@anthonator Just dropping by to say that your approach inspired me to figure out something similar.

I did just slightly differently without a new connection pool module (and thus no delegations).

defmodule DB.SharedConnectionPool do
  alias DBConnection.ConnectionPool

  def child_spec({mod, opts}) do
    opts = Keyword.put_new(opts, :name, pool_name(opts))
    Supervisor.Spec.worker(__MODULE__, [{mod, opts}])
  end

  def start_link({mod, opts}) do
    case GenServer.start_link(ConnectionPool, {mod, opts}, start_opts(opts)) do
      {:ok, pid} -> {:ok, pid}
      {:error, {:already_started, pid}} -> {:ok, pid}
      error -> error
    end
  end

  defp pool_name(opts) do
    case Keyword.fetch(opts, :shared_pool_id) do
      {:ok, pool} -> String.to_atom("#{__MODULE__}-#{pool}")
      :error -> __MODULE__
    end
  end

  # Exact same as `DBConnection.ConnectionPool`
  defp start_opts(opts) do
    Keyword.take(opts, [:name, :spawn_opt])
  end
end

And I have the repos configured like this:

config :my_app, MyApp.Repo,
  pool: DB.SharedConnectionPool,
  shared_pool_id: :my_shared_pool_id

config :my_second_app, MySecondApp.Repo,
  pool: DB.SharedConnectionPool,
  shared_pool_id: :my_shared_pool_id

This way makes it also possible to specifically configure what repos should share a pool, and multiple shared pools can be created.

The downside is that it still relies on the connection info from the configs, which means if two repos with different configs share the same pool, there’s a race condition and one repo config will be taken to start the pool. But otherwise this approach has the least impact on how Ecto works.

Full code here: Allow DB connection pool sharing between repos by unnawut · Pull Request #853 · omgnetwork/ewallet · GitHub (disclaimer: I’m employed by this project)

With db_connection 2.4.0, i get this error

** (UndefinedFunctionError) function Solve.Utils.DB.SharedConnectionPool.checkout/3 is undefined or private
    (utils 0.1.0) Solve.Utils.DB.SharedConnectionPool.checkout(#PID<0.13239.0>, [#PID<0.13592.0>, #PID<0.13591.0>], [log: #Function<14.6198013/1 in Ecto.Adapters.SQL.with_log/3>, source: "query_history_v1", timeout: 15000, pool_size: 10, pool: Solve.Utils.DB.SharedConnectionPool])
    (db_connection 2.4.0) lib/db_connection.ex:1082: DBConnection.checkout/3
    (db_connection 2.4.0) lib/db_connection.ex:1407: DBConnection.run/6
    (db_connection 2.4.0) lib/db_connection.ex:574: DBConnection.parsed_prepare_execute/5
    (db_connection 2.4.0) lib/db_connection.ex:566: DBConnection.prepare_execute/4
    (ecto_sql 3.5.4) lib/ecto/adapters/sql.ex:690: Ecto.Adapters.SQL.execute!/4
    (ecto_sql 3.5.4) lib/ecto/adapters/sql.ex:682: Ecto.Adapters.SQL.execute/5
    (ecto 3.5.6) lib/ecto/repo/queryable.ex:224: Ecto.Repo.Queryable.execute/4
    (ecto 3.5.6) lib/ecto/repo/queryable.ex:19: Ecto.Repo.Queryable.all/3
    (saved_query_service 0.1.0) lib/query_service.ex:55: Solve.SavedQueryService.fetch_history/2
    (query_runner 0.1.0) lib/query_runner/tidy_history_task.ex:7: Solve.QueryRunner.TidyHistory.run/0
    (elixir 1.11.3) lib/task/supervised.ex:90: Task.Supervised.invoke_mfa/2
    (stdlib 3.14) proc_lib.erl:226: :proc_lib.init_p_do_apply/3

Adding:

  defdelegate checkout(pool, callers, opts), to: ConnectionPool

to the connection pool here Sharing a DBConnection pool with multiple Ecto Repo's - #5 by unnawut seems to fix the issue

This is the reason I just share a repo between all umbrella apps, and declare the schemas separately in each app. Otherwise you can easily max out the 200 connection limit on an AWS db.t2.small instance