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#Testing Guide for Developers

##Guiding principles (1) use the Testing Pyramid - you should have many more unit tests than UI tests

Unit tests are good because:

• they are fast
• they are reliable
• they isolate failures

Acceptance tests may be UI tests, but end-to-end tests always are. Minimise UI tests! They are expensive to write, time consuming to run, and sometimes non-deterministic (ie there are often false positives) and brittle (easily broken). They can be useful but we shouldn't have many of them. If they fail and expose a bug, unit test(s) should be written to address the failing scenario first.

Google reported how end-to-end testing had the following problems:

• difficult to find root cause of problems
• partner failures and lab failures ruined the test results
• many smaller bugs were hidden behind bigger bugs.

Ease of fixing bugs (mean time to repair) should be prioritised over having fewer test failures.

Google blog - just say no to more end to end tests End to end testing considered harmful Yet another testing pyramid Ice Cream Cone anti-pattern