How to create and test with mnesia?

Questions / Help
, ,
1

Hello everyone, I’m building an app using mnesia as storage, and I got a some questions:

  1. How to correctly test modules which using the mnesia?
    As example: I have an app with 3 genservers which contain callbacks for CRUD under specific supervisor. Every single genserver works with specific table. Im adding the tests for this 3 genserver, testing scenarios and the setup block (into every script file of course) with :mnesia.clear_table(:my_table) inside.

    • If Im trying to call this scripts one by one - everything is ok. Troubles begins when I’m calling all tests at once. As I understand, :mnesia works asyncroniously, and tables do not have time to change their state between two or more testing scenarios. As result: sometimes testing scripts works correctly, sometimes not.

    • Sometimes i got message like disk_log: repairing '__FILE__'. All appearance, I see this message because :mnesia are not stopped correctly after ExUnit complete (I think it is perceived like a sudden shutting down for mnesia, working ends off and formed the difference between data in file on disk and data in RAM).

      What can I do, for fix this message reason? Maybe exist some way for stop the :mnesia correctly when all test scenarios is complete?

    How do I clean/drop/change state properly the mnesia table(s)/schema(s) for making a tests wich gonna work stable and predictably?

  2. How to correctly create mnesia schema & start the mnesia in app?
    Basically, I have a 2 way, start it manually or add mnesia into extra_applications in mix.exs. I choosed first option. I’ve choosed this way, because if I add mnesia into extra_applications - I will have to stop it before calling :mnesia.create_schema (I’m creating the schema & starting mnesia inside init/1 of supervisor).

    Is it good way or not? I don’t know why, but I feel that it’s - anti-pattern.

    Is this practice if correct? Creating the schema inside supervisor’s init/1. Or I should apart the creating schema/tables from supervisor logic? If yes - how can I create schema/tables correctly in application deployment?

    If you have some advices, examples or something else, which can help me with this questions - I would be appreciative.

2 Likes
2

I won’t be able to address all your questions because my experience is with RAM-only tables, but I can share what works for me anyway in case it helps.

All Mnesia-related code are isolated in it’s own module (it’s called Store in my case). It exposes an init/1 function that creates the tables and a stop/0 function that is only used by the tests, which deletes the tables and stops Mnesia. Since all the tables are RAM-only, I don’t use create_schema.

In each test file, I have something like this:

  setup_all do
    Store.init()
    on_exit(&Store.stop/0)
    :ok
  end

If you are relying on the order of the tests in each test file, you can add the following line to your test_helper.exs file to make sure the tests will run in the given order:

ExUnit.configure(seed: 0)

So each test file starts with an empty database, then I use the functions provided by my Store module to insert, query, update and delete data and test the results.

For production, I have a GenServer that calls that init/1 function in its own init/1 and handles events from Mnesia like node up and node down, but that’s all. The users of the Store module call its functions directly from their processes and not by sending messages to a GenServer.

Finally, I do have :mnesia in my extra_applications.

Hope it helps,

Regards

1 Like
3

I thought about this way, but 1. this solution creates a dependency of implementation from tests (I will need to create specific functions that needs for testing the module and doesn’t useful for business logic. 2. It doesn’t solve the problem with dropping the data from table/schema =(