Configuring proper sequence of Rack middlewares

I am running into a situation where I can’t ensure the proper sequence
Rack middlewares. I have two middlewares in my Rails 3.0.9 stack:

1- ExceptionNotifier
2- ActionDispatch::ShowExceptions

The first one is a
notifies me (via email) of any unhandled exception thrown by my app.
The second one is to handle the display of error pages. The call methods
these two are straightforward. The first one calls its predecessor and
catches resulting exception, sends email and re-raises the same
The second one just enhances already available functionality in Rails to
render properly formatted pages. The second one does not raise any
exception and thus renders the response and everything is all right.

In an initializer, I have a code (link
gist) that inserts the 1st one before the 2nd one.

But in reality, ActionDispatch::ShowException is always called first and
when I get an exceptional condition, the properly formatted error pages
displayed, but email is never sent because the ExceptionNotifier is

Does that make sense? Anybody has any comment about how I can make it do
the right thing? (Call ExceptionNotifier first and ShowExceptions


PS: output of rake middleware with this setup:

use ActionDispatch::Static
use Rack::Lock
use ActiveSupport::Cache::Strategy::LocalCache
use Rack::Runtime
use Rails::Rack::Logger
use ExceptionNotifier
use ActionDispatch::ShowExceptions
use ActionDispatch::ShowExceptions
use ActionDispatch::RemoteIp
use Rack::Sendfile
use ActionDispatch::Callbacks
use ActiveRecord::ConnectionAdapters::ConnectionManagement
use ActiveRecord::QueryCache
use ActionDispatch::Cookies
use ActionDispatch::Session::CookieStore
use ActionDispatch::Flash
use ActionDispatch::ParamsParser
use Rack::MethodOverride
use ActionDispatch::Head
use ActionDispatch::BestStandardsSupport
use OAuth2::Provider::Rack::Middleware
use OmniAuth::Builder
run MyApp::Application.routes

On Monday, 5 November 2012 19:39:45 UTC-5, Kedar M. wrote:

catches resulting exception, sends email and re-raises the same exception.
use Rack::Lock
use ActiveRecord::QueryCache

The best way to visualize the process here is to imagine the request
in at the top of the middleware stack (AD::Static here) and is passed
down to successive middlewares until it generates a response. The
then flows back up the stack until it gets to the top and is handed to
the web server for delivery to the user.

Short story shorter, you need to put them in the opposite order -
ShowExceptions is rescuing the exception so ExceptionNotifier never sees

–Matt J.

ShowExceptions is rescuing the exception so ExceptionNotifier never sees it.

Thank you, Matt J…

I changed the sequence so that ExceptionNotifier middleware is below the
ShowExceptions middleware in the stack (as reported by rake middleware).

In this case however, I am not seeing the desired effect. Is it because
ShowExceptions::render_exeption_with_template, as written, actually
the response (the formatted error page) and does not re-raise the
exception? Should I rewrite
to re-raise the exception in case of certain errors (e.g. 500 and not