BulkCacheFetcher

Bulk Cache Fetcher fills the gap between Russian doll caching and the n+1 queries problem.

Russian doll caching is really great for handling views and partials. When those partials show highly nested objects, though, cache misses are expensive. Usually, you'll either preload the entire object hierarchies in your controller (even on cache hits), or you'll accept the n+1 queries when you miss the cache.

Bulk Cache Fetcher allows you to query the cache for a list of objects, and gives you the opportunity to fetch all of the cache misses at once, using whatever :includes you want.

For example, if you have a list of objects by id to fetch, and a list of keys you want them to be cached under, you'd use Bulk Cache Fetcher like this:

identifiers = {:cache_key_1 => 1, :cache_key_2 => 2, :cache_key_3 => 3}
BulkCacheFetcher.new(Rails.cache).fetch(identifiers) do |uncached_keys_and_ids|
  ids = uncached_keys_and_ids.values
  BlogPost.where(:id => ids).includes([:author, :comments])
end

This will include and cache each BlogPost, with comments and authors, as if you did the .where.includes without caching. If a BlogPost is cached already, it won't fetch it (or its includes) from the database.

Installation

Add this line to your application's Gemfile:

gem 'bulk_cache_fetcher'

And then execute:

$ bundle

Or install it yourself as:

$ gem install bulk_cache_fetcher

Usage

Basic usage is pretty simple:

cache_fetcher = BulkCacheFetcher.new(Rails.cache)
cache_fetcher.fetch([1, 2, 3]) do |identifiers|
  Post.includes(...).find(identifiers)
end

it even returns them in the same order you return them in:

results = cache_fetcher.fetch([2, 1, 3]) do |identifiers|
  expensive_calculation(identifiers) # => returns [result 2, result 1, result 3]
end

results.first # => expensive_calculation([2])
cache.read(1) # => expensive_calculation([1])

Complex identifiers

In a lot of cases, you'll have a cache key along with a little bit of extra data you'll need to find the record or perform a calculation. You can use the cache fetcher for this, with a hash instead of an array for the identifier list:

cache_fetcher.fetch({:one => 1, :two => 2}) do |identifiers|
  expensive_calculation(identifiers.values)
end

cache.read(:one) # => expensive_calculation([1])

Contributing

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request