That logging issue was a mistake. All the access_log directive need to be in a single location/server block.

After a lot of testing. I found an issue.

The processing time of post_action is added to the response time. hence delaying the response.

Here is how i have used post_action to log.

http {

server {

location / {
proxy_pass http://upstream123;
post_action @loglua;
}

location @loglua {
set $log '';
rewrite_by_lua_file /nginx/mylua.lua;
logformat format1 '$log';
access_log /var/log/nginx/newlog.log format1;
}
}
}