pid file deleted briefly when doing hot restart

Petteri Räty betelgeuse at gentoo.org
Mon Nov 26 23:56:18 UTC 2012


On 26.11.2012 20.24, Eric Wong wrote:

> 
>> The use case here is that with health monitors wouldn't have a window
>> where a pid file does not exist. With a hot restart it should always be
>> possible to have a pid file that points to either the old or the new master.
> 
> Then, doesn't nginx have the same problem?
> 

nginx doens't have the same problem as I show later. Even if it I
restart/reload nginx very infrequently. Unicorn has to be hot restarted
every time we change our application code and happens frequently.

>>> I think unicorn differs a bit from nginx here:
>>>
>>> nginx uses rename() to clear the way for a new pid file.  Like unicorn,
>>> this still leaves a window where no pid file exists.
>>>
>>
>> Looking at the inotify log it seems the reason pid file does not exist
>> is an explicit delete and not due to rename. It happens a couple seconds
>> earlier also so the window is possible to hit even with a periodic poller.
> 
> Is matching nginx rename behavior enough to solve the problem?
> 
> Matching nginx behavior can become the default if it solves your problem.
> 

nginx does not explicitly unlink the old pid file before it renames it
out of the way so yes matching nginx in that regard changes the behavior
exactly how I originally asked but you were against that. Maybe the
point is moot though.

This is from a combo of USR2, WINCH, QUIT sent from htop to the master
process:

2012-11-27 01:44:25 +0200
[:moved_from, :move]
"nginx.pid"
2012-11-27 01:44:25 +0200
[:moved_to, :move]
"nginx.pid.oldbin"
2012-11-27 01:44:25 +0200
[:create]
"nginx.pid"
2012-11-27 01:44:25 +0200
[:open]
"nginx.pid"
2012-11-27 01:44:25 +0200
[:modify]
"nginx.pid"
2012-11-27 01:44:25 +0200
[:close_write, :close]
"nginx.pid"
2012-11-27 01:45:31 +0200
[:delete]
"nginx.pid.oldbin"

The window here is much smaller than for the current unicorn behavior.

Regards,
Petteri


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