nginx: what is nodelay in http request limit module? - nginx

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, buy with nodelay nginx process all 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

Answer 2

TL;DR: The nodelay option is useful if you want to impose a rate limit without constraining the allowed spacing between requests.
There's new documentation from Nginx with examples that answers this: https://www.nginx.com/blog/rate-limiting-nginx/
Here's the pertinent part. Given:
limit_req_zone $binary_remote_addr zone=mylimit:10m rate=10r/s;
location /login/ {
limit_req zone=mylimit burst=20;
...
}
The burst parameter defines how many requests a client can make in
excess of the rate specified by the zone (with our sample mylimit
zone, the rate limit is 10 requests per second, or 1 every 100
milliseconds). A request that arrives sooner than 100 milliseconds
after the previous one is put in a queue, and here we are setting the
queue size to 20.
That means if 21 requests arrive from a given IP address
simultaneously, NGINX forwards the first one to the upstream server
group immediately and puts the remaining 20 in the queue. It then
forwards a queued request every 100 milliseconds, and returns 503 to
the client only if an incoming request makes the number of queued
requests go over 20.
If you add nodelay:
location /login/ {
limit_req zone=mylimit burst=20 nodelay;
...
}
With the nodelay parameter, NGINX still allocates slots in the queue
according to the burst parameter and imposes the configured rate
limit, but not by spacing out the forwarding of queued requests.
Instead, when a request arrives “too soon”, NGINX forwards it
immediately as long as there is a slot available for it in the queue.
It marks that slot as “taken” and does not free it for use by another
request until the appropriate time has passed (in our example, after
100 milliseconds).