Trunk-based: features behind flags in main

Шаги

1. 01

   Create a repo with a basic structure

   bashcopy

   cd /home/student/work

   mkdir -p trunk-lab && cd trunk-lab

   git init -b main

   cat > app.py <<'EOF'

   FLAGS = {}

   def greet(name):

       return f"Hello, {name}"

   print(greet("world"))

   EOF

   git add . && git commit -m "init: greet"

   FLAGS = {} is a placeholder for the flags you will add as you go. This is a typical pattern.

   ✓ The base is ready. FLAGS is empty.

2. 02

   Add the first feature behind a flag: one small commit

   The feature is uppercase letters in the greeting. First add it behind a flag that is off: the change lands in main, but the behavior does not change.

   bashcopy

   cd /home/student/work/trunk-lab

   git switch -c feat/uppercase

   cat > app.py <<'EOF'

   FLAGS = {"uppercase": False}

   def greet(name):

       out = f"Hello, {name}"

       if FLAGS.get("uppercase"):

           out = out.upper()

       return out

   print(greet("world"))

   EOF

   git add . && git commit -m "feat(uppercase): scaffold behind flag, default off"

   git switch main

   # --no-ff = always create a merge commit, even when a fast-forward is possible

   git merge --no-ff feat/uppercase -m "merge feat/uppercase"

   git branch -d feat/uppercase  # delete it right away: the branch is merged, no longer needed

   python3 app.py                # output is "Hello, world": flag is off

   The key point: the output Hello, world is the same. The feature is in main, but it is off. The branch lived about 30 seconds.

   ✓ The feature is in main, the behavior did not change.

3. 03

   Turn the flag on in a separate commit

   bashcopy

   cd /home/student/work/trunk-lab

   sed -i 's/"uppercase": False/"uppercase": True/' app.py   # turn the flag on in place

   git commit -am "enable(uppercase): turn on for everyone"

   python3 app.py                # now "HELLO, WORLD"

   The output is now HELLO, WORLD. This is a separate commit: you can roll back only the activation without touching the feature code itself.

   ✓ The flag is on, the behavior changed.

4. 04

   Add a second feature in parallel, also as a small commit

   Picture two developers working in parallel. The second feature is a custom greeting.

   bashcopy

   cd /home/student/work/trunk-lab

   git switch -c feat/greeting

   # add the key "greeting": False to the FLAGS dict

   sed -i 's/FLAGS = {"uppercase": True}/FLAGS = {"uppercase": True, "greeting": False}/' app.py

   # replace the literal "Hello" with a conditional choice based on the flag

   sed -i 's/out = f"Hello, {name}"/g = "Hi" if FLAGS.get("greeting") else "Hello"\n    out = f"{g}, {name}"/' app.py

   git commit -am "feat(greeting): customizable, behind flag"

   git switch main

   git merge --no-ff feat/greeting -m "merge feat/greeting"

   git branch -d feat/greeting

   python3 app.py                # "HELLO, WORLD" again: greeting is off

   The output is HELLO, WORLD again: greeting is off. But the feature is already in main.

   ✓ The second feature is merged, the behavior did not change.

5. 05

   Look at the graph: still almost linear

   bashcopy

   cd /home/student/work/trunk-lab

   git log --oneline --graph --all

   You see 4 commits, two merge commits from --no-ff (this is a convention so the features stay markable). But logically everything moves along main. No parallel long-lived branches.

   ✓ Four commits, a flat structure.

6. 06

   Turn on the second flag

   bashcopy

   cd /home/student/work/trunk-lab

   sed -i 's/"greeting": False/"greeting": True/' app.py

   git commit -am "enable(greeting): GA"

   python3 app.py                # "HI, WORLD": both features are active

   The output is HI, WORLD. Both features are active.

   ✓ Both features work.

7. 07

   Remove the flags: the final step of the lifecycle

   Once the features are stable and there is no reason to turn them off anymore, remove the flags so the code does not accumulate dead code.

   bashcopy

   cd /home/student/work/trunk-lab

   cat > app.py <<'EOF'

   def greet(name):

       return f"Hi, {name}".upper()

   print(greet("world"))

   EOF

   git commit -am "cleanup: remove uppercase + greeting flags (both GA)"

   python3 app.py

   git log --oneline --graph

   The output is the same: HI, WORLD. But the code is clean: no FLAGS, no if. The graph is a linear history of the features. This is trunk-based in its final form.

   ✓ The flags are removed, the code is clean. The history is linear.