foo.rb

class TokenStore
  include Singleton

  attr_reader :params

  def initialize
    @params = {
      client_id: ENV['PUBLIX_AUTH_CLIENT_ID'],
      client_secret: ENV['PUBLIX_AUTH_CLIENT_SECRET'],
      grant_type: "client_credentials",
      scope: ENV['PUBLIX_AUTH_PARAM_SCOPE'],
      requested_token_use: ENV['PUBLIX_AUTH_REQUESTED_TOKEN_USE'],
    }.compact
  end

  def token
    cached_token = cached

    if cached_token.present?
      puts "Returning cached token"
      return cached_token
    end

    puts "Fetching a new token"
    fetch_token!
  end

  def cached
    token_and_expiry = Rails.cache.read(cache_key)
    return nil unless token_and_expiry.present?

    token, expires_at = token_and_expiry
    # In reality, we do a jitter based check, but this is being ignored for simplicity for testing
    return token if (utc_now + 120) > expires_at
  end

  def fetch_token!
    response = Faraday.post(ENV['PUBLIX_AUTH_URL'], params) do |f|
      f.response :json
      f.request :url_encoded
    end

    access_token = response.body['access_token']
    # In reality, we do a jitter based reduction here but this is being ignored for simplicity for testing
    expires_in = response.body["expires_in"].to_i.seconds

    expires_at = (utc_now + expires_in).to_i

    Rails.cache.write(cache_key, [access_token, expires_at])
    access_token
  end

  def cache_key
    Digest::SHA256.hexdigest("access_tokens/publix/dev_test")
  end

  def utc_now
    Time.now.utc.to_i
  end
end