Rack middlewares

The Ruby agent automatically instruments Rack middlewares. If you are unfamiliar with the basics of Rack middlewares, review the resources linked by the Rails on Rack guide. Additionally, the Ruby agent provides some features via Rack middlewares:

New Relic automatically installs these middlewares for Rails and Sinatra.

Rack instrumentation

The two most common ways to configure Rack middlewares are the Rack::Builder API (most often from config.ru) and Rails' middleware stack configuration:


Middlewares in your config.ru file are configured using Rack::Builder. For the Ruby agent to instrument middlewares from Rack::Builder, your app must run version 1.1.0 or higher of the rack gem. This is the most common use of middlewares with Sinatra or pure-rack applications.

Rails middlewares

Rails uses its own class (ActionDispatch::MiddlewareStack) instead of Rack::Builder to configure middlewares. Even if you haven't explicitly added middlewares to your Rails application, many components of Rails itself are implemented as middleware, so middleware data will appear by default.

The Ruby agent automatically instruments middlewares added via ActionDispatch::MiddlewareStack on Rails 3.0 or higher. For more information about configuring middlewares with Rails, see the Ruby on Rails guide.

Viewing middleware data

You can view middleware data in APM.

In the APM Summary page

The main chart on your app's APM Summary page includes a purple bar that shows average time per request spent in all Rack middlewares for your application.

APM > (selected application) > Summary: Middleware time appears in purple on your app's main Overview chart.
In the APM Transactions page

You can also see time for individual middlewares for a specific transaction name from your app's APM Transactions page.


APM > (selected application) > Monitor > Transactions > (selected transaction) > Trace details: Here is an example of middleware time for a selected transaction for your app.

In APM transaction trace details

Transaction traces also capture detailed middleware call information.

APM > (selected application) > Monitor > Transactions > (selected transaction trace): Here is an example of middleware details in a transaction trace.

Disabling Rack instrumentation

If you do not want to instrument Rack middlewares, you may disable Rack middleware instrumentation with the disable_middleware_instrumentation setting. You can also ignore specific transactions.

Installing Ruby agent middlewares manually

The Ruby agent's implementation of New Relic's Cross Application Tracing feature uses Rack middleware instrumentation to read and write HTTP headers that are necessary to pass information between monitored applications. If you have disabled middleware instrumentation as described above and want to use cross application tracing, you must manually add the NewRelic::Rack::AgentHooks middleware to your middleware stack. For more information, see Cross application tracing in Ruby.

Manual Rack instrumentation

Earlier versions of the Ruby agent supported manually instrumenting Rack middlewares via the NewRelic::Agent::Instrumentation::Rack module. This instrumentation is deprecated in Ruby agent versions 3.9.0 or higher, because it is unnecessary with automatic middleware instrumentation. New Relic recommends that you remove references to this module from your code after upgrading to 3.9.0 or higher.

For more help

If you need more help, check out these support and learning resources: