Library configuration - multi-instance

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Hello people!

I am about to write a “SDK” for a service. Before I got started, I was looking for some general patterns and best practices.

One thing I noticed is that most of them don’t worry about multiple “instances”. Almost all of them that I’ve used or have seen are using the single instance approach. By that I mean: they have a config like:

# Configuration
config :my_library, client_id: id, client_key: key

defmodule MyApplication do
  # possibly add MyLibrary to the application supervision
end

# Then on application code
MyLibrary.make_some_call(params)

Examples: :ex_aws, :ex_twilio, :sendgrid, etc…

Ecto uses a different approach that facilitates the multiple instances approach but it makes it a bit more verbose to configure it.

# Configuration
config :my_app, MyRepo1, options

config :my_app, MyRepo2, other_options

defmodule MyRepo1 do
  use Ecto.Repo, opts # where otp_app: :my_app
end

defmodule MyRepo2 do
  use Ecto.Repo, opts # where otp_app: :my_app
end

defmodule MyApplication do
  # add MyRepo1 and MyRepo2 to the application supervision
end

# Then on application code
some_query |> MyRepo1.all()
some_query |> MyRepo2.all()

With the first approach, I don’t think there is a good way to have two instances of the library running at the same time. With the second approach we end up needing some macros…

I would like to hear people’s opinion on this:

  1. Should we, as library authors, strive to always provide a multi-instance configuration?
  2. Are there other ways to do this cleanly? Even if that demands Elixir 1.9+…

Cheers!

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