redis-structured-multi

redis-structured-multi is a Ruby library for assembling Redis's multibulk replies (from the redis-rb gem's Redis#multi) into objects, before they're actually returned, by using promises, thunks, and fmap.

Just require redis/structured-multi, then you can do something like this:

records = [
  {:name => 'bob', :height => 180, :likes => 'carrots'},
  {:name => 'phil', :height => 145, :likes => 'apples'}]

REDIS = Redis.new

full_records = REDIS.structured_multi do
  records.map do |record|
    last_visit = REDIS.get("user:#{record[:name]}:lastvisittime").to_i

    likes = (REDIS.smembers("user:#{record[:name]}:likes") + [record[:likes]]).to_set
    
    left_handed = REDIS.get("user:#{record[:name]}:has:hand:left")
    right_handed = REDIS.get("user:#{record[:name]}:has:hand:right")

    record.merge(
      :last_visit => last_visit,
      :likes => likes,
      :has_both_hands => (left_handed && right_handed))
  end
end

Under the covers, this is a single Redis pipeline. Even though we're able to do things like redis_bool && other_redis_bool, or redis_set_members + static_set_members, we're really just transforming promises into other promises. Here's the strategy:

• Execute a bunch of Redis commands; receive promises in exchange.
• Build the structure you want out of data you have, plus these promises.
• Return this structure as the value of the structured_multi block.

After you do this, structured-multi will actually execute the pipeline, then fmap your structure into an equivalent one with all the same real data, but with the promises replaced with their actual Redis-retrieved values.