I’m working with Cachex to cache several types of data and I started running into problems with duplicate keys. I figured there are 2 solutions to that:

1. Add prefixes to the keys.
2. Start multiple instances of Cachex

Adding prefixes to provide a kind of “namespace” is simple enough, but it’s a bit smelly when using fetch and you provide a callback: the callback function will receive as its argument the namespaced key that was looked for. In my case, I would have to strip off that namespace prefix before I could do anything with that key.

When I tried to start up multiple instance of Cachex in my application, I thought I could just add multiple child workers per https://hexdocs.pm/cachex/getting-started.html#starting-your-cache

children = [
  supervisor(MyApp.Repo, []),
  # Namespaces for cache instances
  worker(Cachex, [:foo_cache, []]),
  worker(Cachex, [:bar_cache, []]),
]

opts = [strategy: :one_for_one, name: MyApp.Supervisor]
Supervisor.start_link(children, opts)

But trying to run the app in that configuration generates an error:

** (Mix) Could not start application my_app: MyApp.Application.start(:normal, []) returned an error: bad child specification, more than one child specification has the id: Cachex.
If using maps as child specifications, make sure the :id keys are unique.
If using a module or {module, arg} as child, use Supervisor.child_spec/2 to change the :id, for example:

children = [
  Supervisor.child_spec({MyWorker, arg}, id: :my_worker_1),
  Supervisor.child_spec({MyWorker, arg}, id: :my_worker_2)
]

I’ve tried adapting my arguments to the format suggested by that error message to something like this:

children = [
    Supervisor.child_spec({Cachex, [:foo_cache, []]}, id: :cachex1),
    Supervisor.child_spec({Cachex, [:bar_cache, []]}, id: :cachex2),
]

but that generates a different error:

** (Mix) Could not start application auth: MyApp.Application.start(:normal, []) returned an error: shutdown: failed to start child: :foo_cache
    ** (EXIT) :invalid_name

Is it bad practice to try and spin up multiple instances of Cachex? Should I just suck it up and deal with manually prefixing keys? Or can someone show me how to properly declare multiple Cachex instances in my supervisor?

Thanks!