Log module question: does the buffer mess up the order of the log entries?

Valentin V. Bartenev vbart at nginx.com
Mon Sep 23 17:48:29 UTC 2013

On Monday 23 September 2013 05:59:29 海峰 刘 wrote:
> Hi experts,
> I am reading the log module source code, there is something difficult to
> make sure, so I ask for your help.
> Access log module use a buffer to buffer log entries before writing to the
> file system, the buffer is initialised before the worker processes are
> forked, so I guess after the fork(), each worker has a copy, this also
> explains why there is no lock-unlock operations while using the buffer. To
> be sure about that, I did a simple test:
> 1, configure nginx to use 16k access log buffer, use the default keep-alive
> time(65), work in master-workers mode with a few worker processes; 2, open
> one browser, access nginx server, refresh a few times, no access log
> generated; 3, open another browser, do the same thing as 2, until the
> access log was flushed;
> I think there is a chance that the two browser was served by different
> worker processes, and log entries may be buffered in different buffers,
> which buffer get full first, which will be flush first. According that,
> the order of the log entries could be messed up. Unfortunately, I didn't
> see that after testing for a few times.
> My question is, Am I wrong about the log module behaviour, or I didn't get
> the right way to test it?

The order of access log entries is undefined anyway if you have more than
one worker processes.

  wbr, Valentin V. Bartenev

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