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# config.threadsafe!: what does it do?
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# Let’s take a look at the threadsafe! method:
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def threadsafe!
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@preload_frameworks = true
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@cache_classes = true
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@dependency_loading = false
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@allow_concurrency = true
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self
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end
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# Calling this method sets four options in our app configuration. Let’s walk
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# through each option and talk about what it does.
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# Preloading Frameworks
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# The first option @preload_frameworks does pretty much what it says, it forces
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# the Rails framework to be eagerly loaded on boot. When this option is not
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# enabled, framework classes are loaded lazily via autoload. In multi-threaded
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# environments, the framework needs to be eagerly loaded before any threads are
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# created because of thread safety issues with autoload. We know that loading
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# the framework isn’t threadsafe, so the strategy is to load it all up before
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# any threads are ready to handle requests.
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# Caching classes
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# The @cache_classes option controls whether or not classes get reloaded.
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# Remember when you’re doing “TDD” in your application? You modify a controller,
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# then reload the page to “test” it and see that things changed? Ya, that’s what
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# this option controls. When this option is false, as in development, your
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# classes will be reloaded when they are modified. Without this option, we
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# wouldn’t be able to do our “F5DD” (yes, that’s F5 Driven Development).
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# In production, we know that classes aren’t going to be modified on the fly,
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# so doing the work to figure out whether or not to reload classes is just
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# wasting resources, so it makes sense to never reload class definitions.
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# Dependency loading
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# This option, @dependency_loading controls code loading when missing constants
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# are encountered. For example, a controller references the User model, but the
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# User constant isn’t defined. In that case, if @dependency_loading is true,
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# Rails will find the file that contains the User constant, and load that file.
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# We already talked about how code loading is not thread safe, so the idea here
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# is that we should load the framework, then load all user code, then disable
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# dependency loading. Once dependency loading is disabled, framework code and
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# app code should be loaded, and any missing constants will just raise an
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# exception rather than attempt to load code.
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# We justify disabling this option in production because (as was mentioned
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# earlier) code loading is not threadsafe, and we expect to have all code loaded
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# before any threads can handle requests.
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# Allowing concurrency
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# @allow_concurrency option controls whether or not the Rack::Lock middleware
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# is used in your stack. Rack::Lock wraps a mutex around your request. The idea
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# being that if you have code that is not threadsafe, this mutex will prevent
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# multiple threads from executing your controller code at the same time. When
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# threadsafe! is set, this middleware is removed, and controller code can be
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# executed in parallel.
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