Blocking mode?

[SvenAlHamad]

Hi,

I'm trying to create a reverse proxy, where the upstream location is defined inside a mysql table. I'm using this module to fetch the target, however I'm having a problem since this module works in a non-blocking mode.

Imagine this code:

set $target "";
access_by_lua_file /etc/openresty/scripts/code-to-set-target.lua;
proxy_pass https://$target;

$target is always empty when executing the proxy_pass line, but actually the problem is that the variable inside the lua scripts is populated only when the mysql result is returned, which can happen after the proxy_pass line is executed.

My question is, how can I force the mysql module to wait until results are returned, before proceeding with the rest of the execution?

[agentzh]

@SvenAlHamad Nonblocking means not blocking any OS threads. This library's API is nonblocking but synchronous, meaning that the current request will not continue without waiting for the MySQL replies.

[SvenAlHamad]

Hmm...but in that case I'm not sure why my code doesn't work:

code-to-set-target.lua;

local mysql = require "resty.mysql"
local db, err = mysql:new()
if not db then
    ngx.say("failed to instantiate mysql: ", err)
    return
end

db:set_timeout(1000) -- 1 sec

local ok, err, errcode, sqlstate = db:connect{
    host = "----",
    port = 3306,
    database = "proxy",
    user = "----",
    password = "-----",
    charset = "utf8",
    max_packet_size = 1024 * 1024,
}

if not ok then
    ngx.say("failed to connect: ", err, ": ", errcode, " ", sqlstate)
    return
end

--ngx.say("connected to mysql.")

local quoted_name = ngx.quote_sql_str(ngx.var.host)
res, err, errcode, sqlstate = db:query("select target from domains where domain = " .. quoted_name, 1)

if not res then
    ngx.say("bad result: ", err, ": ", errcode, ": ", sqlstate, ".")
    return
end

local cjson = require "cjson"
--ngx.say("result: ", cjson.encode(res))
local temp = res[1].target
--ngx.say("temp: ", temp)
ngx.var.target = temp
--ngx.say("ngx var: ", ngx.var.target)

local ok, err = db:set_keepalive(10000, 100)

if not ok then
    ngx.say("failed to set keepalive: ", err)
    return
end

When I uncomment the ngx.say calls, I get an output with the correct values.

But then inside my nginx.conf file, the proxy pass $target value seems to be empty. In case I manually set a hardcoded value for ngx.var.target inside the lua script, that value is passed correctly to the nginx.conf. That's why I thought it's got something to do with the non-blocking nature.

This is the extract from the nginx.conf.

location / {
    set $target;
    rewrite_by_lua_file /etc/openresty/scripts/code-to-set-target.lua;

    # proxy
    proxy_pass https://$host$target;
}

[agentzh]

@SvenAlHamad You cannot use ngx.say() to generate a response in the rewrite phase otherwise your proxy_pass won't have a chance to generate its own response.

[agentzh]

@SvenAlHamad For debugging purposes, you should use ngx.log() or print() instead.

[SvenAlHamad]

@agentzh thanks for the tip!

I've used ngx.say just for testing. But even if replace all the instances of that call with print, I get exactly the same behavior as before. My variable is still not populated, but in the logs, I see the print entries with the correct values. However, I've then tried outputting the value of the $target like so, and I get back an empty response:

nginx.conf

location / {
    set $target;
    rewrite_by_lua_file /etc/openresty/scripts/code-to-set-target.lua;
    return 200 $target;
}

Have any ideas why that might be?

[agentzh]

@SvenAlHamad return directive runs before rewrite_by_lua_file no matter how you arrange them in nginx.conf. I suggest you read my tutorials first to be aware of such pitfalls:

https://openresty.org/download/agentzh-nginx-tutorials-en.html