Nginx lua openresty variable scope

Question

Here is working example of openresty nginx.conf file. In this example I make request to redis multiple times. As you can see I request data from redis first to check if domain can get SSL then again to get what backend to proxy request to, and then I've added S3 proxy and I would need to request data from redis again. I'm new to OpenResty and Lua and I wonder if it is possible to fetch data from redis once and use it multiple times across the script?

user www-data;
worker_processes  auto;
pid /run/openresty.pid;

events {
    worker_connections  1024;
}

error_log /var/log/openresty/error.log debug;

http {
    resolver 127.0.0.53 ipv6=off;
    lua_shared_dict acme 16m;

    init_by_lua_block {
        require("resty.acme.autossl").init({
            tos_accepted = true,
            staging = true,
            account_key_path = "/etc/openresty/account.key",
            account_email = "didnt@forgot.removing",
            domain_whitelist_callback = function(domain)
                local redis = require "resty.redis"
                local rds = redis:new()

                local ok, err = rds:connect("127.0.0.1", 6379)
                if not ok then
                    ngx.log(ngx.ERR, "failed to connect to redis: ", err)
                    return ngx.exit(500)
                end

                local res, err = rds:exists(domain)

                if res == 1 then
                    return true
                end
                if res == 0 then
                    return false
                end
            end
        })
    }

    init_worker_by_lua_block {
        require("resty.acme.autossl").init_worker()
    }

    server {
        access_log /var/log/openresty/access.log;

        listen 80;
        listen 443 ssl;
        server_name _;

        location / {
            set $backend '';
            set $tenant '';

            access_by_lua '
                local domain = ngx.req.get_headers()["Host"]
                local key = "site:" .. domain

                if not domain then
                ngx.log(ngx.ERR, "message 404 missing")
                return ngx.exit(404)
                end

                local redis =  require "resty.redis"
                local rds = redis:new()

                local ok, err = rds:connect("127.0.0.1", 6379)
                if not ok then
                    ngx.log(ngx.ERR, "failed to connect to redis: ", err)
                    return ngx.exit(500)
                end

                local all, err = rds:hgetall(key)
                if not all then
                    ngx.log(ngx.ERR, "no komprende: ", err)
                    return ngx.exit(505)
                end

                if all == ngx.null then
                    ngx.log(ngx.ERR, "no host found for key ", key)
                    return ngx.exit(404)
                end

                local result = {}
                    for i = 1, #all, 2 do
                    result[all[i]] = all[i+1]
                end

                ngx.var.backend = result["backend"]
                ngx.var.tenant = result["tenantID"]

                ngx.log(ngx.ERR, "uhm: ", ngx.var.backend)
            ';

            add_header X-TenantID $tenant always;
            proxy_pass http://$backend;

        }

        location ~* ^/static/(.*) {
            resolver               127.0.0.53 valid=300s;
            resolver_timeout       10s;
            set $s3_bucket        'drasha.ams3.digitaloceanspaces.com';
            set $url_full         '$1';
            proxy_http_version     1.1;
            proxy_set_header       Host $s3_bucket;
            proxy_set_header       Authorization '';
            proxy_hide_header      x-amz-id-2;
            proxy_hide_header      x-amz-request-id;
            proxy_hide_header      Set-Cookie;
            proxy_ignore_headers   "Set-Cookie";
            proxy_buffering        off;
            proxy_intercept_errors on;
            proxy_pass             http://$s3_bucket/AYAYA/$url_full;
        }

        lua_ssl_trusted_certificate /etc/ssl/certs/ca-certificates.crt;
        lua_ssl_verify_depth 2;
        ssl_certificate /etc/openresty/default.pem;
        ssl_certificate_key /etc/openresty/default.key;
        ssl_certificate_by_lua_block {
            require("resty.acme.autossl").ssl_certificate()
        }

        location /.well-known {
            content_by_lua_block {
                require("resty.acme.autossl").serve_http_challenge()
            }
        }
    }
}

Answer 1

OpenResty run Lua hooks in a sandbox, so one cannot use global variables to share data.

You shall use Data Sharing within an Nginx Worker It is usual practice to cache anything on Lua module level, possibly with some reasonable expiration period if data stored in Redis may be changed.

BTW - don't use XXX_by_lua directives - you should take care about nginx escaping rules, use XXX_by_lua_block.

Additional example:

local redis = require"resty-redis"

-- the module
local _M = {}

local hgetall_results = {}
_M.hgetall = function(key)
  if hgetall_results[key] then
    return hgetall_results[key]
  end
  local rds = redis:new()
  local ok, err = rds:connect("127.0.0.1", 6379)
  local all, err = rds:hgetall(key)
  local result = {}
  for i = 1, #all, 2 do
     result[all[i]] = all[i+1]
  end
  -- cache 
  hgetall_results[key] = result
  return result
end

return _M

The example above just illustrates the usual module scoped cache pattern. The error handling is on your own.