Re: Thread.current

[Jimmy Soho <jimmy.soho@gmail.com>]

Some component don't always have to work within a webserver context,
and therefor assume no access to a Rack +env+ hash. Widely used
examples are the i18n and the active_support gem. In our case we have
for example queued jobs that are executed with the full rails
environment loaded, which does not have a rack context. This is not an
issue, but does explain (to some extent) why some components use
thread local storage instead of the rack +env+ context.

I'm not trying to use Thread local storage myself, it is forced upon
us. ;-) I'm trying to determine if the components we must use due to
dependencies, and which do use Thread local storage, if they are
leaking data from one request into the next request if you are within
a unicorn context. There are cases where we want this, and there are
cases where we don't want this leakage.

Take for example activesupport's usage of Time.zone. Under water this
is set in a thread local var. If you set Time.zone in one request, but
not in the next request, using unicorn the next request will use the
time zone of the previous request. Using rack or mongrel (in
multithreaded mode) you don't have this issue perse (though they have
other issues then).

Same for the i18n gem and it's usage of the I18n.locale value, which
is also set in a thread local var.

So yeah, unfortunately I have to take into account this "crappy idiom"
and need to know exactly which thread local vars are set by all the
components we use, and determine which of those must be reset before
each request.


Cheers,
Jimmy


On Wed, Jan 12, 2011 at 10:12 AM, Jordan Ritter <jpr5@darkridge.com> wrote:
> Unicorn is purely about employing a multi-process model, not a multi-thread model; it specifically avoids spawning threads to handle inbound requests.   In fact, I'll bet that inside each request, Thread.current == Thread.main.
>
> Separate from Unicorn, when running a rack-compatilbe app in multithreaded mode (the default when the app is invoked directly via rackup + config.ru), there's no guarantee about which thread will service a given request.  This fact may not matter to you, depending on what you're trying to do.
>
> That said, you *could* use Thread local storage for per-request storage in either unicorn or multithreaded situations, so long as you wiped your storage at the beginning/end of each request -- but that's a crappy idiom, even if it might be "common" (don't know what you're referring to offhand).  Can't suggest a more appropriate pattern without knowing more about what you're actually trying to do.
>
> cheers,
> --jordan
>
> On Jan 11, 2011, at 2:52 PM, Jimmy Soho wrote:
>
>> Hi,
>>
>> Some more questions still:
>>
>> It seems a worker uses the exact same thread to handle each request.
>>
>> Is that guaranteed to happen for the lifetime of a worker? Or are
>> there cases where a unicorn worker might spin a new thread to handle
>> the next requests?
>>
>> If the same thread is always used, isn't that a potential issue when
>> programmers use thread local variables, which are not reset at the
>> next request?  (I know, the usage of thread local variables is not
>> recommended, but take a random rails project, go into their $GEM_HOME
>> and do grep -r Thread.current . , see what I mean..)
>>
>
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