Is it thread safe to use memoization on class variables?

rubyonrails-talk
1 
class Blog
  
  def self.articles
    @@articles ||= Dir.glob(Rails.root.join('app', 'views', 'articles', '*.html.erb')).map do |file|
      parse_file(file).front_matter
    end
  end

end

Is the above code thread safe / safe?

(i.e. I am asking about the use of @@articles ||= to cache the expensive operation)

2

Since ||= is in fact two operations, your code would not prevent multiple executions of Dir.glob... at the same time. In that sense it is not “thread safe”.

Whether this creates a real problem for your application depends on your context (performance constraints, side effects of Dir.glob..., etc.)

Sidekiq wiki and How Do I Know Whether My Rails App Is Thread-safe or Not? both recommend not using class variables, but these are general guidelines and your concrete situation might be different.

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3

For my understanding the worst case scenario is that two threads (or more) start reading the files together and then they both assign (the same computed value) to the variable.

I guess that the assignment operation (=) is atomic, so it should not create a corrupted state for the class variable in my case (the files are static and are only read).

4

That’s right. However, for a slightly more elegant approach, you could load these cached values on initialization stage. For example, if you add a config/initializers/load_articles.rb:

Rails.application.config.to_prepare do
  Blog.articles = Dir.glob … # Assuming you have Blog.articles=.
end

This will not only load them once in a thread-safe manner, but would also (if memory serves right) reload these files in development, if you update/add/remove articles, without having to restart the Rails server.

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