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Basic Concepts of Merge Strategies

Merge strategies in Git determine how to integrate the history of two or more branches. Different strategies are suitable for different scenarios, and understanding these strategies is crucial for using Git efficiently. Core merge strategies include recursive, resolve, octopus, ours, and subtree, among others.

When executing the git merge command, Git automatically selects the appropriate strategy based on the situation, but you can also manually specify it using the -s parameter. For example:

git merge -s recursive branch-name

Recursive Merge Strategy (recursive)

This is Git's default merge strategy, suitable for merging two branches. When multiple common ancestors exist, this strategy recursively finds the best common ancestor to serve as the merge base.

The recursive merge handles conflicts intelligently:

1. Automatically handles renames
2. Detects file movements
3. Resolves simple conflicts automatically

Example scenario:

# Create and switch to a new branch
git checkout -b feature-branch

# Modify a file and commit
echo "New feature code" >> feature.js
git add feature.js
git commit -m "Add new feature"

# Switch back to the main branch and merge
git checkout main
git merge feature-branch

Resolve Merge Strategy (resolve)

This strategy is simpler than the recursive strategy, using only one common ancestor as the merge base. It can be used when the recursive strategy causes performance issues due to complex history.

Applicable scenarios:

• Very complex history
• Slow recursive merges
• When only a simple merge is needed

git merge -s resolve branch-name

Octopus Merge Strategy (octopus)

This strategy is specifically designed to merge more than two branches simultaneously. It cannot handle merges requiring conflict resolution; all changes to be merged must be able to merge automatically.

Typical usage:

git merge branch1 branch2 branch3

Common use cases in development:

• Merging multiple feature branches into the main branch
• Integrating work from multiple developers simultaneously
• Batch merging fix branches

Ours Merge Strategy (ours)

This strategy creates a merge commit but does not actually merge any changes. It preserves the current branch's code state while ignoring all changes from the branch to be merged.

Use cases:

• Recording that a branch was merged without introducing changes
• Resolving conflicts for specific files
• Preserving branch history while ignoring content

git merge -s ours branch-to-ignore

Subtree Merge Strategy (subtree)

The subtree strategy is used to merge one repository as a subdirectory into another. It is more flexible than submodules and is suitable for managing project dependencies.

Implementation steps:

1. Add a remote repository:

git remote add lib-project https://github.com/user/lib-project.git

2. Fetch remote content:

git fetch lib-project

3. Perform a subtree merge:

git merge -s subtree --allow-unrelated-histories lib-project/main

Custom Merge Strategies

Git allows custom merge strategies to be configured. For example, you can specify merge drivers for specific files:

1. Specify in .gitattributes:

*.json merge=jsonMerge

2. Configure a custom merge driver:

git config merge.jsonMerge.name "JSON merge driver"
git config merge.jsonMerge.driver "json-merge-driver %O %A %B"

Advanced Merge Strategy Configuration

You can adjust merge behavior through Git configuration:

1. Set default merge strategies:

git config --global pull.rebase true
git config --global merge.ff only

2. Configure conflict resolution tools:

git config --global merge.tool vscode
git config --global mergetool.vscode.cmd "code --wait $MERGED"

3. Set merge commit message templates:

git config --global merge.log true
git config --global merge.conflictstyle diff3

Choosing Merge Strategies in Real Projects

In large projects, selecting a merge strategy requires consideration of:

1. Feature branch merges:

# Recommended to use recursive merge
git merge --no-ff feature/login

2. Release branch merges:

# Use ours strategy to preserve specific configurations
git merge -s ours release/config

3. Multi-team collaboration:

# Use octopus to merge multiple fixes
git merge hotfix/security hotfix/perf

Special Handling of Merge Conflicts

When conflicts arise during merging, you can combine different strategies:

1. Partially accept changes:

git merge -Xours feature/update
# or
git merge -Xtheirs feature/update

2. Ignore whitespace changes:

git merge -Xignore-all-space branch-name

3. Rename detection:

git merge -Xrename-threshold=50% feature/refactor

Comparison of Merge Strategies and Rebase

Merging and rebasing are the two main ways to integrate changes:

1. Merging preserves complete history:

git merge feature-branch

2. Rebasing creates linear history:

git rebase main

3. Interactive rebase offers more control:

git rebase -i HEAD~3

Automated Merge Strategies

In CI/CD workflows, merging can be automated:

1. Example of a safe merge script:

const { execSync } = require('child_process');

function safeMerge(source, target) {
  try {
    execSync(`git checkout ${target}`);
    execSync(`git merge --no-ff ${source}`);
    console.log(`Successfully merged ${source} into ${target}`);
  } catch (error) {
    console.error(`Merge failed: ${error.message}`);
    execSync('git merge --abort');
    process.exit(1);
  }
}

safeMerge('feature/new-api', 'develop');

2. Pre-merge checks:

git merge --no-commit --no-ff feature/test
git diff --check
git merge --abort

Performance Optimization for Merge Strategies

For large repositories, merge performance is important:

1. Shallow cloning reduces data:

git clone --depth 1 https://repo.url

2. Disable rename detection:

git merge --no-renames feature/branch

3. Use memory caching:

git config --global merge.renamelimit 99999

Merge Strategies and Git Hooks

Git hooks can enhance the merge process:

1. Pre-merge checks:

#!/bin/sh
# .git/hooks/pre-merge
if git diff-index --quiet HEAD --; then
  echo "Working directory clean, ready to merge"
  exit 0
else
  echo "Error: Uncommitted changes in working directory"
  exit 1
fi

2. Post-merge automation:

#!/bin/sh
# .git/hooks/post-merge
npm install

Team Standards for Merge Strategies

Establishing team merge standards is important:

1. Code review requirements:

- At least one reviewer approval
- CI builds must pass
- All comments must be resolved before merging

2. Merge message templates:

Merge [type]: [brief description]

[detailed description]

Related issues: #123

3. Branch protection rules:

# Configure on GitHub/GitLab:
- Require linear history
- Disable force pushes
- Require status checks