Get ArgumentError ets.lookup in Absinthe Middleware using FunWithFlags

Question

I’m trying to use FunWithFlags.enabled? (ref fwf) inside an absinthe middleware/3 callback but I got the following error:

(ArgumentError) argument error
    (stdlib) :ets.lookup(:fun_with_flags_cache, :some_flag)

Stacktrace:
  │ lib/fun_with_flags/store/cache.ex:35: FunWithFlags.Store.Cache.get/1
  │ lib/fun_with_flags/store.ex:12: FunWithFlags.Store.lookup/1
  │ lib/fun_with_flags.ex:77: FunWithFlags.enabled?/2

I haven’t tried too much. I just narrowed down where the issue is happening. I’m guessing the middleware compiles before the :some_flag record (or maybe the :fun_with_flags_cache table) is created and then failed because there is no record/table.

This link says:

It stores flag information in Redis or a relational DB (PostgreSQL or MySQL, with Ecto) for persistence and synchronization across different nodes, but it also maintains a local cache in an ETS table for fast lookups. When flags are added or toggled on a node, the other nodes are notified via PubSub and reload their local ETS caches.

I already tried to disable the ETS with the following config:

config :fun_with_flags, :cache,
  enabled: false,

But I didn't work. :(

Here is an example of how I'm using it:

defmodule MyApp.Schema do
use Absinthe.Schema
...
  def middleware(middleware, field, object) do
  FunWithFlags.enabled?(:some_flag) # <-- (ArgumentError) argument error (stdlib) :ets.lookup...
  ...
  end
end

Thanks!

Answer 1

Since the documentation explicitly mentions possible race conditions, I’d suggest you start the FwF application manually as described in Application Start Behaviour section of the documentation.

In mix.exs:

- {:fun_with_flags, "~> 1.6"},
+ {:fun_with_flags, "~> 1.6", runtime: false},

and in your Application.start/1 callback:

  def start(_type, _args) do
    children = [
      ...,
+     FunWithFlags.Supervisor,
    ]

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