RequestLocals
¶ ↑
If you have ever needed to use a global variable in Rails, you know it sucks.
One of the usual tricks is to go for Thread.current
, or if you have done your homework, to use the awesome {request_store
}.
# Using Thread.current def self.foo Thread.current[:foo] ||= 0 end def self.foo=(value) Thread.current[:foo] = value end # Using RequestStore def self.foo RequestStore.fetch(:foo) { 0 } end def self.foo=(value) RequestStore.store[:foo] = value end
The problem¶ ↑
-
Using
Thread.current
, values can stick around even after the request is over, since some servers have a pool of Threads that they reuse, which can cause bugs. -
Using
request_store
, the storage is not actually request local. Variables are stored inThread.current
, except that the storage is cleared after each request. However, this does not work when you need to use multiple threads per request, different threads access different stores.
The solution¶ ↑
Add this line to your Gemfile:
gem 'request_store_rails'
And change the code to this:
def self.foo RequestLocals.fetch(:foo) { 0 } end def self.foo=(value) RequestLocals.store[:foo] = value end
Oh yeah, everywhere you used Thread.current
or RequestStore.store
just change it to RequestLocals.store
. Now your variables will actually be stored in a true request-local way.
No Rails? No Problem!¶ ↑
A Railtie is added that configures the Middleware for you, but if you're not using Rails, no biggie! Just use the Middleware yourself, however you need. You'll probably have to shove this somewhere:
use RequestStoreRails::Middleware
Multi-Threading¶ ↑
The middleware in the gem sets a thread-local variable :request_store_id
in Thread.current
for the main thread that is executing the request.
If you need to spawn threads within a server that is already using thread-based concurrency, all you need to do is to make sure that the :request_store_id
variable is set for your threads, and you will be able to access the RequestLocals
as usual.
A good way to apply this pattern is by encapsulating it into a helper class:
# Public: Custom thread class that allows us to preserve the request context. class ThreadWithContext # Public: Returns a new Thread that preserves the context of the current request. def ThreadWithContext.new(*args) store_id = RequestLocals.current_store_id Thread.new { RequestLocals.set_current_store_id(store_id) yield *args } end end RequestLocals[:foo] = 1 ThreadWithContext.new { puts RequestLocals[:foo] # => 1 }
The gem does not provide such construct to avoid name collisions, you are free to reuse the snippet above and adjust it to match your use case.
If you are feeling adventurous, you could try using this fire and forget script and make all of your threads request aware, or should I say prepend and forget :smile:? Probably not something to be used in a production environment, but whatever floats your boat :boat:
Atomicity¶ ↑
Have in mind that the RequestLocals.fetch(:foo) { 'default' }
operation is {atomic}[https://github.com/ElMassimo/request_store_rails/blob/master/lib/request_locals.rb#L62], while RequestLocal[:foo] ||= 'default'
is not. In most scenarios, there is not a lot of difference, but if you are in a concurrent environment make sure to use the one that is more suitable for your use case :wink:
Replacing request_store
¶ ↑
While the plan is not to achieve 100% compatibility, this gem usually works well as a drop-in replacement. If you are using gems that rely on RequestStore
but for some reason you need them to use the appropriate request/thread scope, you can try something like this on application.rb
or similar:3
if RequestStore != RequestLocals RequestStore::Railtie.initializers.clear Kernel.suppress_warnings { RequestStore = RequestLocals } end
Usage in Sidekiq¶ ↑
If your code depends on these global variables, it's likely that you'll need to avoid collisions in Sidekiq workers (which would happen if the current store id is nil
).
You can use the following middleware, using the job id to identify the store:
class Sidekiq::Middleware::Server::RequestStoreRails def call(_worker, job, _queue) RequestLocals.set_current_store_id(job['jid']) yield ensure RequestLocals.clear! RequestLocals.set_current_store_id(nil) end end
Make sure to configure it as server middleware:
Sidekiq.configure_server do |config| config.server_middleware do |chain| chain.add Sidekiq::Middleware::Server::RequestStoreRails end end
Special Thanks¶ ↑
The inspiration for this gem, tests, and a big part of the readme were borrowed from the really cool {request_store
} gem. Thanks Steve :smiley:
Contributing¶ ↑
-
Fork it
-
Create your feature branch (
git checkout -b my-new-feature
) -
Commit your changes (
git commit -am 'Add some feature'
) -
Push to the branch (
git push origin my-new-feature
) -
Create new Pull Request
Don't forget to run the tests with rake
.
License¶ ↑
Copyright (c) 2015 Máximo Mussini Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.