Database mocking without requiring an *external* preconfigured database

For some python projects at work we found that there are pytest-{postgres,mysql,redis} and probably more.

These tools take the burden off of you to maintain a Dev database locally which you need to configure your in Dev applications accordingly.

Those are basically plugins for pytest, which use the fixture mechanism of pytest to provide temporary “installations” of the tool in question, “persisting” it’s data into temporary folders and removing after the suite has run.

Even though this has an obvious runtime cost to initialise the tooling for each test run, it really made onboarding the junior quite easy. We just told them to install mysql package for their Ubuntu WSL and told them to run make test which wrapped the actual test run with proper command line arguments. No need to walk them through configuring a local mysql or having remember to run some docker(-compose) whenever changing projects.

Has anyone seen something similar for elixir or is there any interest in it?

I am sorry, what exactly do those packages do? Didn’t get it. What tool? What temporary installations? Why once per test? Could you clarify?

I archive this behavior by using Nix.
At least it works perfectly with running PostgreSQL in a temporary directory.

GitHub - cw789/elixir_nix_example

The probably interesting PostgreSQL part under:

So it means within WSL - install Nix - run nix-shell command x…
If you run the command a second time, runtime cost isn’t anymore much higher.

Yeah, using nix such a thing is easy, I know, though even I wrap most of my stuff in flakes nowadays, I try to make it that potential contributors don’t need nix installed.

That would be wonderful. Difficult onboarding is always an issue.

They start the tool they are made for, eg mysql, and remove its runtime folders afterwards. It does not affect any potentially running instances of that software that might already be on the system.

And it’s not per test, but per run.

So only the first test hitting the database is actually slow as it needs to wait for everything beeing set up.

As I said already it comes with a low cost for setting things up, and might also enable to test behaviour against different database versions.

So I’m not aware of anything like this deticated for Elixir and not using Docker.
But I think without any container or virtual machine sandboxing it is difficult to not affect any other running instances.
But I’m of course interested.

I’m still not sure I get it.

Are you talking about something like the PGDATA environment variable that allows you to specify where will PostgreSQL store its data? I’ve had projects where that was used to kinda sorta enforce some isolation.

We used the MySQL version of those plugins, that basically starts a mysql demon with a configuration pointing the data dir to somewhere in tmp, listening on specified ports or domain sockets.

It manages the full database application lifecycle.


If I understood your requirement correctly, you may be interested in excontainers, a library I developed to tackle exactly this problem.

Unfortunately it has not seen much adoption yet, but in case you decide to give it a try I’d be happy to hear about any problem you may encounter or any feedback you may have :slight_smile:

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