[ruby-core:121421] [Ruby Feature#21194] How to manage application-level information in Ruby application

Issue #21194 has been reported by mame (Yusuke Endoh).

----------------------------------------
Feature #21194: How to manage application-level information in Ruby application
https://bugs.ruby-lang.org/issues/21194

* Author: mame (Yusuke Endoh)
* Status: Open
----------------------------------------
## Goal

I want to manage application-level information (e.g., application configuration) while making it easily accessible from the part classes of the application. Additionally, I want to support multiple instances of the application within a single process.

## Current approach 1: Global variables

The simplest way to achieve this is by using global variables.

```ruby
class MyApp
  class Part1
    def run
      using $config[:part1]...
    end
  end

  class Part2
    def run
      using $config[:part2]...
    end
  end

  def initialize(config)
    $config = config
    @part1 = Part1.new
    @part2 = Part2.new
  end

  def run
    @part1.run
    @part2.run
  end
end

app1 = MyApp.new({ part1: "aaa", part2: "bbb" })
# app2 = MyApp.new({ part1: "AAA", part2: "BBB" }) # Cannot create this

app1.run
```

This code is simple and clear, but it does not allow creating multiple `MyApp` instances with different configurations.
To achieve that, we would need to create separate process using `fork` or `spawn`.

This limitation remains even if we replace global variables with constants (`MyApp::Config`) or class methods (`MyApp.config`).

## Current approach 2: Passing configuration via `initialize`

A textbook and well-structured approach is to explicitly pass configuration through `initialize`.

```ruby
class MyApp
  class Part1
    def initialize(config)
      @config = config
    end

    def run
      using @config[:part1]...
    end
  end

  class Part2
    def initialize(config)
      @config = config
    end

    def run
      using @config[:part2]...
    end
  end

  def initialize(config)
    @part1 = Part1.new(config)
    @part2 = Part2.new(config)
  end

  def run
    @part1.run
    @part2.run
  end
end

app1 = MyApp.new({ part1: ..., part2: ... })
app2 = MyApp.new({ part1: ..., part2: ... })

app1.run
app2.run
```

This approach allows creating multiple MyApp instances with different configurations in a single Ruby process.
However, it has two major drawbacks:

* `config` must be passed explicitly in every `initialize` and `new` call, making the code verbose.
* Both `Part1` and `Part2` instances hold their own `@config` variables, which is redundant -- especially when creating a large number of small instances (e.g., tree nodes).

## Current approach 3: Thread-local storage

Storing configuration in `Thread[:config]` allows multiple application instances without explicit parameter passing.

```ruby
class MyApp
  class Part1
    def run
      using Thread[:config][:part1]...
    end
  end

  class Part2
    def run
      using Thread[:config][:part2]...
    end
  end

  def initialize(config)
    @config = config
    @part1 = Part1.new
    @part2 = Part2.new
  end

  def run
    Thread[:config] = config
    @part1.run
    @part2.run
  end
end

app1 = MyApp.new({ part1: ..., part2: ... })
app2 = MyApp.new({ part1: ..., part2: ... })

app1.run
app2.run
```

This approach is mostly effective but has an issue:

* `Thread[:config] = @config` must be set at the beginning of `MyApp#run`. While this is manageable if there is only one public API, it becomes error-prone when multiple APIs exist.

Note that using `Fiber#[]` instead of `Thread#[]` has the same issue.

## Proposal

Ideally, we want to support multiple application instances while keeping the simplicity of the global variable approach.

To achieve this, I propose introducing a new type variable, such as `$@config`:

* `$@config` belongs to an instance
* When accessing `$@config`, it is looked up not only in `self` but also by traversing the call stack to find the nearest `self` instance that has `$@config`.

With this, the code could be written as follows:

```ruby
class MyApp
  class Part1
    def run
      $@config[:part1] # accesses MyApp's $@config
    end
  end

  class Part2
    def run
      $@config[:part2]
    end
  end

  def initialize(config)
    $@config = config
    @part1 = Part1.new
    @part2 = Part2.new
  end

  def run
    @part1.run
    @part2.run
  end
end

app1 = MyApp.new({ part1: ..., part2: ... })
app2 = MyApp.new({ part1: ..., part2: ... })

app1.run
app2.run
```

This behaves similarly to dynamically scoped variables but differs in that it is resolved through the `self` instances.

(`Thread.new` is a bit problematic: if you use `Thread.new` in a method of `MyApp::Part1`, you wouldn't have access to `$@config` in it. It might be nice to take over all `$@x` variables.)

## Feedback wanted

Whenever I write a large Ruby application, I encounter this problem.
However, TBH, I am not entirely confident that my proposed solution is the best one.

Do you ever encounter this problem? How do you deal with the problem when you do? Is there a better workaround?



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