Rails cache:

The main methods to call are clear, read, write, delete, exist?, and fetch.

class FakeRedis
  def initialize; @data = {}; end

  def read(key, *args) @data[key] end
  def write(key, val, *args) @data[key] = val end
  def delete(key) @data.delete(key) end
  def exists?(key) @data.include?(key) end
  def fetch(key, *args) @data[key] ||= yield if block_given? end
  def clear; @data = {}; end
end

allow(Rails).to receive(:cache).and_return(FakeRedis.new)