Ruby on Rails Background Jobs with Sidekiq

Understanding Idempotence

Idempotence can be a difficult concept to grasp, and it can be equally difficult to determine that a routine or job is idempotent. The key is to ask yourself what will happen if the method is called more than once with the same arguments.

Here is a simplified example, where a job is adding more quantity to a product:

class AddMoreProductsJob
  include Sidekiq::Job
  def perform(product_id, num_new_products)
    product = Product.find(product_id)
    product.update!(
      quantity_remaining: product.quantity_remaining + num_new_products
    )
  end
end

If the job gets retried for some reason, even if every line of code has executed, it will have added twice the number of products as intended. That means ...