A gem to measure production code usage, showing each line of code that is executed. Coverband allows easy configuration to collect and report on production code usage. It can be used as Rack middleware, wrapping a block with sampling, or manually configured to meet any need (like usage during background jobs). I like to think of this as production code coverage, but that implies test coverage to some folks, so being more explicit to say that it shows when a line of code is executed in a given environment is the most accurate way to describe it.

Code Quality Rank: L5
Monthly Downloads: 58,338
Programming language: Ruby
License: MIT License

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Build Status Coverage Status Maintainability

Key Features • Installation • Coverage Report • Advanced Config • License • Change Log / Roadmap • Code of Conduct

A gem to measure production code usage, showing a counter for the number of times each line of code that is executed. Coverband allows easy configuration to collect and report on production code usage. It reports in the background via a thread or can be used as Rack middleware, or manually configured to meet any need.

Note: Coverband is not intended for test code coverage, for that we recommended using SimpleCov.

Key Features

The primary goal of Coverband is giving deep insight into your production runtime usage of your application code, while having the least impact on performance possible.

  • Low performance overhead
  • Simple setup and configuration
  • Out of the box support for all standard code execution paths (web, cron, background jobs, rake tasks, etc)
  • Splits load time (Rails eager load) and Run time metrics
  • Easy to understand actionable insights from the report
  • Tracks Gem usage (still in experimental stages and not recommended for production)
  • Development mode, offers deep insight of code usage details (number of LOC execution during single request, etc) during development.
  • Mountable web interface to easily share reports



Coverband stores coverage data in Redis. The Redis endpoint is looked for in this order:

  3. localhost

The redis store can also be explicitly defined within the coverband.rb. See advanced config.

Gem Installation

Add this line to your application's Gemfile, remember to bundle install after updating:

gem 'coverband'

If tracking gem usage, be sure to include coverband before other gems you would like to track.

Upgrading to Latest

No custom code or middleware required

With older versions of coverband, projects would report to redis using rack or sidekiq middleware. After coverband 4.0, this should no longer be required and could cause performance issues. Reporting to redis is now automatically done within a background thread with no custom code needed.

See changelog.


The Railtie integration means you shouldn't need to do anything anything else other than ensure coverband is required after rails within your Gemfile. The only exception to this is gem tracking of Bundle.require which depends on requiring coverband within the application.rb. See Collecting Gem / Library Usage.


For the best coverage you want this loaded as early as possible. I have been putting it directly in my config.ru but you could use an initializer, though you may end up missing some boot up coverage. To start collection require Coverband as early as possible.

require 'coverband'
require File.dirname(__FILE__) + '/config/environment'

use Coverband::BackgroundMiddleware
run ActionController::Dispatcher.new

Coverage Report

Coverband comes with a mountable rack app for viewing reports. For Rails this can be done in config/routes.rb with:

Rails.application.routes.draw do
  mount Coverband::Reporters::Web.new, at: '/coverage'

But don't forget to protect your source code with proper authentication. Something like this when using devise:

Rails.application.routes.draw do
  authenticate :user, lambda { |u| u.admin? } do
    mount Coverband::Reporters::Web.new, at: '/coverage'

or you can enable basic auth by setting ENV['COVERBAND_PASSWORD'] or via your configuration config.password = 'my_pass'

Coverband Web Endpoint

The web endpoint is a barebones endpoint that you can either expose direct (after authentication) or you can just link to the actions you wish to expose. The index is intended as a example to showcase all the features.


The web index as available on the Coverband Demo site

  • force coverage collection: This triggers coverage collection on the current webserver process
  • clear coverage report: This will clear the coverage data. This wipes out all collected data (dangerous)
  • View individual file details
  • clear individual file coverage: This will clear the details of the file you are looking at. This is helpful if you don't want to loose all Coverage data but made a change that you expect would impact a particular file.

Rake Tasks

The rake task generates a report locally and opens a browser pointing to coverage/index.html.

rake coverband:coverage

This is mostly useful in your local development environment.

Example Output

Since Coverband is Simplecov output compatible it should work with any of the SimpleCov::Formatter's available. The output below is produced using the default Simplecov HTML formatter.

Index Page image

Details on an example Sinatra app image

Coverband Demo

Take Coverband for a spin on the live Heroku deployed Coverband Demo. The full source code for the demo is available to help with installation, configuration, and understanding of basic usage.

Example apps

Advanced Config

If you need to configure coverband, this can be done by creating a config/coverband.rb file relative to your project root.

  • See lib/coverband/configuration.rb for all options
  • By default Coverband will try to stored data to Redis * Redis endpoint is looked for in this order: ENV['COVERBAND_REDIS_URL'], ENV['REDIS_URL'], or localhost

Below is an example config file for a Rails 5 app:

Coverband.configure do |config|
  config.store = Coverband::Adapters::RedisStore.new(Redis.new(url: ENV['MY_REDIS_URL']))
  config.logger = Rails.logger
  # configure S3 integration
  config.s3_bucket = 'coverband-demo'
  config.s3_region = 'us-east-1'
  config.s3_access_key_id = ENV['AWS_ACCESS_KEY_ID']
  config.s3_secret_access_key = ENV['AWS_SECRET_ACCESS_KEY']

  # config options false, true. (defaults to false)
  # true and debug can give helpful and interesting code usage information
  # and is safe to use if one is investigating issues in production, but it will slightly
  # hit perf.
  config.verbose = false

Ignoring Files

Sometimes you have files that are known to be valuable perhaps in other environments or something that is just run very infrequently. Opposed to having to mentally filter them out of the report, you can just have them ignored in the Coverband reporting by using config.ignore as shown below. Ignore takes a string but can also match with regex rules see how below ignores all rake tasks as an example.

config.ignore +=  ['config/application.rb',

Writing Coverband Results to S3

If you add some additional Coverband configuration your coverage html report will be written directly to S3, update config/coverband.rb like below.

  # configure S3 integration
  config.s3_bucket = 'coverband-demo'
  config.s3_region = 'us-east-1'
  config.s3_access_key_id = ENV['AWS_ACCESS_KEY_ID']
  config.s3_secret_access_key = ENV['AWS_SECRET_ACCESS_KEY']

Alternatively, Coverband if you don't set via the config.s3_* accessor methods will look for the standard S3 environment variables.


Coverage Data Migration

Between the release of 4.0 and 4.1 our data format changed. This resets all your coverage data. If you want to restore your previous coverage data, feel free to migrate.

rake coverband:migrate

  • We will be working to support migrations going forward, when possible

Clear Coverage

Now that Coverband uses MD5 hashes there should be no reason to manually clear coverage unless one is testing, changing versions, possibly debugging Coverband itself.

rake coverband:clear

Adding Rake Tasks outside of Rails

Rails apps should automaticallly include the tasks via the Railtie.

For non Rails apps, either add the below to your Rakefile or to a file included in your Rakefile such as lib/tasks/coverband.rake if you want to break it up that way.

require 'coverband'
require 'coverband/utils/tasks'

Verify it works

rake -T coverband
rake coverband:clear         # reset coverband coverage data
rake coverband:coverage      # report runtime coverband code coverage

Forcing Coverband to Track Coverage on files loaded during boot safe_reload_files

Coverband will report code usage for anything required or loaded after calling Coverband.start which happens automatically when coverband is required. This means some of the files loaded before coverband such as the Rails application.rb will be reported as having no coverage.

The safe_reload_files reload option in the configuration options can help to ensure you can track any files regardless of loading before Coverband. For example if I wanted to show the coverage of config/coverband.rb, which has to be loaded before calling Coverband.start, I could do that by adding that path to the safe_reload_files option.

Coverband.configure do |config|
  # ... a bunch of other options
  # using the new safe reload to enforce files loaded
  config.safe_reload_files = ['config/coverband.rb']

By adding any files above you will get reporting on those files as part of your coverage runtime report. The files are reloaded when Coverband first starts, you can also trigger a reload via the web interface.

Collecting Gem / Library Usage

WARNING: Gem Tracking is still in experimental stages and not recommended for production. We have some performance issues when view reports on large applications. Gem tracing also during background thread data collection has HIGH memory requirements, during report merging (seemingly around 128mb of extra memory, which is crazy). We recommend deploying WITHOUT track_gems first and only enabling it after confirming that Coverband is working and performing well.

Gem usage can be tracked by enabling the track_gems config.

Coverband.configure do |config|
  config.track_gems = true

The track_gems feature exposes a Gems tab in the report which prints out the percentage usage of each Gem. See demo here.

When tracking gems, it is important that Coverband#start is called before the gems to be tracked are required. The best way to do this is to require coverband before Bundle.require is called. Within rails, require coverband within the application.rb like so:

require 'coverband'

If you are using the resque integration, resque needs to be required before coverband since the integration will not run unless resque is loaded. Within the application.rb just require resque before coverband.

require 'resque'
require 'coverband'

The track_gems config only exposes the overall usage of a gem. In order to see the detail of each file, enable the gem_details flag.

Coverband.configure do |config|
  config.track_gems = true
  config.gem_details = true

This flag exposes line by line usage of gem files. Unfortunately due to the way the coverband report is currently rendered, enabling gem_details slows down viewing of the coverage report in the browser and is not yet recommended.

Manually Starting Coverband

Coverband starts on require of the the library which is usually done within the Gemfile. This can be disabled by setting the COVERBAND_DISABLE_AUTO_START environment variable. This environment variable can be useful to toggle coverband on and off in certain environments.

In order to start coverband manually yourself when this flag is enabled, call Coverband.configure followed by Coverband.start.


Verbose Debug / Development Mode

Note: To debug issues getting Coverband working. I recommend running in development mode, by turning verbose logging on config.verbose = true and passing in the Rails.logger config.logger = Rails.logger to the Coverband config. We respect the log level, and I would recommend log level info generally, but if you are investigating a prolbem Coverband logs additional data at the debug level. This makes it easy to follow in development mode. Be careful to not leave these on in production as they will affect performance.

If you are trying to debug locally wondering what code is being run during a request. The verbose modes config.verbose = true && Rails.logger.level = :debug. With true set it will output the number of lines executed per file, to the passed in log.


  • Coverband 3.0.X+ requires Ruby 2.3+
  • Coverband currently requires Redis for production usage

Contributing To Coverband

If you are working on adding features, PRs, or bugfixes to Coverband this section should help get you going.

  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Make sure all tests are passing (run bundle install, make sure Redis is running, and then execute rake test)
  6. Create new Pull Request

Tests & Benchmarks

If you submit a change please make sure the tests and benchmarks are passing.

  • run tests:
    • bundle exec rake
    • BUNDLE_GEMFILE=Gemfile.rails4 bundle exec rake (Same tests using rails 4 instead of 5)
  • view test coverage: open coverage/index.html
  • run the benchmarks before and after your change to see impact
    • rake benchmarks
    • run a single test by line number like rspec: bundle exec m test/coverband/reporters/html_test.rb:29

Known Issues

  • total fail on front end code, because of the precompiled template step basically coverage doesn't work well for erb, slim, and the like.
    • related it will try to report something, but the line numbers reported for ERB files are often off and aren't considered useful. I recommend filtering out .erb using the config.ignore option. The default configuration excludes these files
  • coverage doesn't show for Rails config/application.rb or config/boot.rb as they get loaded when loading the Rake environment prior to starting the Coverage library. See reload files section.

Debugging Redis Store

What files have been synced to Redis?


What is the coverage data in Redis?


The Coverband logo was created by Dave Woodall. Thanks Dave!


This is a MIT License project... See the file license.txt for copying permission.

*Note that all licence references and agreements mentioned in the Coverband README section above are relevant to that project's source code only.