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Is config.threadsafe! replaceable with config.allow_concurrency = true on Ruby 2.0?

Maurizio De Santis Mar 21, 2013 9:12 AM
Posted in group: Ruby on Rails: Core
In this article Aaron Patterson explains config.threadsafe! effects.

Since that the not-threadsafe autoload bug is closed on Ruby 2.0, we can reconsider his observations:

  @preload_frameworks = true

The first option @preload_frameworks does pretty much what it says, it forces the Rails framework to be eagerly loaded on boot. When this option is not enabled, framework classes are loaded lazily via autoload. In multi-threaded environments, the framework needs to be eagerly loaded before any threads are created because of thread safety issues with autoload. We know that loading the framework isn’t threadsafe, so the strategy is to load it all up before any threads are ready to handle requests.

The bug is closed on Ruby 2.0, so @preload_frameworks should be safely set to false.

  @cache_classes      = true

The @cache_classes option controls whether or not classes get reloaded. Remember when you’re doing “TDD” in your application? You modify a controller, then reload the page to “test” it and see that things changed? Ya, that’s what this option controls. When this option is false, as in development, your classes will be reloaded when they are modified. Without this option, we wouldn’t be able to do our “F5DD” (yes, that’s F5 Driven Development).

In production, we know that classes aren’t going to be modified on the fly, so doing the work to figure out whether or not to reload classes is just wasting resources, so it makes sense to never reload class definitions.

This consideration is valid, but it is by default set to true in the production environment configuration file, so we can skip it.

  @dependency_loading = false

This option, @dependency_loading controls code loading when missing constants are encountered. For example, a controller references the User model, but the User constant isn’t defined. In that case, if @dependency_loading is true, Rails will find the file that contains the User constant, and load that file. We already talked about how code loading is not thread safe, so the idea here is that we should load the framework, then load all user code, then disable dependency loading. Once dependency loading is disabled, framework code and app code should be loaded, and any missing constants will just raise an exception rather than attempt to load code.

We justify disabling this option in production because (as was mentioned earlier) code loading is not threadsafe, and we expect to have all code loaded before any threads can handle requests.

This is related to the same old bug, so we can set it to true.

@allow_concurrency = true

@allow_concurrency is my favorite option. This option controls whether or not the Rack::Lock middleware is used in your stack. Rack::Lock wraps a mutex around your request. The idea being that if you have code that is not threadsafe, this mutex will prevent multiple threads from executing your controller code at the same time. When threadsafe! is set, this middleware is removed, and controller code can be executed in parallel.

(google groups editing sucks) This is the only option which actually enables Rails multithreading, so we set it to true, like Patterson wrote.

So, summing up:

@preload_frameworks = false
@cache_classes = true
@dependency_loading = true
@allow_concurrency = true

But the first three configurations are the default ones on the production environment configuration file, so on Ruby 2.0 we can use:

# Code is not reloaded between requests
config.cache_classes = true
# Enable threaded mode
config.allow_concurrency = true

Isn't it?