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With the nginx HttpLimitReq module requests can be limited by IP. However, I'm not understanding what the "nodelay" option does.

If the excess requests within the limit burst delay are not necessary, you should use the nodelay

limit_req   zone=one  burst=5  nodelay;
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2 Answers 2

up vote 5 down vote accepted

The documentation here has an explanation that sounds like what you want to know:

The directive specifies the zone (zone) and the maximum possible bursts of requests (burst). If the rate exceeds the demands outlined in the zone, the request is delayed, so that queries are processed at a given speed

From what I understand, requests over the burst will be delayed (take more time and wait until they can be served), with the nodelay options the delay is not used and excess requests are denied with a 503 error.

This blog post (archive.org) gives good explanation how the rate limiting works on nginx:

If you’re like me, you’re probably wondering what the heck burst really means. Here is the trick: replace the word ‘burst’ with ‘bucket’, and assume that every user is given a bucket with 5 tokens. Every time that they exceed the rate of 1 request per second, they have to pay a token. Once they’ve spent all of their tokens, they are given an HTTP 503 error message, which has essentially become the standard for ‘back off, man!’.

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I think you're incorrect, the nginx manual states: "Excessive requests are delayed until their number exceeds the maximum burst size". Note that until exceeds maximum burst is entirely different meaning than over the burst that you said. You also conflated burst with excess requests, I believe excess requests means it's above the zone, while it may still be below the maximum burst. –  Hendy Irawan Dec 19 '14 at 7:09

The way I see it is as follows:

  1. Requests will be served as fast as possible until the zone rate is exceeded. The zone rate is "on average", so if your rate is 1r/s and burst 10 you can have 10 requests in 10 second window.

  2. After the zone rate is exceeded:

    a. Without nodelay, further requests up to burst will be delayed.

    b. With nodelay, further requests up to burst will be served as fast as possible.

  3. After the burst is exceeded, server will return error response until the burst window expires. e.g. for rate 1r/s and burst 10, client will need to wait up to 10 seconds for the next accepted request.

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