The New Relic Rails Agent includes Developer Mode, a built in UI for examining details about web transactions in your application. With Developer Mode you can examine the library and database calls made by the controller actions as they are invoked on the Rails instance.
The free Developer Mode gives you some of the same in-depth transaction visibility you get with Transaction Traces on rpm.newrelic.com, but works only in a single Rails instance, and has much higher overhead than the production version. It is not recommended for use beyond a developer's desktop 'sandbox.'
You can see a demonstration video of Developer Mode here.
Installation
Installing Developer Mode is as simple as installing the New Relic Ruby Agent as a plug-in or gem.
You may need to set the newrelic.yml setting developer to true for the RAILS_ENV in which you are working. It is on by default only when RAILS_ENV = development.
While the newrelic.yml file will be created as part of the installation, it is not necessary to have a valid license.
Using Developer Mode
If you run your application on your desktop using thin, WEBrick or something similar, you just need to open the URL /newrelic on your server to see the Developer Mode UI.
If you are using Passenger, Unicorn, or other forking application servers on your desktop, you may notice some odd behavior if your application server launches multiple instances.
Troubleshooting
Firstly, note that the developer mode in recent agents only works on Rails 2.3 or greater - it is rack based, and earlier versions of the framework did not incorporate rack.
The developer edition installs a middleware in your app that responds to any URL prepended with /newrelic:
/newrelic
or
/newrelic/files/images/foo.png
In some cases the /newrelic URL gets captured by an earlier middleware - you may disable the developer mode route and include the middleware yourself in this case:
in config.ru at the beginning of the file:
require 'newrelic_rpm'
require 'new_relic/rack/developer_mode'
use NewRelic::Rack::DeveloperMode
Getting a routing error? The route installed to access the Developer Mode can sometimes conflict with your own routes. You can disable the /newrelic route and add your own route definition to access the developer mode UI.
Are your requests disappearing and re-appearing? When using Passenger in Developer Mode, you may notice sometimes the list of requests disappear, or that it seems like it misses some requests. You might also get an error message clicking on a request, indicating the trace is not available, suggesting maybe you restarted.
This occurs when Passenger spawns more than one handler process. The transaction traces get put in one of the processes and if you connect to the other one, you won't see it.
The workaround is to set the max number of instances to 1 via the PassengerMaxPoolSize parameter. Also, if you hit refresh several times you have a good chance of getting the requests back.
If you have this problem with unicorn or any other multi-worker dispatcher, try limiting the number of workers to 1.
