I assume, your configuration is not working. Please turn on logging: GitHub - zookzook/elixir-mongodb-driver: MongoDB driver for Elixir · GitHub, then you see more information about.

Thanks for the prompt reply @zookzook! This is interesting. I have verified that BOTH app1 and app2 have the IDENTICAL configuration options, something like this:

[
  name: :my_mongo,
  pool_size: 20,
  url: "mongodb://:@localhost:/app1_dev",
  timeout: 15000,
  ssl: false
]

Note that the URL looks funny in dev because we don’t specify a user, password, port, or parameters (which do get set when connecting to a production mongo instance).

What is really curious is that BOTH app1 AND app2 show errors on startup when I set the config to include log: true:

10:13:33.630 [error] an exception was raised logging %DBConnection.LogEntry{call: :execute, connection_time: 5298000, decode_time: 6000, idle_time: 548338000, params: [], pool_time: 147000, query: %Mongo.Query{action: {:exec_hello, []}}, result: {:ok, %Mongo.Query{action: {:exec_hello, []}}, {%{"code" => 59, "codeName" => "CommandNotFound", "errmsg" => "no such command: 'helloOk'", "ok" => 0.0}, %Mongo.Events.CommandStartedEvent{command: :hello, command_name: :hello, connection_id: #PID<0.815.0>, database_name: "admin", operation_id: nil, request_id: 1}, 0, 5240}}}: ** (BadFunctionError) expected a function, got: true
    (db_connection 2.4.2) lib/db_connection.ex:1532: DBConnection.log/2
    (db_connection 2.4.2) lib/db_connection.ex:1510: DBConnection.log/5
    (mongodb_driver 0.9.2) lib/mongo.ex:1526: Mongo.exec_hello/2
    (stdlib 3.16.1) timer.erl:166: :timer.tc/1
    (mongodb_driver 0.9.2) lib/mongo/monitor.ex:239: Mongo.Monitor.get_server_description/1
    (mongodb_driver 0.9.2) lib/mongo/monitor.ex:164: Mongo.Monitor.update_server_description/1
    (mongodb_driver 0.9.2) lib/mongo/monitor.ex:151: Mongo.Monitor.handle_info/2
    (stdlib 3.16.1) gen_server.erl:695: :gen_server.try_dispatch/4
    (stdlib 3.16.1) gen_server.erl:771: :gen_server.handle_msg/6
    (stdlib 3.16.1) proc_lib.erl:226: :proc_lib.init_p_do_apply/3

app1 throws 3 of these errors, and then they pop up intermittently, but app1 can go on to successfully issue commands, e.g.

# app1
iex> Mongo.insert_one(:my_mongo, "foo", %{what: "up"})
{:ok,
 %Mongo.InsertOneResult{
   acknowledged: true,
   inserted_id: #BSON.ObjectId<633d93aac6e47b35cb0d1fc5>
 }}

whereas app2 just throws an unending stream of these errors into the log. The same command on app2 yields a timeout:

# app2
iex> Mongo.insert_one(:my_mongo, "foo", %{what: "up"})
** (exit) exited in: GenServer.call(:my_mongo, {:checkout_session, :write, [write_counter: 1]}, 60000)
    ** (EXIT) time out
    (elixir 1.13.4) lib/gen_server.ex:1030: GenServer.call/3
    (mongodb_driver 0.9.2) lib/mongo/session.ex:142: Mongo.Session.start_session/3
    (mongodb_driver 0.9.2) lib/mongo/session.ex:694: Mongo.Session.in_session/5
    (mongodb_driver 0.9.2) lib/mongo.ex:1482: Mongo.issue_command/4
    (mongodb_driver 0.9.2) lib/mongo.ex:849: Mongo.insert_one/4

It’s like Mongo “knows” how its server is getting started or something. Very curious. Any other ideas for what could explain this behavior?

I just noticed app1 shows "mongodb_driver": {:hex, :mongodb_driver, "0.9.1" in its mix.lock whereas app2 specifies "mongodb_driver": {:hex, :mongodb_driver, "0.9.2" … so I’m gonna suss that out

If both apps have identical configurations then they started the supervisor related stuff twice. Use only one configuration. A topology should only started as a singleton.