Engineering Craft › Branching & Releases

Trunk-Based Development

Everyone merges small changes to main frequently.

Also known as: trunk based, TBD, trunk development

Trunk-based development is a branching model where everyone integrates into one main line, usually main or trunk, in small, frequent changes. Branches, if used at all, live for hours or a day or two, not weeks. The team’s safety comes from automated tests and fast feedback rather than from long-lived branches.

Unfinished work reaches main behind a feature flag, so the code is merged but turned off for users:

if flags.enabled("new_checkout", user_id):
    return new_checkout(cart)
return old_checkout(cart)      # still the default until the flag is turned on

This keeps main releasable at all times, and it keeps the gap between any two developers’ code small.

The trade-off is discipline. The approach depends on a continuous integration pipeline that is fast and reliable, good test coverage, and a habit of splitting work into small pieces. Without those, merging to main often means merging broken code. Flags also need cleanup, or old branches in the code pile up.

The classic mistake is adopting the model in name while still working on week-long branches, or merging to main without tests and calling it trunk-based. The practice is the small, frequent integration itself. If a change is too big to merge in a day, split it, and use flags to ship it in parts. See long-lived branches for what happens when that doesn’t happen.