Shift Left
A practice of moving testing, quality, and security processes earlier in the software development lifecycle to find and fix issues sooner.
Definition
The "left" refers to the left side of a typical project timeline diagram, where development starts. Shift left means integrating quality gates earlier: writing tests before code (TDD), running security scans on every commit, reviewing architecture before implementation, and validating requirements before development begins. This contrasts with traditional approaches where testing and security happen at the end.
Pourquoi c'est important
Bugs found in production cost 100x more to fix than bugs found during development. The IBM Systems Science Institute found that fixing a defect in design costs 1x, in testing costs 15x, and in production costs 100x. Shift left reduces costs and improves quality.
Exemple
Instead of running security scans monthly, a team "shifts left" by running diffray on every PR, catching vulnerabilities within hours of introduction rather than weeks later.