We’re in the process of switching over to Unicorn here at Revelation and yesterday we started to notice some strange behavior with I18n. The I18n locale we set in a previous request was persisting to the next. Digging into I18n we found out why: I18n holds the current locale on a thread-local variable, and unicorn does not start a new thread for each new request, so the current locale or any other thread-local variable will persist across requests. Here’s a quick example to show the different behavior of Mongrel vs. Unicorn with thread-local variables.

Here’s a really basic Sinatra app:

If we start this app up with Mongrel and go to /portland we get: "Portland: ". The value of our thread-local variable is nil, because mongrel creates a new thread for each request.

If we start this app up with Unicorn and go to /portland we get: “Portland: is awesome”. Unicorn has kept the same thread across both requests so the thread-local variable is still set to “is awesome”.

Since I18n stores locale in a thread-locale variable you have to be very cautious to reset locale for each request or the last requests locale will persist and your users will get some strange behavior.

Cheers,

James

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