This add-on is operated by Rails Autoscale

Automatic web dyno scaling designed for Rails apps

Rails Autoscale

Last updated 06 April 2017

Rails Autoscale is an add-on that automatically scales your Ruby on Rails web dynos.

Autoscaling your application ensures that you can handle increased traffic while only paying for the dynos you need.

How is this different from Heroku’s own autoscaling?

The main difference is how scaling needs are determined. Heroku’s autoscaling (and other autoscaling solutions) use response time to trigger scaling. If your app has consistent response times across all endpoints, this works great. However, it only takes a few poorly-performing endpoints to trigger an unnecessary scale event. Rails Autoscale uses request queue time to ensure dynos are only added when more capacity is needed.

Provisioning the add-on

Rails Autoscale can be installed via the CLI:

$ heroku addons:create rails-autoscale
-----> Adding RAILS_AUTOSCALE_URL to sharp-mountain-4005... done, v18 (free)

After Rails Autoscale is provisioned, a RAILS_AUTOSCALE_URL config var is available in the app configuration. This setting is used by the rails_autoscale_agent gem when communicating with the Rails Autoscale service.

Installing the gem

To enable autoscaling functionality, you must install the rails_autoscale_agent gem in your Rails application. Add this line to your application’s Gemfile and run bundle:

gem 'rails_autoscale_agent'

This automatically inserts the agent into your Rack middleware stack. The agent runs asynchronously and periodically reports request queue information to the Rails Autoscale service. It is active only when the RAILS_AUTOSCALE_URL setting is present.

After installing the Rails Autoscale add-on and the rails_autoscale_agent gem, your application is ready to integrate with the add-on.

Adjust your settings in the Rails Autoscale dashboard

After you deploy your application with the rails_autoscale_agent gem, you’ll begin to see activity in your Rails Autoscale dashboard.

The dashboard can be accessed via the CLI:

$ heroku addons:open rails-autoscale
Opening RAILS_AUTOSCALE_URL for sharp-mountain-4005

or by visiting the Heroku Dashboard and selecting Rails Autoscale from the Add-ons menu.

Request queue time threshold

Rails Autoscale triggers scale events by monitoring the request queue time for every request. This is the time between Heroku accepting a request and your application beginning processing. Queue time will increase when your application servers are too busy to handle all incoming requests.

Rails Autoscale tracks the 95th percentile queue time every minute, which for most applications will hover well below the default threshold of 100ms. This threshold value means that repeated measurements over 100ms will trigger an UPSCALE for your application, adding one web dyno. After 15 minutes of measurements well below this threshold, a DOWNSCALE event is triggered.

The threshold typically needs to be customized for each application. Keep an eye on the dashboard chart to see where your queue times hover, and adjust accordingly via the “edit settings” link. It’s completely normal to see occasional spikes above your threshold, but repeated spikes are usually an indicator that your app needs additional web dynos.

Enable autoscaling

Autoscaling is turned OFF by default to avoid unintended scaling. In your dashboard settings, check the “Enable autoscaling” checkbox to turn it on. Note that you’ll also need to provide your Heroku API key, which is available on your Heroku Manage Account page.

Troubleshooting

If you’re not seeing any activity in the dashboard chart, these steps should resolve the issue:

• Ensure you’ve deployed your application with the rails_autoscale_agent gem installed.
• Ensure your application has received requests since deploying with the agent gem.
• Allow 1-2 minutes for the data to show in the dashboard.

If you’re still not seeing activity in the dashboard, check your application logs. The rails_autoscale_agent gem writes to your application log for each request and when it reports metrics every minute. If the gem is installed correctly, you should see logs preceeded with “[RailsAutoscaleAgent]”.

If you’ve verified the gem installation, and you’re still not seeing queue time info in the dashboard, see the support section below.

Migrating between plans

Use the heroku addons:upgrade command to migrate to a new plan.

$ heroku addons:upgrade rails-autoscale:newplan
-----> Upgrading rails-autoscale:newplan to sharp-mountain-4005... done, v18 ($49/mo)
       Your plan has been updated to: rails-autoscale:newplan

Removing the add-on

Rails Autoscale can be removed via the CLI.

$ heroku addons:destroy rails-autoscale
-----> Removing rails-autoscale from sharp-mountain-4005... done, v20 (free)

Support

All Rails Autoscale support and runtime issues should be submitted via the Heroku Support channels. Send us your suggestions and feedback at feedback@railsautoscale.com.