Slow cache read on first cache fetch in Rails

In Rails, the first cache fetch can be slower than subsequent fetches due to the way caching works by default. When a cache key is requested for the first time, Rails performs the necessary calculations or queries to generate the content and then stores it in the cache. Subsequent requests for the same cache key will then be much faster as the cached content is directly returned.

To address the slow first cache fetch issue, you can use the fetch method of Rails caching to handle the cache fetch in a more efficient way. The fetch method allows you to specify a block of code to execute when the cache key is missing, and the result of the block will be stored in the cache.

Here's how you can use the fetch method to improve the cache read performance:

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# app/controllers/my_controller.rb
class MyController < ApplicationController
  def my_action
    # Set the cache key
    cache_key = "my_cache_key"

    # Fetch the data from the cache, or calculate it and store it in the cache
    @data = Rails.cache.fetch(cache_key) do
      # This block will be executed only when the cache is empty for the given key
      # Calculate or retrieve the data here, e.g., perform complex database queries
      # and return the result that you want to cache
      retrieve_data_from_database_or_complex_operation
    end

    # Render the view with @data
  end

  private

  def retrieve_data_from_database_or_complex_operation
    # Perform complex operations or database queries to get the data
    # Return the data that you want to cache
  end
end

In this example, we use the Rails.cache.fetch method to fetch the data from the cache with the specified cache key (my_cache_key). If the cache is empty for the given key, the block provided to fetch will be executed, and the result of the block will be stored in the cache. On subsequent requests for the same cache key, the data will be directly fetched from the cache.

By using fetch, you can improve the performance of the first cache fetch by making the caching process more efficient and avoiding unnecessary recalculations or queries. Keep in mind that the block provided to fetch should contain the code that calculates or retrieves the data you want to cache.