Dev Thoughts

Musings from my development journey.


Observing Change: A Gold Master Test in Practice - Jake Worth


Jake Worth discusses how developing legacy code is inevitable. Every production application will, at some point, become a legacy application.

In order to properly work on legacy applications, tests are a prerequisite.

  • Unit tests
  • API tests
  • UI tests

Unit tests get the closest to specifying application behavior, but can still fall short.

Gold Master Test

Genesis of idea: Working Effectively with Legacy Code book by Michael Feathers.

"In nearly every legacy system, what the system does is more important what it's supposed to do." - Feathers

Characterization Test

A characterization test is a test that characterizes the actual behavior of the code.


  1. Code in test harness
  2. Write assertion you know will fail
  3. Let failure show you how to proceed
  4. Change the test to match the behavior

This is backwards from normal TDD, but will allow you to build test coverage for a working application for the purposes of being able to refactor code.

After this process is completed, write additional tests on a case by case basis.

Gold Master Test Process

"A gold master test is a regression test for complex, untested systems that asserts a consistent macro level behavior."

The goal of this test is to ensure that the application continues to work, even when making future changes.

Ideal Application

  • It's mature
  • Complex
  • Expects minimal changes to output


  • Rigorous development standards
  • Exposes surprising behavior
  • Useful for forensic analysis

This can help give insight on the underlying behavior when adding new features in the future.

Gold Master Test Code Process


  • Get production database
  • Scrub security specific data, such as emails, security codes, etc. (you can use pg flags for scrubbing)
  • Check the cleaned up database into version control

Test Phase

  • Start with empty test file
  • Pipe cleaned production database into test database
  • Compare current result to production database

What Happens When the Test Fails?


When the test fails:

  1. Check to see if the change is valid
  2. If valid, update the gold master database
  3. If not valid, reevaluate the change


The gold master test provides a high level test for an application's full functionality that includes testing against production data instead of test data, including HTML and actual database.

Code example