Comparison between rebase and merge

Basic Concepts of Rebase and Merge

In Git, rebase and merge are both commonly used methods for integrating branch changes. Rebase maintains a linear history by reapplying commits, while merge creates a new merge commit to preserve the branch structure. Both have their pros and cons and are suitable for different scenarios.

How Rebase Works

The rebase operation "replays" the commits of the current branch onto the latest commit of the target branch. For example:

// The current branch feature is based on an old commit of main
git checkout feature
git rebase main

This process involves:

1. Finding the common ancestor commit of feature and main
2. Temporarily saving the unique commits from the feature branch
3. Moving the feature branch pointer to the latest commit of the main branch
4. Reapplying the saved commits in order

The rebased history becomes a straight line, making it appear as if all changes were made on top of the latest code in the target branch.

How Merge Works

The merge operation creates a new "merge commit" to connect the histories of two branches:

git checkout main
git merge feature

The merge process involves:

1. Finding the common ancestor of the two branches
2. Creating a new commit that includes all changes from both branches
3. Preserving the original branch structure, resulting in a forked and merged history

Advantages of Rebase

1. Clearer History: The rebased history is linear, making it easier to understand the sequence of changes.
2. Avoids Redundant Merge Commits: Particularly suitable for small feature branches that frequently sync with the main branch.
3. Facilitates Code Review: All changes appear as a continuous series of commits.

Example scenario: When the feature branch needs to sync with the latest changes from the main branch:

git checkout feature
git rebase main
// Resolve potential conflicts
git push --force-with-lease

Advantages of Merge

1. Preserves Complete History: Accurately reflects the actual development process's branch structure.
2. Safer Operation: Does not rewrite existing commit history.
3. Team Collaboration Friendly: Suitable for shared branches with multiple collaborators.

Example scenario: Merging a long-term development feature branch back into the main branch:

git checkout main
git merge --no-ff feature
// Resolve conflicts and commit
git push

Risks and Limitations of Rebase

1. Rewrites History: Modifying shared commits can cause collaboration issues.
2. Complex Conflict Resolution: May require resolving the same conflicts multiple times.
3. Loss of Context: Original branch structure information disappears.

Dangerous example:

// If others have already worked based on the original feature branch
git rebase main
git push --force  // This will disrupt others' work

Disadvantages of Merge

1. Complex History: Numerous merge commits can make the history difficult to read.
2. Pollutes History: Includes many merge commits that only sync code.
3. Hard to Trace: Difficult to locate specific changes in complex branch structures.

When to Choose Rebase

1. Local, unpushed personal branches.
2. Before integrating a feature branch into the main branch.
3. Important projects that require a clear linear history.

Example workflow:

// Develop a new feature
git checkout -b feature
// Multiple commits...
// Sync updates from the main branch
git fetch origin
git rebase origin/main
// Resolve conflicts...
// Prepare to merge
git checkout main
git merge feature

When to Choose Merge

1. Shared branches or already pushed branches.
2. Scenarios where complete branch history needs to be preserved.
3. Merging long-lived feature branches.

Team collaboration example:

// Multi-person collaborative feature branch
git checkout feature
git merge main  // Sync updates
// Multiple people continue development...
// Final merge
git checkout main
git merge --no-ff feature  // Explicitly preserve the merge point

Advanced Rebase Techniques

Interactive rebase allows reordering, combining, or modifying commits:

git rebase -i HEAD~3

Typical operations:

• squash: Combine multiple commits.
• reword: Modify commit messages.
• edit: Pause to modify commit content.
• drop: Delete commits.

Example:

pick a1b2c3d Add login feature
squash e4f5g6h Fix login styles
reword i7j8k9l Update login logic

Merge Strategy Options

Git offers multiple merge strategies:

git merge -s recursive -Xours feature  // Recursive merge, use current branch version for conflicts
git merge -s octopus feature1 feature2  // Merge multiple branches simultaneously

Special options:

• --no-ff: Force creation of a merge commit.
• --squash: Combine all changes into a single commit.
• -Xignore-all-space: Ignore whitespace changes.

Decision Factors in Real Projects

1. Team Standards: Follow the project's existing workflow.
2. Branch Lifecycle: Short-lived branches suit rebase; long-lived branches suit merge.
3. Release Strategy: Projects with frequent releases may prefer rebase.
4. Tool Integration: Some CI/CD systems have requirements for history format.

Differences in Conflict Handling

Rebase conflicts:

• Can occur during each commit replay.
• Need to be resolved sequentially.
• Use git rebase --continue to proceed.

Merge conflicts:

• Resolve all conflicts at once.
• Produce a single merge commit.
• Use git merge --continue to complete.

Graphical History Comparison

After rebase:

* a1b2c3d (HEAD -> main) Feature C
* b2c3d4e Feature B
* c3d4e5f Feature A
* d4e5f6g Base commit

After merge:

*   1a2b3c4 (HEAD -> main) Merge feature branch
|\  
| * a1b2c3d (feature) Feature C
| * b2c3d4e Feature B
| * c3d4e5f Feature A
|/  
* d4e5f6g Base commit

Performance Considerations

1. Rebase: May be slower for large repositories or long branches, as each commit is replayed.
2. Merge: Usually faster but can result in a larger repository (more commit objects).

Undoing Operations

Undo an incorrect rebase:

git reflog  # Find the state before rebase
git reset --hard HEAD@{1}

Undo an incorrect merge:

git merge --abort  # Abandon during conflict
git reset --hard ORIG_HEAD  # Revert to pre-merge state

Combining Rebase and Merge

Mixed strategy in complex projects:

// Use rebase locally to maintain cleanliness
git checkout feature
git rebase main
// Use merge for final integration to preserve context
git checkout main
git merge --no-ff feature

Impact of Branching Strategies

1. Git Flow: Tends to use merge to preserve complete release history.
2. Trunk-Based: More likely to use rebase to maintain linear history.
3. GitHub Flow: Mixes both depending on the situation.

Differences in Tool Integration

1. GitHub/GitLab PR: Defaults to creating merge commits.
2. CLI Workflow: More flexible in choosing rebase or merge.
3. GUI Tools: Typically offer both options.

Deep Impact of History Rewriting

Rebase will:

1. Change the SHA-1 hash of commits.
2. Affect references based on commit hashes (e.g., CI build numbers).
3. May require updates to dependent subsystems.

Best Practices for Team Collaboration

1. Clear Agreements: Establish a unified integration strategy for the team.
2. Documentation: Explain in README or contribution guidelines.
3. Consistent Training: Ensure all members understand the workflow.
4. CI Validation: Set up checks to prevent accidental history rewriting.

Handling Special Cases

Partial rebase:

git rebase --onto newbase oldbase feature

Merge only specific commits:

git cherry-pick a1b2c3d

Submodules and Rebase/Merge

Submodule updates are also affected by rebase/merge choices:

git submodule update --merge
git submodule update --rebase

Compatibility with Bisect

Rebased history may affect the use of git bisect, as the commit context has changed. Merge-preserved original commits are better for issue tracking.

Impact of Large File Storage

LFS-tracked files may require extra handling during rebase, as file pointers are rewritten. Merge is generally safer.

Considerations for Hook Scripts

Rebase triggers the post-rewrite hook, while merge triggers post-merge. Custom hooks need to distinguish between these scenarios.