How to Use Feature Flags for Trunk-Based Development

Trunk-based development relies on frequent merges into a single deployable main branch, a practice associated with stronger software delivery performance and reduced merge-conflict risk compared with long-lived branching models. Because substantial features and migrations often take longer than a day to build, feature flags can keep incomplete code deployed but inactive, separating deployment from user release and allowing teams to merge incremental work safely. A flag-first workflow involves creating a flag before coding, testing both enabled and disabled paths in CI, using environment-specific configurations, and conducting dark launches or gradual rollouts to selected users while monitoring results. For complex system replacements, branch by abstraction can allow legacy and new implementations to coexist behind a common interface. The discussion emphasizes that flags require lifecycle management, including owners, removal dates, stale-flag detection, and code cleanup, because unused flags multiply possible code paths and create technical debt. It also warns against adding flags only after code is live, using them to conceal long-lived branches, or failing to remove them after a feature is fully released, while presenting GrowthBook as a platform supporting local flag evaluation, environment controls, staged rollouts, monitoring, and cleanup tools.

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