OpenResty / Lua - Maintain mysql database connection in worker threads - mysql

Question

I have a simple module called "firewall.lua" that I wrote that has a function
firewall.check_ip(ip) which connects to localhost mysql and performs a query and returns the result. The function gets called from within Location / blocks in nginx sites via access_by_lua_block . The module gets initialized by init_worker_by_lua firewall.init().
Everything works as expected.
What I'd like to do however is maintain the database connection on the worker thread(s) so that I don't have to re-connect every time the function is called but instead re-use the existing connection established by the worker during initialization.
I'm not quite sure how to do this or if its actually doable in openresty/lua. I tried initializing the database connection variables outside of the function to give them scope within the module instead of function and I get various API errors that did not point me in the right direction.
Thank you!

Answer 1

This is possible using the OpenResty cosocket API, which gives you the ability to use a pool of non-blocking connections. There's already one MySQL driver (lua-resty-mysql) which uses the cosocket API. Since you didn't provide a code sample, I'm assuming you're not using it.
Example of a connection and query using lua-resty-mysql (untested):
access_by_lua_block {
local mysql = require "resty.mysql";
local db, err = mysql:new()
db:set_timeout(1000) -- 1 second
local ok, err, errcode, sqlstate = db:connect{
host = "127.0.0.1",
port = 3306,
database = "my_db",
user = "my_user",
password = "my_pwd",
}
if not ok then
ngx.say("Connection to MySQL failed: ", err)
return
end
result, err, errcode, sqlstate = db:query("select ...")
if not result then
ngx.say("MySQL error: ", err, ".")
return
end
db:close()
}
In case, e.g., you want to control the pool name or use other options, you can pass on additional parameters to connect:
...
local ok, err, errcode, sqlstate = db:connect{
host = "127.0.0.1",
port = 3306,
database = "my_db",
user = "my_user",
password = "my_pwd",
pool = "my_connection_pool",
}
...
You can find more information in the official docs:
lua-resty-mysql: https://github.com/openresty/lua-resty-mysql
Cosocket API: https://openresty-reference.readthedocs.io/en/latest/Lua_Nginx_API/#ngxsockettcp