Scale up Heroku dynos with interleaved load tests to find out how many dynos you need
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How many dynos do I need on Heroku?

We set out to answer this problem by combining the ruby gem from and the Heroku::Client which allows you to change the number of dynos for your app instantly.

The syntax is pretty simple

$ ./blitzer.rb
Usage: blitzer user pass heroku-app [target]
user   - Your heroku account name (email)
pass   - Heroku API key (from the accounts page)
app    - The name of your Heroku app
target - How many users do you want to get to? default = 1000

The app is just the name of your Heroku app which resolves to You can set the target concurrency to a specific value, with the default being 1,000 users. When you run this, we keep adding dynos (up to 32 dynos) until the percentage of errors during the load test is greater than 2%.

The output from running the blitzer looks something like this:

Blitzing with a target of 1000 users

starting 1 dyno(s) .done
  rushing ..322 hits | 0 errors | 1 timeouts @ 167 users
starting 2 dyno(s) ....done
  rushing .......6259 hits | 1 errors | 0 timeouts @ 670 users
starting 4 dyno(s) ...done
  rushing .......4784 hits | 1 errors | 0 timeouts @ 591 users
starting 8 dyno(s) .......done
  rushing .......4781 hits | 1 errors | 0 timeouts @ 589 users
starting 16 dyno(s) ...done
  rushing .............done

>> You need 16 dyno(s) to handle 1,000 users

Integrating load and performance testing into your continuous integration process doesn't get simpler than this!