Redis get unique user identity by key

getuserid.lua

local appPrefix   = ARGV[1]
local userId      = KEYS[1]

if not appPrefix then
  appPrefix = "trak"
end

if not userId then
  return "error: userId/keys[1] is required"
end

local ukey        = appPrefix .. "::users::" .. userId
local uid = redis.call("GET", ukey)

if not uid then
  uid = redis.call("INCR", appPrefix .. "::identity::users") 
  redis.call("SET", ukey, uid)
end

return uid .. ""