Atomic Redis INCRBY on a specific field of all JSON-encoded elements of a Redis list.

json_list_incrby.lua

-- Let's say you've got a Redis list that's composed of JSON values:
-- -- {"a": 5.2, "b": "something blah blah"}
-- -- {"a": 4.3, "b": "something blah blah"}
-- -- {"a": 125.2, "b": "something blah blah"}
-- And let's say that you want to increment all of the "a" fields by n.
-- Call the below script with:
-- -- KEYS = [key_of_list_to_update]
-- -- ARGV = [json_key_to_incr, value_to_add]

local num = redis.call('llen', KEYS[1])
for i = 1,num do
    local obj = cjson.decode(redis.call('lpop', KEYS[1]))
    obj[ARGV[1]] = obj[ARGV[1]] + ARGV[2]
    redis.call('rpush', KEYS[1], cjson.encode(obj))
end
return num