Is there a way to trigger a GenServer restart when phoenix_live_reload fires?

[konstantine]

Hi. I am using a GenServer to store a number of global constants on ETS. These constants combine data from a variable set of Elixir modules. The constants are derived after compilation, with :application.get_key(:my_app, :modules) supplying the module names (from which the data is extracted). Prior to compilation, get_key/2 seems to return nil.

This approach might work in production; in development, however, I must remember to restart the server manually, every time a change is made to the data-containing modules. I was therefore wondering whether there might be a way of triggering a GenServer restart, every time phoenix_live_reload fires following the detection of a change in my Elixir code.

[LostKobrakai]

Phoenix.CodeReloader is what you need to look at. It looks like you could decorate the reload callback used by the plug.

phoenixframework/phoenix/blob/v1.4.11/lib/phoenix/code_reloader.ex#L55

[konstantine]

Thanks for the pointer. I was hoping for a “fix and forget” type of solution, that there might be an obvious mechanism that I had somehow missed. If I decorate the reload callback, presumably I will have to repeat this every time CodeReloader is updated. If my understanding is correct, I would rather opt for a runtime calculation of each constant during development – and accept the risk of a discrepancy with the production environment.

Best wishes for 2020!

[OvermindDL1]

@konstantine

:application.get_key(:my_app, :modules)

Why not just call this every time it’s needed? It’s very cheap and you can change the values at runtime with ease.

[konstantine]

Indeed, :application.get_key(:my_app, :modules) does the job perfectly, but I was hoping for a built-in way to trigger it. The problem with its returning nil prior to compilation was not the cost of the runtime operation, it was that I could not use the re-compilation to reset my (runtime) constants. So, unless dev and prod logic is kept separate, I can be left with a GenServer preserving stale “constants” on ETS, while phoenix_live_reload is giving me the false impression that my changes are taking effect.

Clearly, this is not a huge issue, I just wanted to make sure that I am not missing an obvious solution.