The summary.
NEVER COPY PRODUCTION DATA TO A TEST ENVIRONMENT WITHOUT ANONYMISING
Test environments are less protected, and this is a common cause of breaches. Names removed while addresses, dates and relationships remain is not anonymisation.
TESTS THAT CREATE THEIR OWN DATA ARE INDEPENDENT AND RELIABLE
Shared fixed datasets couple every test to them, and break together when they change.
BEHAVIOUR AT A HUNDRED RECORDS DIFFERS ENTIRELY FROM A MILLION
Missing indexes are invisible until volume makes them fatal. Generate data matching production distribution, not only count.
KEEP SECURITY SETTINGS IDENTICAL TO PRODUCTION
Testing with them relaxed hides exactly the problems that appear on release.
ENVIRONMENTS CREATED PER TEST RUN AND DISPOSED AFTERWARDS REMOVE INTERFERENCE
Shared environments mean one person's testing affects another's results.
NO ENVIRONMENT REPRODUCES PRODUCTION ENTIRELY
Which is why synthetic monitoring, feature flags, canary releases and a working rollback path are part of testing rather than separate from it.
