Rajeev Kannav Sharma

Posts About

Ordering Rack Middleware Rails Application

19 Sep 2015

Rack middleware is a simply bigger thing then rails. It’s look-alike rack application that you can use to filter, process, or otherwise mess with responses.There is middleware like warden - devise, to work on catch and log exceptions, deal with cookies, SSO, and pretty much anything else you can think of. And in case you missed it, rails is a rack application. Create a new rails app and run

    rake middleware

It will display results like this :

    use Rack::Cors
    use Rack::Sendfile
    use ActionDispatch::Static
    use Rack::Lock
    use #<ActiveSupport::Cache::Strategy::LocalCache::Middleware:0x00000007238168>
    use Rack::Runtime
    use Rack::MethodOverride
    use ActionDispatch::RequestId
    use Rails::Rack::Logger
    use ActionDispatch::ShowExceptions
    use ActionDispatch::DebugExceptions
    use ActionDispatch::RemoteIp
    use ActionDispatch::Reloader
    use ActionDispatch::Callbacks
    use ActiveRecord::Migration::CheckPending
    use ActiveRecord::ConnectionAdapters::ConnectionManagement
    use ActiveRecord::QueryCache
    use ActionDispatch::Cookies
    use ActionDispatch::Session::CookieStore
    use ActionDispatch::Flash
    use ActionDispatch::ParamsParser
    use Remotipart::Middleware
    use Rack::Head
    use Rack::ConditionalGet
    use Rack::ETag
    run Banana::Application.routes

Okay so what’s the problem?

let’s you have an middleware say AwesomeMe. The AwesomeMe is calculated after the response time is injected, so that’s fine (imagine if the etag middleware was at the bottom). What about poor ExceptionNotifier? What if there is an exception thrown in the ResponseTimeInjector or AwesomeMe middleware? ExceptionNotifier isn’t going to catch it! The ExceptionNotifier middleware doesn’t modify anything in the response, so it needs to be up higher; it needs to be first.


You can add a new middleware to the middleware stack using any of the following methods:

  • config.middleware.use(new_middleware, args) - Adds the new middleware at the bottom of the middleware stack.

  • config.middleware.insert_before(existing_middleware, new_middleware, args) - Adds the new middleware before the specified existing middleware in the middleware stack.

  • config.middleware.insert_after(existing_middleware, new_middleware, args) - Adds the new middleware after the specified existing middleware in the middleware stack.

# config/application.rb

# Push Rack::BounceFavicon at the bottom
config.middleware.use Rack::BounceFavicon

# Add Lifo::Cache after ActiveRecord::QueryCache.
# Pass { page_cache: false } argument to Lifo::Cache.
config.middleware.insert_after ActiveRecord::QueryCache, Lifo::Cache, page_cache: false
And if you get stuck… Ask Here

email me rajeevsharma86@gmail.com

Tweet to me at @rajeevkannav.