🐥 Using multiple cachex instances in your Elixir Phoenix application

In web applications, cache is often used to temporarily store things in memory in order to speed things up.

In Elixir, the go-to library for this is Cachex.

In my use case, I wanted to setup different instances of cachex so that I can selectively clean them them up.

The way you normally set it up is by adding a child to the supervision tree in lib/my_app/application.ex:

children = [
  {Cachex, [:my_cache]}
]

To setup multiple caches, my first approach was to do this:

children = [
  {Cachex, [:my_first_cache]},
  {Cachex, [:my_second_cache]}
]

However, when you start the application, you'll be greeted by 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)
    ]

As the error message indicates, the way to fix it is to define them like this:

children = [
  Supervisor.child_spec({Cachex, [:my_first_cache]}, id: :my_first_cache),
  Supervisor.child_spec({Cachex, [:my_second_cache]}, id: :my_second_cache)
]

You can read more about Supervisor.child_spec/2 in the documentation.

If you don't want to set the value for id manually, you can also use the make_ref/0 function:

children = [
  Supervisor.child_spec({Cachex, [:my_first_cache]}, id: make_ref()),
  Supervisor.child_spec({Cachex, [:my_second_cache]}, id: make_ref())
]