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Rack middleware to help measure production code coverage


A gem to measure production code coverage. Coverband allows easy configuration to collect and report on production code coverage. It can be used as Rack middleware, wrapping a block with sampling, or manually configured to meet any need (like coverage on background jobs).

  • Allow sampling to avoid the performance overhead on every request.
  • Ignore directories to avoid overhead data collection on vendor, lib, etc.
  • Take a baseline to get initial app loading coverage.

At the moment, Coverband relies on Ruby's set_trace_func hook. I attempted to use the standard lib's Coverage support but it proved buggy when sampling or stopping and starting collection. When Coverage is patched in future Ruby versions it would likely be better. Using set_trace_func has some limitations where it doesn't collect covered lines, but I have been impressed with the coverage it shows for both Sinatra and Rails applications.


After running in production for 30 minutes, we were able very easily delete 2000 LOC after looking through the data. We expect to be able to clean up much more after it has collected more data.

Performance Impact

At the moment the performance impact of standard Ruby runtime coverage can be pretty large. Once getting things working. I highly recommend adding coverband_ext to the project which should shave the performance overhead down to something very reasonable. The two ways to deal with performance right now are lowering the sample rate and using the C extension. Often for smaller projects using the C extension makes 100% coverage possible.


Add this line to your application's Gemfile:

gem 'coverband'

And then execute:

$ bundle

Or install it yourself as:

$ gem install coverband

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 a example Sinatra app image


  • Coverband has been running in production on Ruby 1.9.3, 2.x, 2.1.x on Sinatra, Rails 2.3.x, Rails 3.0.x, Rails 3.1.x, and Rails 3.2.x
  • No 1.8.7 support, Coverband requires Ruby 1.9.3+
  • There is a performance impact which is why the gem supports sampling. On low traffic sites I am running a sample rate of 20% and on very high traffic sites I am sampling at 1%, which still gives useful data
    • The impact with the pure Ruby coverband can't be rather significant on sampled requests
    • Most of the overhead is in the Ruby coverage collector, you can now use coverband_ext to run a C extension collector which is MUCH faster.
    • Using Redis 2.x gem, while supported, is slow and not recommended. It will have a larger impact on overhead performance. Although the Ruby collection dwarfs the redis time, so it likely doesn't matter much.
    • Make sure to ignore any folders like vendor and possibly lib as it can help reduce performance overhead. Or ignore specific frequently hit in app files for better perf.


After installing the gem. There are a few steps to gather data, view reports, and for cleaning up the data.

See an example Sinatra app and example non rack ruby app configured with coverband.

  1. First configure cover band options using the config file, See the section below
  2. Then configure Rake, with helpful tasks. Make sure things are working by recording your Coverband baseline. See the section below
  3. Setup the rack middleware, the middleware is what makes Coverband gather metrics when your app runs. See below for details
    • I setup Coverband in my rackup you can also set it up in rails middleware, but it may miss recording some code coverage early in the rails process. It does improve performance to have it later in the middleware stack. So there is a tradeoff there.
    • To debug in development mode, I recommend turning verbose logging on config.verbose = true and passing in the Rails.logger config.logger = Rails.logger to the Coverband config. This makes it easy to follow in development mode. Be careful to not leave these on in production as they will effect performance.
  4. Start your server with rackup If you use rails s make sure it is using your or Coverband won't be recording any data.
  5. Hit your development server exercising the endpoints you want to verify Coverband is recording.
  6. Now to view changes in live coverage run rake coverband:coverage again, previously it should have only shown the baseline data of your app initializing. After using it in development it should show increased coverage from the actions you have exercised.

Configure Coverband Options

You need to configure cover band you can either do that passing in all configuration options to Coverband.configure in block format, or a simpler style is to call Coverband.configure with nothing while will load config/coverband.rb expecting it to configure the app correctly. Below is an example config file for a Sinatra app:

require 'json'

baseline = Coverband.parse_baseline

Coverband.configure do |config|
  config.root              = Dir.pwd
  if defined? Redis
    config.redis           = => '', :port => 49182, :db => 1)
  config.coverage_baseline = baseline
  config.root_paths        = ['/app/'] # /app/ is needed for heroku deployments
  # regex paths can help if you are seeing files duplicated for each capistrano deployment release
  #config.root_paths       = ['/server/apps/my_app/releases/\d+/'] 
  config.ignore            = ['vendor','lib/scrazy_i18n_patch_thats_hit_all_the_time.rb']
  # Since rails and other frameworks lazy load code. I have found it is bad to allow
  # initial requests to record with coverband. This ignores first 15 requests
  config.startup_delay     = Rails.env.production? ? 15 : 2
  config.percentage        = Rails.env.production? ? 30.0 : 100.0

  config.logger            = Rails.logger

  #stats help you collect how often you are sampling requests and other info
  if defined? Statsd
    config.stats           ='', 8125)
  # config options false, true, or 'debug'. Always use false in production
  # true and debug can give helpful and interesting code usage information
  # they both increase the performance overhead of the gem a little.
  # they can also help with initially debugging the installation.
  config.verbose           = Rails.env.production? ? false : true

Configuring Rake

Either add the below to your Rakefile or to a file included in your Rakefile such as lib/tasks/coverband if you want to break it up that way.

require 'coverband'
require 'coverband/tasks'

This should give you access to a number of cover band tasks

bundle exec rake -T coverband
rake coverband:baseline      # record coverband coverage baseline
rake coverband:clear         # reset coverband coverage data
rake coverband:coverage      # report runtime coverband code coverage

The default Coverband baseline task will try to detect your app as either Rack or Rails environment. It will load the app to take a baseline reading. If the baseline task doesn't load your app well you can override the default baseline to create a better baseline yourself. Below for example is how I take a baseline on a pretty simple Sinatra app.

namespace :coverband do
  desc "get coverage baseline"
  task :baseline_app do
    Coverband::Reporter.baseline {
      require 'sinatra'
      require './app.rb'

To verify that rake is working run rake coverband:baseline then run rake coverband:coverage to view what your baseline coverage looks like before any runtime traffic has been recorded.

Configure rack middleware

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

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

require 'coverband'

use Coverband::Middleware

Configure Manually (for example for background jobs)

It is easy to use Coverband outside of a Rack environment. Make sure you configure Coverband in whatever environment you are using (such as config/initializers/*.rb). Then you can hook into before and after events to add coverage around background jobs, or for any non Rack code.

For example if you had a base Resque class, you could use the before_perform and after_perform hooks to add Coverband

require 'coverband'

def before_perform(*args)
  if (rand * 100.0) <= Coverband.configuration.percentage
    @recording_samples = true
    @recording_samples = false

def after_perform(*args)
  if @recording_samples

In general you can run Coverband anywhere by using the lines below. This can be useful to wrap all cron jobs, background jobs, or other code run outside of web requests. I recommend trying to run both background and cron jobs at 100% coverage as the performance impact is less important and often old code hides around those jobs.

require "coverband"

coverband = Coverband::Base.instance


coverband.sample {
  #code to sample coverband

Clearing Line Coverage Data

After a deploy where code has changed. The line numbers previously recorded in Redis may no longer match the current state of the files. If being slightly out of sync isn't as important as gathering data over a long period, you can live with minor inconsistency for some files.

As often as you like or as part of a deploy hook you can clear the recorded Coverband data with the following command.

# defaults to the currently configured Coverband.configuration.redis
# or pass in the current target redis
Coverband::Reporter.clear_coverage( => '', :port => 6789))

You can also do this with the included rake tasks.

Verbose debug mode for development

If you are trying to debug locally wondering what code is being run during a request. The verbose modes config.verbose = true and config.verbose = 'debug' can be useful. With true set it will output the number of lines executed per file, to the passed in log. The files are sorted from least used file to most active file. I have even run that mode in production without much of a problem. The debug verbose mode outputs both file usage and provides the number of calls per line of code. For example if you see something like below which indicates that the application_helper has 43150 lines executed. That might seem odd. Then looking at the breakdown of application_helper we can see that line 516 was executed 38,577 times. That seems bad, and is likely worth investigating perhaps memoizing or cacheing is required.

config.verbose = 'debug'

coverband file usage:
  [["/Users/danmayer/projects/app_name/lib/facebook.rb", 6],
  ["/Users/danmayer/projects/app_name/app/models/some_modules.rb", 9],
  ["/Users/danmayer/projects/app_name/app/models/user.rb", 2606],

  /Users/danmayer/projects/app_name/app/helpers/application_helper.rb =>
  [[448, 1], [202, 1],
 [517, 1617], [516, 38577]]

Merge coverage data over time

If you are clearing data on every deploy. You might want to write the data out to a file first. Then you could merge the data into the final results later.

data = JSON.generate Coverband::Reporter.get_current_scov_data
File.write("blah.json", data)  
# Then later on, pass it in to the html reporter:
data = JSON.parse("blah.json")) :additional_scov_data => [data]   

Known issues

  • set_trace_func isn't perfect in sending each line of code executed and can be a bit wonky in a few places. Such as missing the end lines in code blocks. If you notice examples of this send them to me.
  • If you don't have a baseline recorded your coverage can look odd like you are missing a bunch of data. It would be good if coverband gave a more actionable warning in this situation.
  • If you have simplecov filters, you need to clear them prior to generating your coverage report. As the filters will be applied to coverband as well and can often filter out everything we are recording.
  • 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.


  • Fix network performance by logging to files that purge later (like NR) (far more time lost in set_trace_func than sending files, hence not a high priority, but would be cool)
  • Add support for zadd so one could determine single call versus multiple calls on a line, letting us determine the most executed code in production.
  • Possibly add ability to record code run for a given route
  • Improve client code api, around manual usage of sampling (like event usage)
  • Provide a better lighter example app, to show how to use Coverband.
    • blank rails app
    • blank Sinatra app
  • report on Coverband files that haven't recorded any coverage (find things like events and crons that aren't recording, or dead files)
  • ability to change the coverband config at runtime by changing the config pushed to the Redis hash. In memory cache around the changes to only make that call periodically.


  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. Create new Pull Request


These notes of kind of for myself, but if anyone is seriously interested in contributing to the project, these resorces might be helpfu. I learned a lot looking at various existing projects and open source code.

Ruby Std-lib Coverage

MIT License

See the file license.txt for copying permission.

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