Blog: Redis as a mutex service, Code: cache_lookup

gistfile1.rb

# CAVEAT EMPTOR: This is excerpted code that I pulled out of context,
# re-arranged, renamed the variables of, and otherwise dicked around with
# a lot without having put it back into a Ruby interpreter, so it's
# possible that there's some gnarly typo or bug in here. In short, don't
# cut-and-paste this code.

# In our example, "cache" is some resource accessible by all of our
# processes and provides a run-time interface similar to Ruby's Hash
# class.
def cache_lookup(redis, resource_handle, cache, &run_on_cache_miss)
  begin
    cache.fetch(resource_handle)
  rescue KeyError
    redis_spinlock(redis, resource_handle) do |spin_count|
      if spin_count > 0
        # We had to wait before acquiring the lock, which means something
        # probably is in the cache now. We should check the cache again.
        begin
          return cache.fetch(resource_handle)
        rescue KeyError
          # do nothing
        end
      
      cache[resource_handle] = run_on_cache_miss.call
    end
  end   
end