Defining multiple instances of Cachex?

Questions / Help
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1

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!

2

Ah, I think figured it out:

worker(Cachex, [:foo_cache, []], id: :cachex_1),
worker(Cachex, [:bar_cache, []], id: :cachex_2),

But is that a good idea?

1 Like
3

Had this problem too and Whitfin helped me get it going. You need to give the workers an id after the options that matches the cache name. Do something like this:

worker(Cachex, [:temp_cache, [expiration: expiration(default: :timer.minutes(10))]], id: :temp_cache),
worker(Cachex, [:main_cache, []], id: :main_cache),

This worked fine in my app. I needed on with a fixed/short TTL and another that was long-lived.

5 Likes
4

It’s a fine idea. I have like 4 cache’s in my big project. ^.^;

1 Like
5

I have like at least 12. haha

1 Like
6

Lol, wow, nice! ^.^