {:ok, agent} = A.start_link(stack)
+    {:ok, pool} = P.start_link(agent: agent)
+
+    assert P.execute(pool, %Q{}, [:param]) == {:ok, %Q{}, %R{}}
+    P.disconnect_all(pool, 0)
+    assert P.execute(pool, %Q{}, [:param]) == {:ok, %Q{}, %R{}}
+    assert P.execute(pool, %Q{}, [:param]) == {:ok, %Q{}, %R{}}
+    assert P.execute(pool, %Q{}, [:param]) == {:ok, %Q{}, %R{}}
+
+    err = %DBConnection.ConnectionError{message: "disconnect_all requested", severity: :debug}
+
+    assert [
+             connect: [_],
+             handle_execute: [_, _, _, :state],
+             handle_execute: [_, _, _, :new_state1],
+             disconnect: [^err, :new_state2],
+             connect: [_],
+             handle_execute: [_, _, _, :final_state],
+             handle_execute: [_, _, _, :final_state1]
+           ] = A.record(agent)
+  end
+
+  test "reconnects when the client exits" do
+    stack = [
+      {:ok, :state},
+      {:idle, :state},
+      :ok,
+      fn opts ->
+        send(opts[:parent], :reconnected)
+        {:ok, :state}
+      end,
+      {:idle, :state},
+      {:idle, :state}
+    ]
+
+    {:ok, agent} = A.start_link(stack)
+    {:ok, pool} = P.start_link(agent: agent, parent: self())
+
+    _ =
+      spawn(fn ->
+        _ = Process.put(:agent, agent)
+
+        P.run(pool, fn _ ->
+          Process.exit(self(), :shutdown)
+        end)
+      end)
+
+    assert_receive :reconnected
+    assert P.run(pool, fn _ -> :result end) == :result
+  end
+end
diff --git a/test/test_helper.exs b/test/test_helper.exs
index 989768b2..1c6ee2ee 100644
--- a/test/test_helper.exs
+++ b/test/test_helper.exs
@@ -5,3 +5,7 @@ Code.require_file("test_support.exs", __DIR__)
 defmodule TestPool do
   use TestConnection, pool: DBConnection.ConnectionPool, pool_size: 1
 end
+
+defmodule TestSingleConnection do
+  use TestConnection, pool: DBConnection.SingleConnection, pool_size: 1
+end

From 3f860a3b7c3040e8e2e6ee17e44a8701fe9d0973 Mon Sep 17 00:00:00 2001
From: KinutaDev <development.tanuki2024@gmail.com>
Date: Mon, 17 Aug 2026 17:06:59 +0900
Subject: [PATCH 2/5] feat: adapt DBConnection.Holder for AtomVM ETS

- Change to use set tables instead of ordered_set
- Change to send ETS-TRANSFER when give_away is missing
- Change to look up owner without ets.info as a fallback
---
 lib/db_connection/connection_pool.ex   |  2 +-
 lib/db_connection/holder.ex            | 78 ++++++++++++++++++++++----
 lib/db_connection/single_connection.ex |  2 +-
 3 files changed, 68 insertions(+), 14 deletions(-)

diff --git a/lib/db_connection/connection_pool.ex b/lib/db_connection/connection_pool.ex
index e760a46e..989a9e13 100644
--- a/lib/db_connection/connection_pool.ex
+++ b/lib/db_connection/connection_pool.ex
@@ -161,7 +161,7 @@ defmodule DBConnection.ConnectionPool do
       :checkin ->
         owner = self()
 
-        case :ets.info(holder, :owner) do
+        case Holder.owner(holder) do
           ^owner ->
             {interval, max_lifetime} = ts
 
diff --git a/lib/db_connection/holder.ex b/lib/db_connection/holder.ex
index 0d69a51d..8a31a769 100644
--- a/lib/db_connection/holder.ex
+++ b/lib/db_connection/holder.ex
@@ -15,7 +15,8 @@ defmodule DBConnection.Holder do
     :lock,
     :connected_at,
     deadline: nil,
-    status: :ok
+    status: :ok,
+    owner: nil
   ])
 
   Record.defrecord(:pool_ref, [:pool, :reference, :deadline, :holder, :lock])
@@ -28,13 +29,21 @@ defmodule DBConnection.Holder do
   @spec new(pid, reference, module, term) :: t
   @spec new(pid, reference, module, term, integer) :: t
   def new(pool, ref, mod, state, connected_at \\ System.monotonic_time()) do
-    # Insert before setting heir so that pool can't receive empty table
-    holder = :ets.new(__MODULE__, [:public, :ordered_set, decentralized_counters: true])
+    # Insert before setting heir so that pool can't receive empty table.
+    # AtomVM has no ordered_set; this table holds a single :conn row so :set is enough.
+    holder = :ets.new(__MODULE__, [:public, :set, decentralized_counters: true])
+
+    conn =
+      conn(
+        connection: self(),
+        module: mod,
+        state: state,
+        connected_at: connected_at,
+        owner: self()
+      )
 
-    conn = conn(connection: self(), module: mod, state: state, connected_at: connected_at)
     true = :ets.insert_new(holder, conn)
-
-    :ets.setopts(holder, {:heir, pool, ref})
+    maybe_set_heir(holder, pool, ref)
     holder
   end
 
@@ -44,7 +53,7 @@ defmodule DBConnection.Holder do
     holder = new(pool, ref, mod, state, connected_at)
 
     try do
-      :ets.give_away(holder, pool, {:checkin, ref, System.monotonic_time()})
+      transfer(holder, pool, {:checkin, ref, System.monotonic_time()})
       {:ok, holder}
     rescue
       ArgumentError -> :error
@@ -207,12 +216,25 @@ defmodule DBConnection.Holder do
     :ok
   end
 
+  @spec owner(t) :: pid | :undefined
+  def owner(holder) do
+    if function_exported?(:ets, :info, 2) do
+      :ets.info(holder, :owner)
+    else
+      try do
+        :ets.lookup_element(holder, :conn, conn(:owner) + 1)
+      rescue
+        ArgumentError -> :undefined
+      end
+    end
+  end
+
   @spec handle_checkout(t, {pid, reference}, reference, checkin_time) :: boolean
   def handle_checkout(holder, {pid, mref}, ref, checkin_time) do
-    :ets.give_away(holder, pid, {mref, ref, checkin_time})
+    transfer(holder, pid, {mref, ref, checkin_time})
   rescue
     ArgumentError ->
-      if Process.alive?(pid) or :ets.info(holder, :owner) != self() do
+      if Process.alive?(pid) or owner(holder) != self() do
         raise ArgumentError, no_holder(holder, pid)
       else
         false
@@ -362,7 +384,7 @@ defmodule DBConnection.Holder do
 
   defp no_holder(holder, maybe_pid) do
     reason =
-      case :ets.info(holder, :owner) do
+      case owner(holder) do
         :undefined -> "does not exist"
         ^maybe_pid -> "is being given to its current owner"
         owner when owner != self() -> "does not belong to the giving process"
@@ -382,7 +404,7 @@ defmodule DBConnection.Holder do
     #{call_reason}.
 
     SELF: #{Util.inspect_pid(self())}
-    ETS INFO: #{inspect(:ets.info(holder))}
+    ETS INFO: #{inspect(ets_info(holder))}
 
     Please report at https://github.com/elixir-ecto/db_connection/issues"
     """
@@ -431,7 +453,7 @@ defmodule DBConnection.Holder do
 
     try do
       :ets.update_element(holder, :conn, [{conn(:deadline) + 1, nil} | ops])
-      :ets.give_away(holder, pool, {tag, ref, info})
+      transfer(holder, pool, {tag, ref, info})
     rescue
       ArgumentError -> :ok
     else
@@ -485,4 +507,36 @@ defmodule DBConnection.Holder do
     hash = :erlang.phash2(pid, interval_ms)
     System.convert_time_unit(hash, :millisecond, :native)
   end
+
+  defp maybe_set_heir(holder, pool, ref) do
+    if function_exported?(:ets, :setopts, 2) do
+      :ets.setopts(holder, {:heir, pool, ref})
+    else
+      :ok
+    end
+  end
+
+  defp transfer(table, pid, gift_data) do
+    if function_exported?(:ets, :give_away, 3) do
+      :ets.give_away(table, pid, gift_data)
+    else
+      _ = put_owner(table, pid)
+      send(pid, {:"ETS-TRANSFER", table, self(), gift_data})
+      true
+    end
+  end
+
+  defp put_owner(table, pid) do
+    :ets.update_element(table, :conn, {conn(:owner) + 1, pid})
+  rescue
+    ArgumentError -> false
+  end
+
+  defp ets_info(holder) do
+    if function_exported?(:ets, :info, 1) do
+      :ets.info(holder)
+    else
+      %{owner: owner(holder)}
+    end
+  end
 end
diff --git a/lib/db_connection/single_connection.ex b/lib/db_connection/single_connection.ex
index 204c6025..d541a4f0 100644
--- a/lib/db_connection/single_connection.ex
+++ b/lib/db_connection/single_connection.ex
@@ -96,7 +96,7 @@ defmodule DBConnection.SingleConnection do
       :checkin ->
         owner = self()
 
-        case :ets.info(holder, :owner) do
+        case Holder.owner(holder) do
           ^owner ->
             {interval, max_lifetime} = state.ts
 

From ab4a894f1c69f384e03b45d31b815e85fec155fb Mon Sep 17 00:00:00 2001
From: KinutaDev <development.tanuki2024@gmail.com>
Date: Mon, 17 Aug 2026 18:49:40 +0900
Subject: [PATCH 3/5] feat: run DBConnection.Connection as a GenServer - Change
 to use GenServer instead of gen_statem handle_event_function - Change connect
 and disconnect to handle_continue - Change backoff from a gen_statem named
 timeout to Process.send_after - Change pool check-in to return without
 hibernate

---
 lib/db_connection/connection.ex | 178 +++++++++++++++-----------------
 1 file changed, 83 insertions(+), 95 deletions(-)

diff --git a/lib/db_connection/connection.ex b/lib/db_connection/connection.ex
index 5b6abf7a..55fb28a3 100644
--- a/lib/db_connection/connection.ex
+++ b/lib/db_connection/connection.ex
@@ -1,7 +1,7 @@
 defmodule DBConnection.Connection do
   @moduledoc false
 
-  @behaviour :gen_statem
+  use GenServer
 
   require Logger
   alias DBConnection.Backoff
@@ -14,7 +14,7 @@ defmodule DBConnection.Connection do
   def start_link(mod, opts, pool, tag) do
     start_opts = Keyword.take(opts, [:debug, :spawn_opt])
     sensitive_options = %DBConnection.SensitiveData{data: opts}
-    :gen_statem.start_link(__MODULE__, {mod, sensitive_options, pool, tag}, start_opts)
+    GenServer.start_link(__MODULE__, {mod, sensitive_options, pool, tag}, start_opts)
   end
 
   @doc false
@@ -27,27 +27,20 @@ defmodule DBConnection.Connection do
 
   @doc false
   def disconnect({pid, ref}, err, state) do
-    :gen_statem.cast(pid, {:disconnect, ref, err, state})
+    GenServer.cast(pid, {:disconnect, ref, err, state})
   end
 
   @doc false
   def stop({pid, ref}, err, state) do
-    :gen_statem.cast(pid, {:stop, ref, err, state})
+    GenServer.cast(pid, {:stop, ref, err, state})
   end
 
   @doc false
   def ping({pid, ref}, state) do
-    :gen_statem.cast(pid, {:ping, ref, state})
+    GenServer.cast(pid, {:ping, ref, state})
   end
 
-  ## gen_statem API
-
-  @doc false
-  @impl :gen_statem
-  def callback_mode, do: :handle_event_function
-
-  @doc false
-  @impl :gen_statem
+  @impl GenServer
   def init({mod, %DBConnection.SensitiveData{data: opts}, pool, tag}) do
     pool_index = Keyword.get(opts, :pool_index)
     label = if pool_index, do: "db_conn_#{pool_index}", else: "db_conn"
@@ -61,6 +54,7 @@ defmodule DBConnection.Connection do
       pool: pool,
       tag: tag,
       timer: nil,
+      backoff_ref: nil,
       connected_at: nil,
       backoff: Backoff.new(opts),
       connection_listeners: Keyword.get(opts, :connection_listeners, []),
@@ -68,13 +62,11 @@ defmodule DBConnection.Connection do
       after_connect_timeout: Keyword.get(opts, :after_connect_timeout, @timeout)
     }
 
-    {:ok, :no_state, s, {:next_event, :internal, {:connect, :init}}}
+    {:ok, s, {:continue, {:connect, :init}}}
   end
 
-  @impl :gen_statem
-  def handle_event(type, info, state, s)
-
-  def handle_event(:internal, {:connect, _info}, :no_state, s) do
+  @impl GenServer
+  def handle_continue({:connect, _info}, s) do
     %{mod: mod, opts: opts, backoff: backoff, after_connect: after_connect} = s
 
     try do
@@ -87,16 +79,19 @@ defmodule DBConnection.Connection do
       {:ok, state} when after_connect != nil ->
         ref = make_ref()
         connected_at = System.monotonic_time()
-        :gen_statem.cast(self(), {:after_connect, ref})
-        {:keep_state, %{s | state: state, client: {ref, :connect}, connected_at: connected_at}}
+        GenServer.cast(self(), {:after_connect, ref})
+        s = cancel_backoff(s)
+
+        {:noreply, %{s | state: state, client: {ref, :connect}, connected_at: connected_at}}
 
       {:ok, state} ->
         backoff = backoff && Backoff.reset(backoff)
         ref = make_ref()
         connected_at = System.monotonic_time()
-        :gen_statem.cast(self(), {:connected, ref})
+        GenServer.cast(self(), {:connected, ref})
+        s = cancel_backoff(s)
 
-        {:keep_state,
+        {:noreply,
          %{
            s
            | state: state,
@@ -135,12 +130,11 @@ defmodule DBConnection.Connection do
           crash_reason: {err, []}
         )
 
-        {timeout, backoff} = Backoff.backoff(backoff)
-        {:keep_state, %{s | backoff: backoff}, {{:timeout, :backoff}, timeout, nil}}
+        {:noreply, schedule_backoff(s)}
     end
   end
 
-  def handle_event(:internal, {:disconnect, {log, err}}, :no_state, %{mod: mod} = s) do
+  def handle_continue({:disconnect, {log, err}}, %{mod: mod} = s) do
     if log == :log do
       severity =
         case err do
@@ -174,33 +168,29 @@ defmodule DBConnection.Connection do
         {:stop, {:shutdown, err}, s}
 
       {_, :after_connect} ->
-        {timeout, backoff} = Backoff.backoff(backoff)
-        {:keep_state, %{s | backoff: backoff}, {{:timeout, :backoff}, timeout, nil}}
+        {:noreply, schedule_backoff(s)}
 
       _ ->
-        {:keep_state, s, {:next_event, :internal, {:connect, :disconnect}}}
+        {:noreply, s, {:continue, {:connect, :disconnect}}}
     end
   end
 
-  def handle_event({:timeout, :backoff}, _content, :no_state, s) do
-    {:keep_state, s, {:next_event, :internal, {:connect, :backoff}}}
-  end
-
-  def handle_event(:cast, {:ping, ref, state}, :no_state, %{client: {ref, :pool}, mod: mod} = s) do
+  @impl GenServer
+  def handle_cast({:ping, ref, state}, %{client: {ref, :pool}, mod: mod} = s) do
     case apply(mod, :ping, [state]) do
       {:ok, state} ->
         pool_update(state, s)
 
       {:disconnect, err, state} ->
-        {:keep_state, %{s | state: state}, {:next_event, :internal, {:disconnect, {:log, err}}}}
+        {:noreply, %{s | state: state}, {:continue, {:disconnect, {:log, err}}}}
     end
   end
 
-  def handle_event(:cast, {:disconnect, ref, err, state}, :no_state, %{client: {ref, _}} = s) do
-    {:keep_state, %{s | state: state}, {:next_event, :internal, {:disconnect, {:log, err}}}}
+  def handle_cast({:disconnect, ref, err, state}, %{client: {ref, _}} = s) do
+    {:noreply, %{s | state: state}, {:continue, {:disconnect, {:log, err}}}}
   end
 
-  def handle_event(:cast, {:stop, ref, err, state}, :no_state, %{client: {ref, _}} = s) do
+  def handle_cast({:stop, ref, err, state}, %{client: {ref, _}} = s) do
     {_, stack} = :erlang.process_info(self(), :current_stacktrace)
 
     case err do
@@ -228,11 +218,11 @@ defmodule DBConnection.Connection do
     {:stop, {err, stack}, %{s | state: state}}
   end
 
-  def handle_event(:cast, {tag, _, _, _}, :no_state, s) when tag in [:disconnect, :stop] do
-    handle_timeout(s)
+  def handle_cast({tag, _, _, _}, s) when tag in [:disconnect, :stop] do
+    {:noreply, s}
   end
 
-  def handle_event(:cast, {:after_connect, ref}, :no_state, %{client: {ref, :connect}} = s) do
+  def handle_cast({:after_connect, ref}, %{client: {ref, :connect}} = s) do
     %{
       mod: mod,
       state: state,
@@ -249,18 +239,18 @@ defmodule DBConnection.Connection do
         {pid, ref} = DBConnection.Task.run_child(mod, state, after_connect, opts)
         timer = start_timer(pid, timeout)
         s = %{s | client: {ref, :after_connect}, timer: timer, state: state}
-        {:keep_state, s}
+        {:noreply, s}
 
       {:disconnect, err, state} ->
-        {:keep_state, %{s | state: state}, {:next_event, :internal, {:disconnect, {:log, err}}}}
+        {:noreply, %{s | state: state}, {:continue, {:disconnect, {:log, err}}}}
     end
   end
 
-  def handle_event(:cast, {:after_connect, _}, :no_state, _s) do
-    :keep_state_and_data
+  def handle_cast({:after_connect, _}, s) do
+    {:noreply, s}
   end
 
-  def handle_event(:cast, {:connected, ref}, :no_state, %{client: {ref, :connect}} = s) do
+  def handle_cast({:connected, ref}, %{client: {ref, :connect}} = s) do
     %{mod: mod, state: state} = s
 
     notify_connection_listeners(:connected, s)
@@ -270,45 +260,39 @@ defmodule DBConnection.Connection do
         pool_update(state, s)
 
       {:disconnect, err, state} ->
-        {:keep_state, %{s | state: state}, {:next_event, :internal, {:disconnect, {:log, err}}}}
+        {:noreply, %{s | state: state}, {:continue, {:disconnect, {:log, err}}}}
     end
   end
 
-  def handle_event(:cast, {:connected, _}, :no_state, _s) do
-    :keep_state_and_data
+  def handle_cast({:connected, _}, s) do
+    {:noreply, s}
   end
 
-  def handle_event(
-        :info,
-        {:DOWN, ref, _, pid, reason},
-        :no_state,
-        %{client: {ref, :after_connect}} = s
-      ) do
+  @impl GenServer
+  def handle_info(:backoff, s) do
+    {:noreply, %{s | backoff_ref: nil}, {:continue, {:connect, :backoff}}}
+  end
+
+  def handle_info({:DOWN, ref, _, pid, reason}, %{client: {ref, :after_connect}} = s) do
     message =
       "client #{Util.inspect_pid(pid)} exited: " <> Exception.format_exit(reason)
 
     err = DBConnection.ConnectionError.exception(message)
 
-    {:keep_state, %{s | client: {nil, :after_connect}},
-     {:next_event, :internal, {:disconnect, {down_log(reason), err}}}}
+    {:noreply, %{s | client: {nil, :after_connect}},
+     {:continue, {:disconnect, {down_log(reason), err}}}}
   end
 
-  def handle_event(:info, {:DOWN, mon, _, pid, reason}, :no_state, %{client: {ref, mon}} = s) do
+  def handle_info({:DOWN, mon, _, pid, reason}, %{client: {ref, mon}} = s) do
     message =
       "client #{Util.inspect_pid(pid)} exited: " <> Exception.format_exit(reason)
 
     err = DBConnection.ConnectionError.exception(message)
 
-    {:keep_state, %{s | client: {ref, nil}},
-     {:next_event, :internal, {:disconnect, {down_log(reason), err}}}}
+    {:noreply, %{s | client: {ref, nil}}, {:continue, {:disconnect, {down_log(reason), err}}}}
   end
 
-  def handle_event(
-        :info,
-        {:timeout, timer, {__MODULE__, pid, timeout}},
-        :no_state,
-        %{timer: timer} = s
-      )
+  def handle_info({:timeout, timer, {__MODULE__, pid, timeout}}, %{timer: timer} = s)
       when is_reference(timer) do
     message =
       "client #{Util.inspect_pid(pid)} timed out because it checked out " <>
@@ -326,13 +310,11 @@ defmodule DBConnection.Connection do
       end
       |> DBConnection.ConnectionError.exception()
 
-    {:keep_state, %{s | timer: nil}, {:next_event, :internal, {:disconnect, {:log, exc}}}}
+    {:noreply, %{s | timer: nil}, {:continue, {:disconnect, {:log, exc}}}}
   end
 
-  def handle_event(
-        :info,
+  def handle_info(
         {:"ETS-TRANSFER", holder, _pid, {msg, ref, extra}},
-        :no_state,
         %{client: {ref, :after_connect}, timer: timer} = s
       ) do
     {_, state} = Holder.delete(holder)
@@ -341,31 +323,27 @@ defmodule DBConnection.Connection do
 
     case msg do
       :checkin -> handle_checkin(state, s)
-      :disconnect -> handle_event(:cast, {:disconnect, ref, extra, state}, :no_state, s)
-      :stop -> handle_event(:cast, {:stop, ref, extra, state}, :no_state, s)
+      :disconnect -> handle_cast({:disconnect, ref, extra, state}, s)
+      :stop -> handle_cast({:stop, ref, extra, state}, s)
     end
   end
 
-  # We discard EXIT messages which may arrive if the process is trapping exits
-  def handle_event(:info, {:EXIT, _, _}, :no_state, s) do
-    handle_timeout(s)
+  def handle_info({:EXIT, _, _}, s) do
+    {:noreply, s}
   end
 
-  def handle_event(:info, msg, :no_state, %{mod: mod} = s) do
+  def handle_info(msg, %{mod: mod} = s) do
     Logger.info(fn ->
       [inspect(mod), ?\s, ?(, Util.inspect_pid(self()), ") missed message: " | inspect(msg)]
     end)
 
-    handle_timeout(s)
+    {:noreply, s}
   end
 
-  @doc false
-  @impl :gen_statem
-  # If client is :closed then the connection was previously disconnected
-  # and cleanup is not required.
-  def terminate(_, _, %{client: :closed}), do: :ok
+  @impl GenServer
+  def terminate(_reason, %{client: :closed}), do: :ok
 
-  def terminate(reason, _, s) do
+  def terminate(reason, s) do
     %{mod: mod, state: state} = s
     msg = "connection exited: " <> Exception.format_exit(reason)
 
@@ -374,16 +352,16 @@ defmodule DBConnection.Connection do
     |> mod.disconnect(state)
   end
 
-  @doc false
-  @impl :gen_statem
-  def format_status(info, [_, :no_state, %{client: :closed, mod: mod}]) do
-    case info do
-      :normal -> [{:data, [{~c"Module", mod}]}]
-      :terminate -> mod
-    end
+  @impl GenServer
+  def format_status(:normal, [_pdict, %{client: :closed, mod: mod}]) do
+    [{:data, [{~c"Module", mod}]}]
+  end
+
+  def format_status(:terminate, [_pdict, %{client: :closed, mod: mod}]) do
+    mod
   end
 
-  def format_status(info, [pdict, :no_state, %{mod: mod, state: state}]) do
+  def format_status(info, [pdict, %{mod: mod, state: state}]) do
     case function_exported?(mod, :format_status, 2) do
       true when info == :normal ->
         normal_status(mod, pdict, state)
@@ -399,8 +377,6 @@ defmodule DBConnection.Connection do
     end
   end
 
-  ## Helpers
-
   defp maybe_sanitize_exception(e, stack, opts) do
     if Keyword.get(opts, :show_sensitive_data_on_connection_error, false) do
       {e, stack}
@@ -436,8 +412,6 @@ defmodule DBConnection.Connection do
   defp down_log({:shutdown, _}), do: :nolog
   defp down_log(_), do: :log
 
-  defp handle_timeout(s), do: {:keep_state, s}
-
   defp demonitor({_, mon}) when is_reference(mon) do
     Process.demonitor(mon, [:flush])
   end
@@ -474,6 +448,20 @@ defmodule DBConnection.Connection do
     end
   end
 
+  defp schedule_backoff(%{backoff: backoff} = s) do
+    {timeout, backoff} = Backoff.backoff(backoff)
+    s = cancel_backoff(s)
+    ref = Process.send_after(self(), :backoff, timeout)
+    %{s | backoff: backoff, backoff_ref: ref}
+  end
+
+  defp cancel_backoff(%{backoff_ref: ref} = s) when is_reference(ref) do
+    _ = Process.cancel_timer(ref)
+    %{s | backoff_ref: nil}
+  end
+
+  defp cancel_backoff(s), do: s
+
   defp handle_checkin(state, s) do
     %{backoff: backoff, client: client} = s
     backoff = backoff && Backoff.reset(backoff)
@@ -484,7 +472,7 @@ defmodule DBConnection.Connection do
   defp pool_update(state, %{pool: pool, tag: tag, mod: mod, connected_at: connected_at} = s) do
     case Holder.update(pool, tag, mod, state, connected_at) do
       {:ok, ref} ->
-        {:keep_state, %{s | client: {ref, :pool}, state: state}, :hibernate}
+        {:noreply, %{s | client: {ref, :pool}, state: state}}
 
       :error ->
         {:stop, {:shutdown, :no_more_pool}, s}

From 2dcae802a11982be5953ab129a1340082b17ab5d Mon Sep 17 00:00:00 2001
From: KinutaDev <development.tanuki2024@gmail.com>
Date: Mon, 17 Aug 2026 18:56:17 +0900
Subject: [PATCH 4/5] feat: replace DynamicSupervisor and Task.Supervisor for
 AtomVM

- Change to add DBConnection.DynamicSupervisor, which starts children later via Supervisor
- Change to add DBConnection.TaskSupervisor, which starts after_connect processes via Supervisor
- Change App, Watcher, and Ownership to start children through those modules
---
 lib/db_connection/app.ex                | 15 ++++------
 lib/db_connection/dynamic_supervisor.ex | 34 +++++++++++++++++++++
 lib/db_connection/ownership/manager.ex  |  2 +-
 lib/db_connection/task.ex               |  2 +-
 lib/db_connection/task_supervisor.ex    | 39 +++++++++++++++++++++++++
 lib/db_connection/watcher.ex            |  2 +-
 test/db_connection/watcher_test.exs     |  4 +--
 7 files changed, 83 insertions(+), 15 deletions(-)
 create mode 100644 lib/db_connection/dynamic_supervisor.ex
 create mode 100644 lib/db_connection/task_supervisor.ex

diff --git a/lib/db_connection/app.ex b/lib/db_connection/app.ex
index 5bf64664..3927609c 100644
--- a/lib/db_connection/app.ex
+++ b/lib/db_connection/app.ex
@@ -5,19 +5,14 @@ defmodule DBConnection.App do
   @impl true
   def start(_type, _args) do
     children = [
-      {Task.Supervisor, name: DBConnection.Task},
-      dynamic_supervisor(DBConnection.Ownership.Supervisor),
-      dynamic_supervisor(DBConnection.ConnectionPool.Supervisor),
+      {DBConnection.TaskSupervisor, name: DBConnection.Task},
+      {DBConnection.DynamicSupervisor,
+       name: DBConnection.Ownership.Supervisor, strategy: :one_for_one},
+      {DBConnection.DynamicSupervisor,
+       name: DBConnection.ConnectionPool.Supervisor, strategy: :one_for_one},
       DBConnection.Watcher
     ]
 
     Supervisor.start_link(children, strategy: :one_for_all, name: __MODULE__)
   end
-
-  defp dynamic_supervisor(name) do
-    Supervisor.child_spec(
-      {DynamicSupervisor, name: name, strategy: :one_for_one},
-      id: name
-    )
-  end
 end
diff --git a/lib/db_connection/dynamic_supervisor.ex b/lib/db_connection/dynamic_supervisor.ex
new file mode 100644
index 00000000..a173ecb8
--- /dev/null
+++ b/lib/db_connection/dynamic_supervisor.ex
@@ -0,0 +1,34 @@
+defmodule DBConnection.DynamicSupervisor do
+  @moduledoc false
+
+  use Supervisor
+
+  def child_spec(opts) do
+    name = Keyword.fetch!(opts, :name)
+
+    %{
+      id: name,
+      start: {__MODULE__, :start_link, [opts]},
+      type: :supervisor
+    }
+  end
+
+  def start_link(opts) do
+    name = Keyword.fetch!(opts, :name)
+    Supervisor.start_link(__MODULE__, opts, name: name)
+  end
+
+  def start_child(supervisor, child_spec) do
+    spec = Supervisor.child_spec(child_spec, id: make_ref())
+    Supervisor.start_child(supervisor, spec)
+  end
+
+  def which_children(supervisor) do
+    Supervisor.which_children(supervisor)
+  end
+
+  @impl true
+  def init(opts) do
+    Supervisor.init([], strategy: Keyword.get(opts, :strategy, :one_for_one))
+  end
+end
diff --git a/lib/db_connection/ownership/manager.ex b/lib/db_connection/ownership/manager.ex
index a6f31031..cd5f07af 100644
--- a/lib/db_connection/ownership/manager.ex
+++ b/lib/db_connection/ownership/manager.ex
@@ -258,7 +258,7 @@ defmodule DBConnection.Ownership.Manager do
     %{pool: pool, checkouts: checkouts, owners: owners, ets: ets, log: log, mode: mode} = state
 
     {:ok, proxy} =
-      DynamicSupervisor.start_child(
+      DBConnection.DynamicSupervisor.start_child(
         DBConnection.Ownership.Supervisor,
         {DBConnection.Ownership.Proxy, {caller, pool, opts}}
       )
diff --git a/lib/db_connection/task.ex b/lib/db_connection/task.ex
index 6d6671f7..acd2e8b7 100644
--- a/lib/db_connection/task.ex
+++ b/lib/db_connection/task.ex
@@ -6,7 +6,7 @@ defmodule DBConnection.Task do
 
   def run_child(mod, state, fun, opts) do
     arg = [fun, self(), %DBConnection.SensitiveData{data: opts}]
-    {:ok, pid} = Task.Supervisor.start_child(@name, __MODULE__, :init, arg)
+    {:ok, pid} = DBConnection.TaskSupervisor.start_child(@name, __MODULE__, :init, arg)
     ref = Process.monitor(pid)
     _ = DBConnection.Holder.update(pid, ref, mod, state)
     {pid, ref}
diff --git a/lib/db_connection/task_supervisor.ex b/lib/db_connection/task_supervisor.ex
new file mode 100644
index 00000000..ad74618f
--- /dev/null
+++ b/lib/db_connection/task_supervisor.ex
@@ -0,0 +1,39 @@
+defmodule DBConnection.TaskSupervisor do
+  @moduledoc false
+
+  use Supervisor
+
+  def child_spec(opts) do
+    name = Keyword.fetch!(opts, :name)
+
+    %{
+      id: name,
+      start: {__MODULE__, :start_link, [opts]},
+      type: :supervisor
+    }
+  end
+
+  def start_link(opts) do
+    name = Keyword.fetch!(opts, :name)
+    Supervisor.start_link(__MODULE__, :ok, name: name)
+  end
+
+  def start_child(supervisor, module, function_name, args) do
+    spec = %{
+      id: make_ref(),
+      start: {__MODULE__, :start_task, [module, function_name, args]},
+      restart: :temporary
+    }
+
+    Supervisor.start_child(supervisor, spec)
+  end
+
+  def start_task(module, function_name, args) do
+    {:ok, :proc_lib.spawn_link(module, function_name, args)}
+  end
+
+  @impl true
+  def init(:ok) do
+    Supervisor.init([], strategy: :one_for_one)
+  end
+end
diff --git a/lib/db_connection/watcher.ex b/lib/db_connection/watcher.ex
index 547add0a..eb9761b4 100644
--- a/lib/db_connection/watcher.ex
+++ b/lib/db_connection/watcher.ex
@@ -24,7 +24,7 @@ defmodule DBConnection.Watcher do
 
   @impl true
   def handle_call({:watch, supervisor, args}, {caller_pid, _ref}, {caller_refs, started_refs}) do
-    case DynamicSupervisor.start_child(supervisor, args) do
+    case DBConnection.DynamicSupervisor.start_child(supervisor, args) do
       {:ok, started_pid} ->
         Process.link(caller_pid)
         caller_ref = Process.monitor(caller_pid)
diff --git a/test/db_connection/watcher_test.exs b/test/db_connection/watcher_test.exs
index f31a718b..be4be399 100644
--- a/test/db_connection/watcher_test.exs
+++ b/test/db_connection/watcher_test.exs
@@ -29,7 +29,7 @@ defmodule DBConnection.WatcherTest do
     # Find the pool sup for pool1 by matching the owner pid in child spec ids.
     pool_sup_ref =
       DBConnection.ConnectionPool.Supervisor
-      |> DynamicSupervisor.which_children()
+      |> DBConnection.DynamicSupervisor.which_children()
       |> Enum.find_value(fn {_, sup_pid, _, _} ->
         try do
           has_pool1? =
@@ -54,7 +54,7 @@ defmodule DBConnection.WatcherTest do
         C.start_link(agent: agent2, pool_size: 1, idle_interval: 100_000)
       end)
 
-    # If the DynamicSupervisor or the watcher is blocked, this hangs
+    # If the supervisor or the watcher is blocked, this hangs
     assert {:ok, _pool2} = Task.await(task, 1_000)
 
     # Ensure the pool supervisor actually terminates

From 553fd57b23a5fc644626c9b8fb2bfd877aade654 Mon Sep 17 00:00:00 2001
From: BlackTanuTanuNinjas <challenge8tanu@gmail.com>
Date: Tue, 15 Sep 2026 18:47:27 +0900
Subject: [PATCH 5/5] feat: fold SingleConnection into ConnectionPool and
 harden Holder

- Change to absorb single-connection pool behavior into ConnectionPool
- Change to delete DBConnection.SingleConnection and ConnectionPool.Pool
- Change to soft-give-away holders with heir reclaim on client down
- Change to use relative timers and millisecond monotonic units
- Change to log connection stop via :erlang.display instead of error_logger
---
 lib/db_connection.ex                          |   2 -
 lib/db_connection/connection.ex               |   8 +-
 lib/db_connection/connection_pool.ex          | 385 +++++-------------
 lib/db_connection/connection_pool/pool.ex     |  48 ---
 lib/db_connection/holder.ex                   | 141 +++++--
 lib/db_connection/single_connection.ex        | 238 -----------
 ...tion_test.exs => connection_pool_test.exs} |   4 +-
 test/test_helper.exs                          |   4 -
 8 files changed, 221 insertions(+), 609 deletions(-)
 delete mode 100644 lib/db_connection/connection_pool/pool.ex
 delete mode 100644 lib/db_connection/single_connection.ex
 rename test/db_connection/{single_connection_test.exs => connection_pool_test.exs} (98%)

diff --git a/lib/db_connection.ex b/lib/db_connection.ex
index 629958bf..58718943 100644
--- a/lib/db_connection.ex
+++ b/lib/db_connection.ex
@@ -76,8 +76,6 @@ defmodule DBConnection do
   DBConnection connections support using different pools via the `:pool` option
   passed to `start_link/2`. The default pool is `DBConnection.ConnectionPool`.
   Another supported pool that is commonly used for tests is `DBConnection.Ownership`.
-  `DBConnection.SingleConnection` is a one-connection pool without CoDel
-  queueing or `DynamicSupervisor`, intended for constrained runtimes.
 
   For now, using *custom* pools is not supported since the API for pools is not
   public.
diff --git a/lib/db_connection/connection.ex b/lib/db_connection/connection.ex
index 55fb28a3..555fe578 100644
--- a/lib/db_connection/connection.ex
+++ b/lib/db_connection/connection.ex
@@ -207,12 +207,7 @@ defmodule DBConnection.Connection do
             _other -> "** #{inspect(err)}"
           end
 
-        format =
-          ~c"** State machine ~p terminating~n" ++
-            ~c"** Reason for termination ==~n" ++
-            ~c"~s~n"
-
-        :error_logger.format(format, [self(), reason])
+        :erlang.display({:db_connection_terminating, self(), reason})
     end
 
     {:stop, {err, stack}, %{s | state: state}}
@@ -472,6 +467,7 @@ defmodule DBConnection.Connection do
   defp pool_update(state, %{pool: pool, tag: tag, mod: mod, connected_at: connected_at} = s) do
     case Holder.update(pool, tag, mod, state, connected_at) do
       {:ok, ref} ->
+        # Omit :hibernate — AtomVM gen_server does not support it.
         {:noreply, %{s | client: {ref, :pool}, state: state}}
 
       :error ->
diff --git a/lib/db_connection/connection_pool.ex b/lib/db_connection/connection_pool.ex
index 989a9e13..8ee12d4c 100644
--- a/lib/db_connection/connection_pool.ex
+++ b/lib/db_connection/connection_pool.ex
@@ -1,8 +1,9 @@
 defmodule DBConnection.ConnectionPool do
   @moduledoc """
-  The default connection pool.
+  The default connection pool for AtomVM.
 
-  The queueing algorithm is based on [CoDel](https://queue.acm.org/appendices/codel.html).
+  Keeps exactly one connection. Does not use CoDel queueing, idle polling,
+  or a nested connection supervisor.
 
   You're not supposed to call any functions on this pool directly, but only pass this
   as the value of the `:pool` option in functions such as `DBConnection.start_link/2`.
@@ -14,12 +15,6 @@ defmodule DBConnection.ConnectionPool do
 
   @behaviour DBConnection.Pool
 
-  @queue_target 50
-  @queue_interval 2000
-  @idle_interval 1000
-  @time_unit 1000
-  @watcher_ref {__MODULE__, :watcher_ref}
-
   @doc false
   def start_link({mod, opts}) do
     GenServer.start_link(__MODULE__, {mod, opts}, start_opts(opts))
@@ -43,150 +38,105 @@ defmodule DBConnection.ConnectionPool do
     GenServer.call(pool, :get_connection_metrics, :infinity)
   end
 
-  ## GenServer api
-
   @impl GenServer
   def init({mod, opts}) do
     Process.flag(:trap_exit, true)
     DBConnection.register_as_pool(mod)
 
-    queue = :ets.new(__MODULE__.Queue, [:protected, :ordered_set, decentralized_counters: true])
-
-    max_lifetime =
-      case Keyword.fetch(opts, :max_lifetime) do
-        {:ok, %Range{first: first, last: last, step: 1}} when first >= 0 and last >= first ->
-          {System.convert_time_unit(first, :millisecond, :native), last - first}
-
-        {:ok, invalid} ->
-          raise ArgumentError,
-                "invalid value for :max_lifetime, expected a non-negative step-1 range, got: #{inspect(invalid)}"
-
-        :error ->
-          nil
-      end
-
-    ts = {nil, max_lifetime}
-    {:ok, watcher_ref} = DBConnection.ConnectionPool.Pool.start_supervised(queue, mod, opts)
-    Process.put(@watcher_ref, watcher_ref)
-    target = Keyword.get(opts, :queue_target, @queue_target)
-    interval = Keyword.get(opts, :queue_interval, @queue_interval)
-    idle_interval = Keyword.get(opts, :idle_interval, @idle_interval)
-    idle_limit = Keyword.get_lazy(opts, :idle_limit, fn -> Keyword.get(opts, :pool_size, 1) end)
-    now_in_native = System.monotonic_time()
-    now_in_ms = System.convert_time_unit(now_in_native, :native, @time_unit)
-
-    codel = %{
-      target: target,
-      interval: interval,
-      delay: 0,
-      slow: false,
-      next: now_in_ms,
-      poll: nil,
-      idle_interval: idle_interval,
-      idle_limit: idle_limit,
-      idle: nil
+    size = Keyword.get(opts, :pool_size, 1)
+
+    if size < 1 do
+      raise ArgumentError, "pool size must be greater or equal to 1, got #{size}"
+    end
+
+    tag = make_ref()
+
+    state = %{
+      mod: mod,
+      opts: opts,
+      tag: tag,
+      holder: nil,
+      out: nil,
+      waiters: :queue.new(),
+      ts: {nil, max_lifetime(opts)},
+      conn: nil,
+      restarts: [],
+      max_restarts: Keyword.get(opts, :max_restarts, 3),
+      max_seconds: Keyword.get(opts, :max_seconds, 5)
     }
 
-    codel = start_idle(now_in_native, start_poll(now_in_ms, now_in_ms, codel))
-    {:ok, {:busy, queue, codel, ts}}
+    {:ok, pid} = start_connection(state)
+    {:ok, %{state | conn: pid}}
   end
 
   @impl GenServer
-  def handle_call(:get_connection_metrics, _from, {status, queue, _, _} = state) do
-    {ready_conn_count, checkout_queue_length} =
-      case status do
-        :busy ->
-          {0, :ets.select_count(queue, [{{{:_, :_, :_}}, [], [true]}])}
-
-        :ready ->
-          {:ets.select_count(queue, [{{{:_, :_}}, [], [true]}]), 0}
-      end
-
+  def handle_call(:get_connection_metrics, _from, state) do
     metrics = %{
       source: {:pool, self()},
-      ready_conn_count: ready_conn_count,
-      checkout_queue_length: checkout_queue_length
+      ready_conn_count: if(state.holder, do: 1, else: 0),
+      checkout_queue_length: :queue.len(state.waiters)
     }
 
     {:reply, [metrics], state}
   end
 
-  def handle_call({:disconnect_all, interval}, _from, {type, queue, codel, ts}) do
-    {_, max_lifetime} = ts
+  def handle_call({:disconnect_all, interval}, _from, state) do
+    {_, max_lifetime} = state.ts
     ts = {{System.monotonic_time(), interval}, max_lifetime}
-    {:reply, :ok, {type, queue, codel, ts}}
+    {:reply, :ok, %{state | ts: ts}}
   end
 
   @impl GenServer
-  def handle_info(
-        {:db_connection, from, {:checkout, _caller, now, queue?}},
-        {:busy, queue, _, _} = busy
-      ) do
-    case queue? do
-      true ->
-        :ets.insert(queue, {{now, System.unique_integer(), from}})
-        {:noreply, busy}
-
-      false ->
-        message = "connection not available and queuing is disabled"
-        err = DBConnection.ConnectionError.exception(message)
-        Holder.reply_error(from, err)
-        {:noreply, busy}
-    end
+  def handle_info({:db_connection, from, {:checkout, _callers, _now, queue?}}, state) do
+    {:noreply, checkout_request(from, queue?, state)}
   end
 
-  def handle_info(
-        {:db_connection, from, {:checkout, _caller, _now, _queue?}} = checkout,
-        {:ready, queue, _codel, _ts} = ready
-      ) do
-    case :ets.first(queue) do
-      {queued_in_native, holder} = key ->
-        Holder.handle_checkout(holder, from, queue, queued_in_native) and :ets.delete(queue, key)
-        {:noreply, ready}
-
-      :"$end_of_table" ->
-        handle_info(checkout, put_elem(ready, 0, :busy))
-    end
-  end
-
-  def handle_info({:"ETS-TRANSFER", holder, pid, queue}, {_, queue, _, _} = data) do
-    message = "client #{Util.inspect_pid(pid)} exited"
-    err = DBConnection.ConnectionError.exception(message: message, severity: :info)
-    Holder.handle_disconnect(holder, err)
-    {:noreply, data}
-  end
-
-  def handle_info({:"ETS-TRANSFER", holder, _, {msg, queue, extra}}, {_, queue, _, ts} = data) do
+  def handle_info({:"ETS-TRANSFER", holder, _pid, {msg, tag, extra}}, %{tag: tag} = state) do
     case msg do
       :checkin ->
         owner = self()
+        _ = Holder.unwatch_client(holder)
 
         case Holder.owner(holder) do
           ^owner ->
-            {interval, max_lifetime} = ts
+            {interval, max_lifetime} = state.ts
 
             if Holder.maybe_disconnect(holder, interval, max_lifetime) do
-              {:noreply, data}
+              {:noreply, %{state | holder: nil, out: nil}}
             else
-              handle_checkin(holder, extra, data)
+              handle_checkin(holder, extra, %{state | out: nil})
             end
 
           :undefined ->
-            {:noreply, data}
+            {:noreply, %{state | out: nil}}
         end
 
       :disconnect ->
+        _ = Holder.unwatch_client(holder)
         Holder.handle_disconnect(holder, extra)
-        {:noreply, data}
+        {:noreply, %{state | holder: nil, out: nil}}
 
       :stop ->
+        _ = Holder.unwatch_client(holder)
         Holder.handle_stop(holder, extra)
-        {:noreply, data}
+        {:noreply, %{state | holder: nil, out: nil}}
     end
   end
 
-  def handle_info({:timeout, deadline, {queue, holder, pid, len}}, {_, queue, _, _} = data) do
-    # Check that timeout refers to current holder (and not previous)
+  def handle_info({:"ETS-TRANSFER", holder, pid, tag}, %{tag: tag} = state) do
+    _ = Holder.unwatch_client(holder)
+    message = "client #{Util.inspect_pid(pid)} exited"
+    err = DBConnection.ConnectionError.exception(message: message, severity: :info)
+    Holder.handle_disconnect(holder, err)
+    {:noreply, %{state | holder: nil, out: nil}}
+  end
+
+  def handle_info({:DOWN, mon, :process, pid, _reason}, %{out: {holder, mon}} = state) do
+    _ = Holder.reclaim_on_client_down(holder, pid)
+    {:noreply, %{state | out: nil}}
+  end
+
+  def handle_info({:timeout, deadline, {tag, holder, pid, len}}, %{tag: tag} = state) do
     if Holder.handle_deadline(holder, deadline) do
       message =
         "client #{Util.inspect_pid(pid)} timed out because " <>
@@ -207,29 +157,11 @@ defmodule DBConnection.ConnectionPool do
       Holder.handle_disconnect(holder, exc)
     end
 
-    {:noreply, data}
+    {:noreply, state}
   end
 
-  def handle_info({:timeout, poll, {time, last_sent}}, {_, _, %{poll: poll}, _} = data) do
-    {status, queue, codel, ts} = data
-
-    # If no queue progress since last poll check queue
-    case :ets.first(queue) do
-      {sent, _, _} when sent <= last_sent and status == :busy ->
-        delay = time - sent
-        timeout(delay, time, queue, start_poll(time, sent, codel), ts)
-
-      {sent, _, _} ->
-        {:noreply, {status, queue, start_poll(time, sent, codel), ts}}
-
-      _ ->
-        {:noreply, {status, queue, start_poll(time, time, codel), ts}}
-    end
-  end
-
-  def handle_info({:timeout, idle, past_in_native}, {_, _, %{idle: idle}, _} = data) do
-    {status, queue, %{idle_limit: limit} = codel, ts} = data
-    drop_idle(past_in_native, limit, status, queue, codel, ts)
+  def handle_info({:EXIT, pid, reason}, %{conn: pid} = state) do
+    restart_connection(reason, state)
   end
 
   def handle_info({:EXIT, _pid, reason}, state) do
@@ -237,182 +169,83 @@ defmodule DBConnection.ConnectionPool do
   end
 
   @impl GenServer
-  def terminate(_reason, _state) do
-    if watcher_ref = Process.get(@watcher_ref) do
-      DBConnection.ConnectionPool.Pool.stop_supervised(watcher_ref)
-    end
-
+  def terminate(_reason, %{conn: pid}) when is_pid(pid) do
+    Process.exit(pid, :shutdown)
     :ok
   end
 
-  defp drop_idle(past_in_native, limit, status, queue, codel, ts) do
-    with true <- status == :ready and limit > 0,
-         {queued_in_native, holder} = key when queued_in_native <= past_in_native <-
-           :ets.first(queue) do
-      :ets.delete(queue, key)
-      {interval, max_lifetime} = ts
-      Holder.maybe_disconnect(holder, interval, max_lifetime) or Holder.handle_ping(holder)
-      drop_idle(past_in_native, limit - 1, status, queue, codel, ts)
-    else
-      _ ->
-        {:noreply, {status, queue, start_idle(System.monotonic_time(), codel), ts}}
-    end
-  end
+  def terminate(_reason, _state), do: :ok
 
-  defp timeout(delay, time, queue, codel, ts) do
-    case codel do
-      %{delay: min_delay, next: next, target: target, interval: interval}
-      when time >= next and min_delay > target ->
-        codel = %{codel | slow: true, delay: delay, next: time + interval}
-        drop_slow(time, target * 2, queue)
-        {:noreply, {:busy, queue, codel, ts}}
-
-      %{next: next, interval: interval} when time >= next ->
-        codel = %{codel | slow: false, delay: delay, next: time + interval}
-        {:noreply, {:busy, queue, codel, ts}}
-
-      _ ->
-        {:noreply, {:busy, queue, codel, ts}}
+  defp checkout_request(from, queue?, %{holder: {holder, checked_in_at}} = state) do
+    if Holder.handle_checkout(holder, from, state.tag, checked_in_at) do
+      {client, _} = from
+      mon = Holder.watch_client(holder, client)
+      %{state | holder: nil, out: {holder, mon}}
+    else
+      checkout_request(from, queue?, %{state | holder: nil})
     end
   end
 
-  defp drop_slow(time, timeout, queue) do
-    min_sent = time - timeout
-    match = {{:"$1", :_, :"$2"}}
-    guards = [{:<, :"$1", min_sent}]
-    select_slow = [{match, guards, [{{:"$1", :"$2"}}]}]
-
-    for {sent, from} <- :ets.select(queue, select_slow) do
-      drop(time - sent, from)
-    end
-
-    :ets.select_delete(queue, [{match, guards, [true]}])
+  defp checkout_request(from, true, state) do
+    %{state | waiters: :queue.in(from, state.waiters)}
   end
 
-  defp handle_checkin(holder, now_in_native, {:ready, queue, _, _} = data) do
-    :ets.insert(queue, {{now_in_native, holder}})
-    {:noreply, data}
+  defp checkout_request(from, false, state) do
+    message = "connection not available and queuing is disabled"
+    Holder.reply_error(from, DBConnection.ConnectionError.exception(message))
+    state
   end
 
-  defp handle_checkin(holder, now_in_native, {:busy, queue, codel, ts}) do
-    now_in_ms = System.convert_time_unit(now_in_native, :native, @time_unit)
-
-    case dequeue(now_in_ms, holder, queue, codel, ts) do
-      {:busy, _, _, _} = busy ->
-        {:noreply, busy}
+  defp handle_checkin(holder, now_in_native, state) do
+    case :queue.out(state.waiters) do
+      {:empty, waiters} ->
+        {:noreply, %{state | holder: {holder, now_in_native}, waiters: waiters}}
 
-      {:ready, _, _, _} = ready ->
-        :ets.insert(queue, {{now_in_native, holder}})
-        {:noreply, ready}
-    end
-  end
+      {{:value, from}, waiters} ->
+        state = %{state | waiters: waiters}
 
-  defp dequeue(time, holder, queue, codel, ts) do
-    case codel do
-      %{next: next, delay: delay, target: target} when time >= next ->
-        dequeue_first(time, delay > target, holder, queue, codel, ts)
-
-      %{slow: false} ->
-        dequeue_fast(time, holder, queue, codel, ts)
-
-      %{slow: true, target: target} ->
-        dequeue_slow(time, target * 2, holder, queue, codel, ts)
-    end
-  end
-
-  defp dequeue_first(time, slow?, holder, queue, codel, ts) do
-    %{interval: interval} = codel
-    next = time + interval
-
-    case :ets.first(queue) do
-      {sent, _, from} = key ->
-        :ets.delete(queue, key)
-        delay = time - sent
-        codel = %{codel | next: next, delay: delay, slow: slow?}
-        go(delay, from, time, holder, queue, codel, ts)
-
-      :"$end_of_table" ->
-        codel = %{codel | next: next, delay: 0, slow: slow?}
-        {:ready, queue, codel, ts}
+        if Holder.handle_checkout(holder, from, state.tag, now_in_native) do
+          {client, _} = from
+          mon = Holder.watch_client(holder, client)
+          {:noreply, %{state | holder: nil, out: {holder, mon}}}
+        else
+          handle_checkin(holder, now_in_native, state)
+        end
     end
   end
 
-  defp dequeue_fast(time, holder, queue, codel, ts) do
-    case :ets.first(queue) do
-      {sent, _, from} = key ->
-        :ets.delete(queue, key)
-        go(time - sent, from, time, holder, queue, codel, ts)
+  defp restart_connection(reason, state) do
+    now = System.monotonic_time(:second)
+    cutoff = now - state.max_seconds
+    restarts = Enum.filter(state.restarts, &(&1 >= cutoff))
 
-      :"$end_of_table" ->
-        {:ready, queue, %{codel | delay: 0}, ts}
+    if length(restarts) >= state.max_restarts do
+      {:stop, reason, state}
+    else
+      {:ok, pid} = start_connection(state)
+      {:noreply, %{state | conn: pid, holder: nil, out: nil, restarts: [now | restarts]}}
     end
   end
 
-  defp dequeue_slow(time, timeout, holder, queue, codel, ts) do
-    case :ets.first(queue) do
-      {sent, _, from} = key when time - sent > timeout ->
-        :ets.delete(queue, key)
-        drop(time - sent, from)
-        dequeue_slow(time, timeout, holder, queue, codel, ts)
-
-      {sent, _, from} = key ->
-        :ets.delete(queue, key)
-        go(time - sent, from, time, holder, queue, codel, ts)
-
-      :"$end_of_table" ->
-        {:ready, queue, %{codel | delay: 0}, ts}
-    end
+  defp start_connection(%{mod: mod, opts: opts, tag: tag}) do
+    DBConnection.Connection.start_link(mod, Keyword.put(opts, :pool_index, 1), self(), tag)
   end
 
-  defp go(delay, from, time, holder, queue, %{delay: min} = codel, ts) do
-    case Holder.handle_checkout(holder, from, queue, 0) do
-      true when delay < min ->
-        {:busy, queue, %{codel | delay: delay}, ts}
+  defp max_lifetime(opts) do
+    case Keyword.fetch(opts, :max_lifetime) do
+      {:ok, %Range{first: first, last: last, step: 1}} when first >= 0 and last >= first ->
+        {System.convert_time_unit(first, :millisecond, :native), last - first}
 
-      true ->
-        {:busy, queue, codel, ts}
+      {:ok, invalid} ->
+        raise ArgumentError,
+              "invalid value for :max_lifetime, expected a non-negative step-1 range, got: #{inspect(invalid)}"
 
-      false ->
-        dequeue(time, holder, queue, codel, ts)
+      :error ->
+        nil
     end
   end
 
-  defp drop(delay, from) do
-    message = """
-    [#{ancestor()}] connection not available and request was dropped from queue after #{delay}ms. \
-    This means requests are coming in and your connection pool cannot serve them fast enough. \
-    You can address this by:
-
-      1. Ensuring your database is available and that you can connect to it
-      2. Tracking down slow queries and making sure they are running fast enough
-      3. Increasing the pool_size (although this increases resource consumption)
-      4. Allowing requests to wait longer by increasing :queue_target and :queue_interval
-
-    See DBConnection.start_link/2 for more information
-    """
-
-    err = DBConnection.ConnectionError.exception(message, :queue_timeout)
-
-    Holder.reply_error(from, err)
-  end
-
-  defp ancestor do
-    Process.get(:"$ancestors", []) |> Enum.find(&is_atom/1)
-  end
-
   defp start_opts(opts) do
     Keyword.take(opts, [:name, :spawn_opt])
   end
-
-  defp start_poll(now, last_sent, %{interval: interval} = codel) do
-    timeout = now + interval
-    poll = :erlang.start_timer(timeout, self(), {timeout, last_sent}, abs: true)
-    %{codel | poll: poll}
-  end
-
-  defp start_idle(now_in_native, %{idle_interval: interval} = codel) do
-    timeout = System.convert_time_unit(now_in_native, :native, :millisecond) + interval
-    idle = :erlang.start_timer(timeout, self(), now_in_native, abs: true)
-    %{codel | idle: idle}
-  end
 end
diff --git a/lib/db_connection/connection_pool/pool.ex b/lib/db_connection/connection_pool/pool.ex
deleted file mode 100644
index 8da2a0d3..00000000
--- a/lib/db_connection/connection_pool/pool.ex
+++ /dev/null
@@ -1,48 +0,0 @@
-defmodule DBConnection.ConnectionPool.Pool do
-  @moduledoc false
-  use Supervisor, restart: :temporary
-
-  def start_supervised(tag, mod, opts) do
-    DBConnection.Watcher.watch(
-      DBConnection.ConnectionPool.Supervisor,
-      {DBConnection.ConnectionPool.Pool, {self(), tag, mod, opts}}
-    )
-  end
-
-  def stop_supervised(ref) do
-    case DBConnection.Watcher.unwatch(ref) do
-      pid when is_pid(pid) -> wait_until_down(pid)
-      nil -> :ok
-    end
-  end
-
-  def start_link(arg) do
-    Supervisor.start_link(__MODULE__, arg)
-  end
-
-  @impl true
-  def init({owner, tag, mod, opts}) do
-    size = Keyword.get(opts, :pool_size, 1)
-
-    if size < 1, do: raise(ArgumentError, "pool size must be greater or equal to 1, got #{size}")
-
-    children = for id <- 1..size, do: conn(owner, tag, id, mod, opts)
-    sup_opts = [strategy: :one_for_one] ++ Keyword.take(opts, [:max_restarts, :max_seconds])
-    Supervisor.init(children, sup_opts)
-  end
-
-  ## Helpers
-
-  defp conn(owner, tag, id, mod, opts) do
-    child_opts = [id: {mod, owner, id}] ++ Keyword.take(opts, [:shutdown])
-    DBConnection.Connection.child_spec(mod, [pool_index: id] ++ opts, owner, tag, child_opts)
-  end
-
-  defp wait_until_down(pid) do
-    ref = Process.monitor(pid)
-
-    receive do
-      {:DOWN, ^ref, _, _, _} -> :ok
-    end
-  end
-end
diff --git a/lib/db_connection/holder.ex b/lib/db_connection/holder.ex
index 8a31a769..6351f450 100644
--- a/lib/db_connection/holder.ex
+++ b/lib/db_connection/holder.ex
@@ -6,7 +6,9 @@ defmodule DBConnection.Holder do
 
   @queue true
   @timeout 15000
-  @time_unit 1000
+  # AtomVM System.monotonic_time/1 only accepts atom units (:millisecond etc.),
+  # not integer "parts per second" like 1000.
+  @time_unit :millisecond
 
   Record.defrecord(:conn, [
     :connection,
@@ -16,7 +18,10 @@ defmodule DBConnection.Holder do
     :connected_at,
     deadline: nil,
     status: :ok,
-    owner: nil
+    owner: nil,
+    heir_pid: nil,
+    heir_data: nil,
+    client_mon: nil
   ])
 
   Record.defrecord(:pool_ref, [:pool, :reference, :deadline, :holder, :lock])
@@ -39,7 +44,9 @@ defmodule DBConnection.Holder do
         module: mod,
         state: state,
         connected_at: connected_at,
-        owner: self()
+        owner: self(),
+        heir_pid: pool,
+        heir_data: ref
       )
 
     true = :ets.insert_new(holder, conn)
@@ -218,15 +225,63 @@ defmodule DBConnection.Holder do
 
   @spec owner(t) :: pid | :undefined
   def owner(holder) do
-    if function_exported?(:ets, :info, 2) do
-      :ets.info(holder, :owner)
-    else
-      try do
-        :ets.lookup_element(holder, :conn, conn(:owner) + 1)
-      rescue
-        ArgumentError -> :undefined
-      end
+    soft_owner(holder)
+  end
+
+  @doc false
+  @spec watch_client(t, pid) :: reference | nil
+  def watch_client(holder, client_pid) when is_pid(client_pid) do
+    mon = Process.monitor(client_pid)
+    true = :ets.update_element(holder, :conn, {conn(:client_mon) + 1, mon})
+    mon
+  rescue
+    ArgumentError -> nil
+  end
+
+  @doc false
+  @spec unwatch_client(t) :: :ok
+  def unwatch_client(holder) do
+    case soft_client_mon(holder) do
+      mon when is_reference(mon) ->
+        Process.demonitor(mon, [:flush])
+        _ = :ets.update_element(holder, :conn, {conn(:client_mon) + 1, nil})
+        :ok
+
+      _ ->
+        :ok
     end
+  rescue
+    ArgumentError -> :ok
+  end
+
+  @doc false
+  @spec reclaim_on_client_down(t, pid) :: boolean
+  def reclaim_on_client_down(holder, client_pid) when is_pid(client_pid) do
+    case :ets.lookup(holder, :conn) do
+      [
+        conn(
+          owner: ^client_pid,
+          heir_pid: heir_pid,
+          heir_data: heir_data,
+          client_mon: mon
+        )
+      ]
+      when is_pid(heir_pid) ->
+        if is_reference(mon), do: Process.demonitor(mon, [:flush])
+        _ = :ets.update_element(holder, :conn, {conn(:client_mon) + 1, nil})
+
+        if put_owner(holder, heir_pid) do
+          send(heir_pid, {:"ETS-TRANSFER", holder, client_pid, heir_data})
+          true
+        else
+          false
+        end
+
+      _ ->
+        false
+    end
+  rescue
+    ArgumentError -> false
   end
 
   @spec handle_checkout(t, {pid, reference}, reference, checkin_time) :: boolean
@@ -486,9 +541,9 @@ defmodule DBConnection.Holder do
   end
 
   defp start_deadline(timeout, pid, ref, holder, start) do
-    deadline =
-      :erlang.start_timer(timeout, pid, {ref, holder, self(), timeout - start}, abs: true)
-
+    now = System.monotonic_time(@time_unit)
+    delay = max(timeout - now, 0)
+    deadline = :erlang.start_timer(delay, pid, {ref, holder, self(), timeout - start})
     {deadline, [{conn(:deadline) + 1, deadline}]}
   end
 
@@ -497,7 +552,8 @@ defmodule DBConnection.Holder do
   end
 
   defp cancel_deadline(deadline) do
-    :erlang.cancel_timer(deadline, async: true, info: false)
+    _ = :erlang.cancel_timer(deadline)
+    :ok
   end
 
   defp hash_holder(_holder, 0), do: 0
@@ -508,21 +564,32 @@ defmodule DBConnection.Holder do
     System.convert_time_unit(hash, :millisecond, :native)
   end
 
-  defp maybe_set_heir(holder, pool, ref) do
-    if function_exported?(:ets, :setopts, 2) do
-      :ets.setopts(holder, {:heir, pool, ref})
-    else
-      :ok
-    end
-  end
+  defp maybe_set_heir(_holder, _pool, _ref), do: :ok
 
   defp transfer(table, pid, gift_data) do
-    if function_exported?(:ets, :give_away, 3) do
-      :ets.give_away(table, pid, gift_data)
-    else
-      _ = put_owner(table, pid)
-      send(pid, {:"ETS-TRANSFER", table, self(), gift_data})
-      true
+    soft_give_away(table, pid, gift_data)
+  end
+
+  # Approximate :ets.give_away/3 failure conditions so checkout/checkin
+  # rescue paths behave like OTP when give_away is missing.
+  defp soft_give_away(table, pid, gift_data) do
+    cond do
+      not is_pid(pid) or not Process.alive?(pid) ->
+        raise ArgumentError
+
+      pid == self() ->
+        raise ArgumentError
+
+      soft_owner(table) != self() ->
+        raise ArgumentError
+
+      true ->
+        if put_owner(table, pid) do
+          send(pid, {:"ETS-TRANSFER", table, self(), gift_data})
+          true
+        else
+          raise ArgumentError
+        end
     end
   end
 
@@ -532,11 +599,19 @@ defmodule DBConnection.Holder do
     ArgumentError -> false
   end
 
+  defp soft_owner(holder) do
+    :ets.lookup_element(holder, :conn, conn(:owner) + 1)
+  rescue
+    ArgumentError -> :undefined
+  end
+
+  defp soft_client_mon(holder) do
+    :ets.lookup_element(holder, :conn, conn(:client_mon) + 1)
+  rescue
+    ArgumentError -> nil
+  end
+
   defp ets_info(holder) do
-    if function_exported?(:ets, :info, 1) do
-      :ets.info(holder)
-    else
-      %{owner: owner(holder)}
-    end
+    %{owner: owner(holder)}
   end
 end
diff --git a/lib/db_connection/single_connection.ex b/lib/db_connection/single_connection.ex
deleted file mode 100644
index d541a4f0..00000000
--- a/lib/db_connection/single_connection.ex
+++ /dev/null
@@ -1,238 +0,0 @@
-defmodule DBConnection.SingleConnection do
-  @moduledoc """
-  A single-connection pool for constrained runtimes.
-
-  Unlike `DBConnection.ConnectionPool`, this pool keeps exactly one connection,
-  does not use CoDel queueing, idle polling, or `DynamicSupervisor`.
-
-  Pass this module as the `:pool` option to `DBConnection.start_link/2`.
-  The default pool remains `DBConnection.ConnectionPool`. `:pool_size` is
-  ignored except that values below 1 raise, same as the default pool.
-  """
-
-  use GenServer
-  alias DBConnection.Holder
-  alias DBConnection.Util
-
-  @behaviour DBConnection.Pool
-
-  @doc false
-  def start_link({mod, opts}) do
-    GenServer.start_link(__MODULE__, {mod, opts}, start_opts(opts))
-  end
-
-  @doc false
-  @impl DBConnection.Pool
-  def checkout(pool, callers, opts) do
-    Holder.checkout(pool, callers, opts)
-  end
-
-  @doc false
-  @impl DBConnection.Pool
-  def disconnect_all(pool, interval, _opts) do
-    GenServer.call(pool, {:disconnect_all, interval}, :infinity)
-  end
-
-  @doc false
-  @impl DBConnection.Pool
-  def get_connection_metrics(pool) do
-    GenServer.call(pool, :get_connection_metrics, :infinity)
-  end
-
-  @impl GenServer
-  def init({mod, opts}) do
-    Process.flag(:trap_exit, true)
-    DBConnection.register_as_pool(mod)
-
-    size = Keyword.get(opts, :pool_size, 1)
-
-    if size < 1 do
-      raise ArgumentError, "pool size must be greater or equal to 1, got #{size}"
-    end
-
-    tag = make_ref()
-
-    state = %{
-      mod: mod,
-      opts: opts,
-      tag: tag,
-      holder: nil,
-      waiters: :queue.new(),
-      ts: {nil, max_lifetime(opts)},
-      conn: nil,
-      restarts: [],
-      max_restarts: Keyword.get(opts, :max_restarts, 3),
-      max_seconds: Keyword.get(opts, :max_seconds, 5)
-    }
-
-    {:ok, pid} = start_connection(state)
-    {:ok, %{state | conn: pid}}
-  end
-
-  @impl GenServer
-  def handle_call(:get_connection_metrics, _from, state) do
-    metrics = %{
-      source: {:pool, self()},
-      ready_conn_count: if(state.holder, do: 1, else: 0),
-      checkout_queue_length: :queue.len(state.waiters)
-    }
-
-    {:reply, [metrics], state}
-  end
-
-  def handle_call({:disconnect_all, interval}, _from, state) do
-    {_, max_lifetime} = state.ts
-    ts = {{System.monotonic_time(), interval}, max_lifetime}
-    {:reply, :ok, %{state | ts: ts}}
-  end
-
-  @impl GenServer
-  def handle_info({:db_connection, from, {:checkout, _callers, _now, queue?}}, state) do
-    {:noreply, checkout_request(from, queue?, state)}
-  end
-
-  def handle_info({:"ETS-TRANSFER", holder, _pid, {msg, tag, extra}}, %{tag: tag} = state) do
-    case msg do
-      :checkin ->
-        owner = self()
-
-        case Holder.owner(holder) do
-          ^owner ->
-            {interval, max_lifetime} = state.ts
-
-            if Holder.maybe_disconnect(holder, interval, max_lifetime) do
-              {:noreply, %{state | holder: nil}}
-            else
-              handle_checkin(holder, extra, state)
-            end
-
-          :undefined ->
-            {:noreply, state}
-        end
-
-      :disconnect ->
-        Holder.handle_disconnect(holder, extra)
-        {:noreply, %{state | holder: nil}}
-
-      :stop ->
-        Holder.handle_stop(holder, extra)
-        {:noreply, %{state | holder: nil}}
-    end
-  end
-
-  def handle_info({:"ETS-TRANSFER", holder, pid, tag}, %{tag: tag} = state) do
-    message = "client #{Util.inspect_pid(pid)} exited"
-    err = DBConnection.ConnectionError.exception(message: message, severity: :info)
-    Holder.handle_disconnect(holder, err)
-    {:noreply, %{state | holder: nil}}
-  end
-
-  def handle_info({:timeout, deadline, {tag, holder, pid, len}}, %{tag: tag} = state) do
-    if Holder.handle_deadline(holder, deadline) do
-      message =
-        "client #{Util.inspect_pid(pid)} timed out because " <>
-          "it queued and checked out the connection for longer than #{len}ms"
-
-      exc =
-        case Process.info(pid, :current_stacktrace) do
-          {:current_stacktrace, stacktrace} ->
-            message <>
-              "\n\n#{Util.inspect_pid(pid)} was at location:\n\n" <>
-              Exception.format_stacktrace(stacktrace)
-
-          _ ->
-            message
-        end
-        |> DBConnection.ConnectionError.exception()
-
-      Holder.handle_disconnect(holder, exc)
-    end
-
-    {:noreply, state}
-  end
-
-  def handle_info({:EXIT, pid, reason}, %{conn: pid} = state) do
-    restart_connection(reason, state)
-  end
-
-  def handle_info({:EXIT, _pid, reason}, state) do
-    {:stop, reason, state}
-  end
-
-  @impl GenServer
-  def terminate(_reason, %{conn: pid}) when is_pid(pid) do
-    Process.exit(pid, :shutdown)
-    :ok
-  end
-
-  def terminate(_reason, _state), do: :ok
-
-  defp checkout_request(from, queue?, %{holder: {holder, checked_in_at}} = state) do
-    if Holder.handle_checkout(holder, from, state.tag, checked_in_at) do
-      %{state | holder: nil}
-    else
-      checkout_request(from, queue?, %{state | holder: nil})
-    end
-  end
-
-  defp checkout_request(from, true, state) do
-    %{state | waiters: :queue.in(from, state.waiters)}
-  end
-
-  defp checkout_request(from, false, state) do
-    message = "connection not available and queuing is disabled"
-    Holder.reply_error(from, DBConnection.ConnectionError.exception(message))
-    state
-  end
-
-  defp handle_checkin(holder, now_in_native, state) do
-    case :queue.out(state.waiters) do
-      {:empty, waiters} ->
-        {:noreply, %{state | holder: {holder, now_in_native}, waiters: waiters}}
-
-      {{:value, from}, waiters} ->
-        state = %{state | waiters: waiters}
-
-        if Holder.handle_checkout(holder, from, state.tag, now_in_native) do
-          {:noreply, %{state | holder: nil}}
-        else
-          handle_checkin(holder, now_in_native, state)
-        end
-    end
-  end
-
-  defp restart_connection(reason, state) do
-    now = System.monotonic_time(:second)
-    cutoff = now - state.max_seconds
-    restarts = Enum.filter(state.restarts, &(&1 >= cutoff))
-
-    if length(restarts) >= state.max_restarts do
-      {:stop, reason, state}
-    else
-      {:ok, pid} = start_connection(state)
-      {:noreply, %{state | conn: pid, holder: nil, restarts: [now | restarts]}}
-    end
-  end
-
-  defp start_connection(%{mod: mod, opts: opts, tag: tag}) do
-    DBConnection.Connection.start_link(mod, Keyword.put(opts, :pool_index, 1), self(), tag)
-  end
-
-  defp max_lifetime(opts) do
-    case Keyword.fetch(opts, :max_lifetime) do
-      {:ok, %Range{first: first, last: last, step: 1}} when first >= 0 and last >= first ->
-        {System.convert_time_unit(first, :millisecond, :native), last - first}
-
-      {:ok, invalid} ->
-        raise ArgumentError,
-              "invalid value for :max_lifetime, expected a non-negative step-1 range, got: #{inspect(invalid)}"
-
-      :error ->
-        nil
-    end
-  end
-
-  defp start_opts(opts) do
-    Keyword.take(opts, [:name, :spawn_opt])
-  end
-end
diff --git a/test/db_connection/single_connection_test.exs b/test/db_connection/connection_pool_test.exs
similarity index 98%
rename from test/db_connection/single_connection_test.exs
rename to test/db_connection/connection_pool_test.exs
index 35c2a1b0..8af32de0 100644
--- a/test/db_connection/single_connection_test.exs
+++ b/test/db_connection/connection_pool_test.exs
@@ -1,7 +1,7 @@
-defmodule DBConnection.SingleConnectionTest do
+defmodule DBConnection.ConnectionPoolTest do
   use ExUnit.Case, async: true
 
-  alias TestSingleConnection, as: P
+  alias TestPool, as: P
   alias TestAgent, as: A
   alias TestQuery, as: Q
   alias TestResult, as: R
diff --git a/test/test_helper.exs b/test/test_helper.exs
index 1c6ee2ee..989768b2 100644
--- a/test/test_helper.exs
+++ b/test/test_helper.exs
@@ -5,7 +5,3 @@ Code.require_file("test_support.exs", __DIR__)
 defmodule TestPool do
   use TestConnection, pool: DBConnection.ConnectionPool, pool_size: 1
 end
-
-defmodule TestSingleConnection do
-  use TestConnection, pool: DBConnection.SingleConnection, pool_size: 1
-end