Wanted: working example of app with Mnesia with volumes and small machine count

Questions / Help
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I’m working to deploy a Phoenix application where I would really like to use :mnesia for some data. I’m using Memento right now, though that’s not a hard dependency.

I was wondering if anyone had a public example of an application that uses Mnesia/Memento in fly with a small number of nodes. How are you starting it, creating tables, and joining/re-joining nodes to your cluster?

Right now I have two machines for this application, with min_machines_running = 1. I have created a volume with a count of 2, mounted it in my machines, and configured :mnesia, :dir to point to that volume.

I find that with my current setup, with 1-2 nodes running on localhost, I can start and stop nodes and data is retained. I do see that on the first node, my table is included in :local_tables, whereas on my second node it is in :tables but not in :local_tables.

I find that in my fly deployment, every deployment kills all my data.

Right now I’m setting up my tables as follows. I’m not attached to any of this, so if there’s something else that actually works I’m happy to throw any or all of it away.

  @tables [Schema.MyTable]

  @impl GenServer
  def init(_opts \\ []) do
    Logger.info("[#{inspect(__MODULE__)}] init")

    disc_nodes = [node() | Node.list()]

    setup!(disc_nodes)
    Memento.wait(@tables, :infinity)
    Memento.add_nodes(Node.list() -- Memento.system()[:running_db_nodes])

    {:ok, nil}
  end

  def setup!(disc_nodes \\ [node()]) do
    local_disc = node() in disc_nodes
    if local_disc, do: File.mkdir_p!(Application.get_env(:mnesia, :dir))
    if local_disc, do: Memento.Schema.set_storage_type(node(), :disc_copies)

    for table <- @tables -- Memento.system()[:local_tables],
        do: Memento.Table.create(table)

    for table <- @tables do
      if local_disc, do: Memento.Table.set_storage_type(table, node(), :disc_copies)
    end
  end
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Locally, because the table is not local, I find that when the second node is started in isolation that :mnesia waits indefinitely for the table to become available.

In my fly deployment, when I force both machines to be started and ssh into them, I see that my table is local on both machines. I don’t know what I did to get that to happen.

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