nginx: what is nodelay in http request limit module?

Question

I am learning nginx httprequestlimitmodule. i am not getting the concept of nodelay in httprequestmodule. i have tried below two configuration with nodelay and without nodelay. in both cases with nodelay and without nodelay i am hitting 10 request in 1 seconds and getting 503 temporary service unavailable error for 6 requests and 4 requests are successful. my question is if the result is same with nodelay and without nodelay then what is the use of nodelay option here.

limit_req_zone $binary_remote_addr zone=one:10m rate=2r/s;
limit_req zone=one burst=2 nodelay;

---

limit_req_zone $binary_remote_addr zone=one:10m rate=2r/s;
limit_req zone=one burst=2 ;

Answer 1

Let's take this config:

limit_req_zone $binary_remote_addr zone=one:10m rate=1r/s;

server {
    listen 127.0.0.1:81;

    location / {
        limit_req zone=one burst=5;
        echo 'OK';
    }

    location /nodelay {
        limit_req zone=one burst=5 nodelay;
        echo 'OK';
    }
}

and test it with nodelay

$ siege -q -b -r 1 -c 10 http://127.0.0.1:81/nodelay
      done.

Transactions:                  6 hits
Availability:              60.00 %
Elapsed time:               0.01 secs
Data transferred:           0.00 MB
Response time:              0.00 secs
Transaction rate:         600.00 trans/sec
Throughput:             0.09 MB/sec
Concurrency:                0.00
Successful transactions:           6
Failed transactions:               4
Longest transaction:            0.00
Shortest transaction:           0.00

And without nodelay:

$ siege -q -b -r 1 -c 10 http://127.0.0.1:81/
      done.

Transactions:                  6 hits
Availability:              60.00 %
Elapsed time:               5.00 secs
Data transferred:           0.00 MB
Response time:              2.50 secs
Transaction rate:           1.20 trans/sec
Throughput:             0.00 MB/sec
Concurrency:                3.00
Successful transactions:           6
Failed transactions:               4
Longest transaction:            5.00
Shortest transaction:           0.00

They both passed 6 request, but with nodelay Nginx process all the burst requests instantly and without this option Nginx makes excessive requests to wait so that overall rate would be no more than 1 request per second and last successful request took 5 seconds to complete.

EDIT: rate=6r/s actually means one request in 1/6th of a second. So if you send 6 request simultaneously you'll get 5 of them with 503.

There is a good answer with “bucket” explanation: https://serverfault.com/a/247302/211028