load balancing using lua

load_balancing.lua

REDIS_HOST = "127.0.0.1"
REDIS_PORT = "6379"
DEFAULT_UPSTREAM_SERVER = "127.0.0.1"
local redis = require "resty.redis"
local red = redis:new()
red:set_timeout(1000) -- 1 second
local ok, err = red:connect(REDIS_HOST,REDIS_PORT)
if not ok then
  ngx.var.target = DEFAULT_UPSTREAM_SERVER
else
  local key = "service_discovery"
  -- Services are idenified by the port they listen to.
  -- Can also be identified by service name or any other information which can be extracted from the request
  local host_list_str, err = red:hmget(key, ngx.var.server_port)
  if not host_list_str or host_list_str[1] == ngx.null then
    ngx.var.target =  DEFAULT_UPSTREAM_SERVER
  else
    local hosts = {}
    local index = 1
    -- Comma seperated host list for a given service
    for value in string.gmatch(host_list_str[1], "[^,]+") do
      hosts[index] = value
      index = index + 1
    end
    -- Pick a random host, assuming randomness will also bring some form of roundrobin-ness
    ngx.var.target = hosts[math.random(#hosts)]
    ngx.log(ngx.ERR, "selected host is: " .. ngx.var.target)
  end
 end