Lua in Redis and Nginx

Redis: atomic scripts

Redis runs Lua scripts atomically — no other command runs while a script executes — which makes multi-step operations safe without transactions.

-- illustrative
-- rate_limit.lua: allow `limit` requests per `window` seconds
local key = KEYS[1]
local limit = tonumber(ARGV[1])
local window = tonumber(ARGV[2])

local current = redis.call("INCR", key)
if current == 1 then
  redis.call("EXPIRE", key, window)
end
if current > limit then
  return 0
end
return 1

$ redis-cli --eval rate_limit.lua ratelimit:user:42 , 5 60
(integer) 1

• KEYS and ARGV hold the keys and arguments passed to EVAL.
• redis.call runs Redis commands (and raises errors); redis.pcall returns them instead.
• Keep scripts short — while one runs, the whole server waits.
• Redis 7 adds Functions (FUNCTION LOAD), named Lua libraries stored on the server.

OpenResty: Lua inside Nginx

OpenResty bundles Nginx with LuaJIT, so request handling can be scripted — API gateways, authentication, routing, and caching, at Nginx speed.

# nginx.conf
http {
    lua_shared_dict counters 10m;

    server {
        listen 8080;

        location /hello {
            default_type application/json;
            content_by_lua_block {
                local cjson = require "cjson"
                local name = ngx.var.arg_name or "world"
                local counters = ngx.shared.counters
                local visits = counters:incr("visits", 1, 0)
                ngx.say(cjson.encode({message = "Hello, " .. name, visits = visits}))
            }
        }
    }
}

$ curl "localhost:8080/hello?name=Ada"
{"message":"Hello, Ada","visits":1}

• Each request runs in its own coroutine; network calls (ngx.socket, lua-resty-http, lua-resty-redis) yield instead of blocking, so one worker handles thousands of connections.
• ngx.shared dictionaries share data between workers.
• Kong and APISIX, two popular API gateways, are built on OpenResty.

Other servers with Lua scripting include HAProxy, Envoy, and Apache (mod_lua).