Git Rebase vs Merge : Quand utiliser chaque approche

Comprendre la difference entre git rebase et git merge est essentiel pour tout developpeur.

Comment fonctionne Git Merge

Git merge cree un nouveau commit de merge avec deux parents.

Lorsque vous executez git merge feature sur la branche principale, Git trouve l'ancetre commun.

Comment fonctionne Git Rebase

Git rebase reecrit l'historique en deplacant vos commits.

Git identifie l'ancetre commun et re-applique vos commits un par un.

Comparaison visuelle

BEFORE (both branches have diverged from common ancestor C2):

          C5---C6---C7  (feature)
         /
  C1---C2---C3---C4     (main)

AFTER git merge feature (from main):

          C5---C6---C7
         /            \
  C1---C2---C3---C4----M8  (main) ← merge commit M8 has 2 parents
                            (feature still points to C7)

AFTER git rebase main (from feature):

  C1---C2---C3---C4                     (main)
                   \
                    C5'---C6'---C7'     (feature) ← new commits (different SHAs)

  Then fast-forward merge:
  C1---C2---C3---C4---C5'---C6'---C7'  (main, feature) ← linear history

Comparaison des fonctionnalites

Aspect	git merge	git rebase
Historique	Preserve tout l'historique	Historique lineaire
SHA des commits	SHA originaux preserves	Nouveaux SHA crees
Securite	Non destructif	Destructif
Resolution des conflits	Resoudre une fois	Resoudre par commit
Equipe	Sur pour les branches partagees	Dangereux sur branches partagees
Annulation	Facile	Difficile
git bisect	Bruyant avec les merges	Historique propre
Graphe	Graphe complexe	Ligne unique propre

Git Merge en detail

Fast-Forward Merge

Quand la branche cible n'a pas de nouveaux commits.

# Fast-forward merge (no merge commit created)
git checkout main
git merge feature

# Before:
# C1---C2 (main)
#        \
#         C3---C4 (feature)

# After:
# C1---C2---C3---C4 (main, feature)
# main pointer simply moved forward

Merge a trois voies

Quand les deux branches ont diverge.

# Three-way merge (creates merge commit)
git checkout main
git merge feature

# Before:
#        C3---C4 (feature)
#       /
# C1---C2---C5---C6 (main)

# After:
#        C3---C4
#       /       \
# C1---C2---C5---C6---M7 (main)  ← merge commit M7

--no-ff

Force la creation d'un commit de merge.

# Force merge commit even when fast-forward is possible
git checkout main
git merge --no-ff feature

# Before:
# C1---C2 (main)
#        \
#         C3---C4 (feature)

# After (with --no-ff):
# C1---C2---------M5 (main)  ← merge commit preserves branch history
#        \       /
#         C3---C4 (feature)

# After (without --no-ff, default):
# C1---C2---C3---C4 (main, feature)  ← no evidence of branch

Git Rebase en detail

Rebase basique

Re-applique vos commits sur la branche cible.

# Basic rebase workflow
git checkout feature
git rebase main

# Before:
#        C3---C4 (feature)
#       /
# C1---C2---C5---C6 (main)

# After rebase:
#                     C3'---C4' (feature)  ← new commits!
#                    /
# C1---C2---C5---C6 (main)

# Then merge (fast-forward):
git checkout main
git merge feature
# C1---C2---C5---C6---C3'---C4' (main, feature)  ← linear!

Rebase interactif

Modifier, squasher, reordonner ou supprimer des commits.

# Interactive rebase - clean up last 4 commits
git rebase -i HEAD~4

# Editor opens with:
pick abc1234 Add user model
pick def5678 Fix typo in user model
pick ghi9012 Add user validation
pick jkl3456 Fix validation edge case

# Change to:
pick abc1234 Add user model
fixup def5678 Fix typo in user model        # squash into previous, discard message
pick ghi9012 Add user validation
fixup jkl3456 Fix validation edge case      # squash into previous, discard message

# Result: 2 clean commits instead of 4
# "Add user model" (includes typo fix)
# "Add user validation" (includes edge case fix)

# Interactive rebase commands:
# pick   = use commit as-is
# reword = use commit but edit message
# edit   = use commit but stop for amending
# squash = meld into previous commit (keep message)
# fixup  = meld into previous commit (discard message)
# drop   = remove commit entirely

--onto Rebase

Rebase un sous-ensemble de commits.

# --onto: Move a branch to a different base
# Scenario: feature-b was branched from feature-a by mistake
#           You want feature-b based on main instead

#        D---E (feature-b)
#       /
# A---B---C (feature-a)
#     |
#     F---G (main)

git rebase --onto main feature-a feature-b

# Result:
#     D'---E' (feature-b)  ← now based on main
#    /
# A---B---C (feature-a)
#     |
#     F---G (main)

Resolution des conflits

Les deux peuvent produire des conflits.

Conflits Merge

Tous les conflits sont presentes en une fois.

# Merge conflict workflow
git checkout main
git merge feature
# CONFLICT (content): Merge conflict in src/app.ts
# Automatic merge failed; fix conflicts and then commit

# 1. Open conflicting files and resolve
# 2. Stage resolved files
git add src/app.ts
# 3. Complete the merge
git commit  # creates merge commit with conflict resolution

# Abort merge if needed
git merge --abort

Conflits Rebase

Les conflits apparaissent par commit.

# Rebase conflict workflow
git checkout feature
git rebase main
# CONFLICT in commit C3: Merge conflict in src/app.ts

# 1. Resolve conflict in src/app.ts
git add src/app.ts
# 2. Continue rebase to next commit
git rebase --continue
# May hit another conflict in C4...

# CONFLICT in commit C4: Merge conflict in src/utils.ts
git add src/utils.ts
git rebase --continue

# Abort rebase if it gets too complex
git rebase --abort  # returns to pre-rebase state

# Skip a problematic commit during rebase
git rebase --skip

Strategies de workflow

Workflow base sur Merge

Le workflow le plus courant.

# GitHub Flow (merge-based)
git checkout -b feature/add-auth
# ... make commits ...
git push -u origin feature/add-auth
# Open PR on GitHub
# Review + approve
# Click "Merge pull request" (creates merge commit)
# Or "Squash and merge" (single commit)

Workflow Rebase puis Merge

Rebase pour mettre a jour, puis merge.

# Rebase-before-merge workflow
git checkout feature/add-auth
# ... make commits ...

# Before opening PR, update with latest main
git fetch origin
git rebase origin/main

# Force push (safe because it's your own branch)
git push --force-with-lease origin feature/add-auth

# Open PR on GitHub
# Merge with --no-ff to record integration point
git checkout main
git merge --no-ff feature/add-auth

Workflow Squash and Merge

Combine tous les commits en un seul.

# Squash and merge (via GitHub UI or CLI)
git checkout main
git merge --squash feature/add-auth
git commit -m "feat: add authentication system"

# Before:
# main: A---B---C
# feature: A---B---D---E---F---G

# After squash-merge:
# main: A---B---C---H  ← H contains all changes from D+E+F+G
# (feature branch can be deleted)

La regle d'or du Rebase

Ne jamais rebase des commits deja pushes sur une branche partagee.

# DANGEROUS: Rebasing a shared branch
git checkout shared-feature
git rebase main
git push --force  # !! This rewrites history for everyone!

# SAFE: Rebasing your own local branch
git checkout my-local-feature
git rebase main
# No push yet, or push --force-with-lease to your own branch

# SAFE: Using --force-with-lease instead of --force
git push --force-with-lease origin my-feature
# Fails if remote has commits you haven't seen

Bonnes pratiques

Git Rebase vs Merge Best Practices:

1. Use merge for integrating feature branches into main
   - Creates clear integration points
   - Safe for shared branches
   - Easy to revert entire features

2. Use rebase to keep feature branches up-to-date
   - git rebase main (before opening PR)
   - Creates clean, linear history
   - Makes code review easier

3. Use interactive rebase to clean up before PR
   - Squash fixup commits
   - Reword unclear commit messages
   - Drop debugging commits

4. Use git pull --rebase as default
   - Avoids unnecessary merge commits
   - git config --global pull.rebase true

5. Never rebase shared/pushed commits
   - Only rebase your own unpushed work
   - Use --force-with-lease, never --force

6. Use squash-merge for feature branches
   - One clean commit per feature on main
   - Detailed commits preserved in PR history

7. Use --no-ff for important merges
   - Preserves the fact that a branch existed
   - Makes git log --first-parent useful

Questions frequentes

Quand utiliser rebase vs merge ?

Rebase pour mettre a jour localement, merge pour integrer.

Le rebase est-il dangereux ?

Sur les branches partagees oui, sur les branches locales non.

Qu'est-ce que squash and merge ?