Simple Lua script for our test

msgpack-increment.lua

--[[
ARGUMENTS
  KEYS:
    1 - key
  ARGV:
    1 - sub-key to increment
--]]

-- unpack data, or empty data
local data = redis.call("get", KEYS[1])
if data == false then
  data = {}
else
  data = cmsgpack.unpack(data)
end

-- initialize
if type(data[ARGV[1]]) == "nil" then
  data[ARGV[1]] = 0
end

-- increment
data[ARGV[1]] = data[ARGV[1]] + 1

-- write data
redis.call("set", KEYS[1], cmsgpack.pack(data))

-- return, for consistency
return cmsgpack.pack(data)