Class variables not classy!

(actually)

Class variables caches are not so classy

(umm, in fact)

Class variables are, well, controversial,

class variables used for caching = bad

Class variables bad?

Main Reason

class A
  @@x = 2  

  def say
   @@x
  end
end

class B < A
  def haha=y
   @@x=y
  end
end

class C < A
  def haha=y
    @@x=y
  end
end


b = B.new
c = C.new

b.say #=> 2
c.say #=> 2

b.haha = 'because I can'
c.say #=> 'because I can'

Other reasons

Thread safety
Its a global variable

But still,

they are useful.

Letsbe practical

And my code has no threads !

class A

  @@a_variable_shared_across_all_instances_of_A = nil
  

  def my_method
    # and it is accessible within the scope
    # just like that
    @a_variable_shared_across_all_instances_of_A = 'shared-data'
  end

end

So whats not classy?

class Shippping

  @@countries = Country.all

  def label_the_parcel(address)
    country = @@countries.detect{|name| name == address.country_name}
    print_label country.shipping_code
  end

end

So whats not classy?

class Shippping

  @@california_code = State.where(country: 'U.S.A').last.shipping_code

  def label_for_california(address)
    print_label address
    print_label @@california_code
  end

end

This can fail during class load

This can fail rails s

So whats not classy?

class Shippping

  def label_for_california(address)
    print_label address
    print_label Shipping.california_code
  end

  def self.california_code
    @@california_code ||= State.where(country: 'U.S.A').last.shipping_code
  end
end

The instance-variable cache pattern

Because why not

So whats not classy?

class A

  def slowly_loaded_data
   @slowly_loaded_data ||= slow_queries
  end

end

The actual instance-variable cache pattern

So whats not classy?

class Shippping

  def label_for_california(address)
    print_label address
    print_label Shipping.california_code
  end

  @@california_code = nil
  def self.california_code
    @@california_code ||= State.where(country: 'U.S.A').last.shipping_code
  end
end

That, right there, is a cache

So whats not classy?

And caches have to expire sometime.

Some day, the california_code will change.

So for that day, we have -

So whats not classy?

class Shippping

  @@california_code = nil
  def self.california_code
    @@california_code ||= State.where(country: 'U.S.A').last.shipping_code
  end

  def self.flush_california_code
    @@california_code = nil
  end
end

class State
  after_save :flush1

  def flush1
    Shipping.flush_california_cache if name=='california'
  end
end

So whats not classy?

And I have no threads

What could possibly go wrong?

So whats not classy?

drum-roll...

Enter Sidekiq

Cache invalidation will not be carried forward
The flush1 action will occur only in the web (Unicorn), and not in the Sidekiq
In fact it will not occur in other unicorn workers too
It is an in-process cache

What can be done?

In-process caches can cause subtle bugs
Using Rails.cache with memcache or File cache (default) is safer
Instance variable caches are still ok

Who else does it



cd discourse
ag @@ . | wc -l  #=>


cd gitlab
ag @@ . | wc -l  #=>


cd spree
ag @@ . | wc -l  #=>


cd rails
ag @@ . | wc -l  #=>

@h6165

Thanks