git-workshop/01-essentials/04-merge-conflict/README.md

# Module 04: Merge Conflicts

## Learning Objectives

By the end of this module, you will:
- Understand what merge conflicts are and why they occur
- Use `git diff` to discover changes between branches
- Identify merge conflicts in your repository
- Read and interpret conflict markers (`<<<<<<<`, `=======`, `>>>>>>>`)
- Resolve merge conflicts manually
- Complete a merge after resolving conflicts

## Setup

Create the challenge environment:

```bash
.\setup.ps1
```

This creates a repository with two feature branches that have conflicting changes.

## Overview

A **merge conflict** occurs when Git cannot automatically combine changes because both branches modified the same part of the same file in different ways.

**When do conflicts happen?**
- ✅ Two branches modify the same lines in a file
- ✅ One branch deletes a file that another branch modifies
- ✅ Complex changes Git can't merge automatically
- ❌ Different files are changed (no conflict!)
- ❌ Different parts of the same file are changed (no conflict!)

**Don't fear conflicts!** They're a normal part of collaborative development. Git just needs your help to decide what the final code should look like.

## Your Task

###  Part 1: Discover the Changes

Before merging, it's good practice to see what each branch changed:

```bash
cd challenge

# Check which branch you're on
git branch

# View all branches
git branch --all
```

You'll see three branches: `main`, `add-timeout`, and `add-debug`.

**Discover what each branch changed:**

```bash
# Compare main with add-timeout
git diff main add-timeout

# Compare main with add-debug
git diff main add-debug

# Compare the two feature branches directly
git diff add-timeout add-debug
```

**What did you discover?**
- Both branches modified `config.json`
- They both added a line in the same location (after `"port": 3000`)
- One adds `"timeout": 5000`
- The other adds `"debug": true`

This is a recipe for a conflict!

### Part 2: Merge the First Branch (No Conflict)

Let's merge `add-timeout` first:

```bash
# Make sure you're on main
git switch main

# Merge the first branch
git merge add-timeout
```

✅ **Success!** This merge works because main hasn't changed since add-timeout was created.

```bash
# View the updated config
cat config.json

# Check the history
git log --oneline --graph --all
```

### Part 3: Try to Merge the Second Branch (Conflict!)

Now let's try to merge `add-debug`:

```bash
# Still on main
git merge add-debug
```

💥 **Boom!** You'll see:

```
Auto-merging config.json
CONFLICT (content): Merge conflict in config.json
Automatic merge failed; fix conflicts and then commit the result.
```

**Don't panic!** This is expected. Git is asking for your help.

### Part 4: Check the Status

```bash
git status
```

You'll see:

```
On branch main
You have unmerged paths.
  (fix conflicts and run "git commit")
  (use "git merge --abort" to abort the merge)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
        both modified:   config.json
```

This tells you that `config.json` needs your attention!

### Part 5: Open and Examine the Conflicted File

Open `config.json` in your text editor:

```bash
# On Windows
notepad config.json

# Or use VS Code
code config.json
```

You'll see special **conflict markers**:

```json
{
  "app": {
    "name": "MyApp",
    "version": "1.0.0",
    "port": 3000,
<<<<<<< HEAD
    "timeout": 5000
=======
    "debug": true
>>>>>>> add-debug
  }
}
```

### Part 6: Understand the Conflict Markers

Let's break down what you're seeing:

```
<<<<<<< HEAD
    "timeout": 5000        ← Your current branch (main, which has add-timeout merged)
=======
    "debug": true          ← The branch you're merging (add-debug)
>>>>>>> add-debug
```

**What each marker means:**
- `<<<<<<< HEAD` - Start of your changes (current branch)
- `=======` - Separator between the two versions
- `>>>>>>> add-debug` - End of their changes (branch being merged)

### Part 7: Resolve the Conflict

You have three options:

**Option 1: Keep ONLY your changes (timeout)**
```json
    "timeout": 5000
```

**Option 2: Keep ONLY their changes (debug)**
```json
    "debug": true
```

**Option 3: Keep BOTH changes** ← **Do this!**
```json
    "timeout": 5000,
    "debug": true
```

### Part 8: Edit the File

For this challenge, we want **both settings**, so:

1. Delete ALL the conflict markers:
   - Remove `<<<<<<< HEAD`
   - Remove `=======`
   - Remove `>>>>>>> add-debug`

2. Keep both settings:

**Before (with conflict markers):**
```json
{
  "app": {
    "name": "MyApp",
    "version": "1.0.0",
    "port": 3000,
<<<<<<< HEAD
    "timeout": 5000
=======
    "debug": true
>>>>>>> add-debug
  }
}
```

**After (resolved):**
```json
{
  "app": {
    "name": "MyApp",
    "version": "1.0.0",
    "port": 3000,
    "timeout": 5000,
    "debug": true
  }
}
```

**Important:**
- Remove ALL markers
- Add a comma after `"timeout": 5000` (for valid JSON)
- Ensure the file is valid JSON

3. Save the file

### Part 9: Mark the Conflict as Resolved

Tell Git you've resolved the conflict:

```bash
# Stage the resolved file
git add config.json

# Check status
git status
```

You should see:

```
On branch main
All conflicts fixed but you are still merging.
  (use "git commit" to conclude merge)
```

Perfect! Git confirms the conflict is resolved.

### Part 10: Complete the Merge

Commit the merge:

```bash
git commit
```

Git will open an editor with a default merge message. You can accept it or customize it, then save and close.

**Done!** Your merge is complete!

```bash
# View the final result
cat config.json

# View the history
git log --oneline --graph --all
```

You should see both `timeout` and `debug` in the config!

### Part 11: Verify Your Solution

From the module directory (not inside challenge/):

```bash
.\verify.ps1
```

## Understanding Conflict Markers

### Anatomy of a Conflict

```
<<<<<<< HEAD            ← Marker: Start of your version
Your changes here
=======                 ← Marker: Separator
Their changes here
>>>>>>> branch-name     ← Marker: End of their version
```

### Common Conflict Patterns

**Simple conflict:**
```
<<<<<<< HEAD
print("Hello")
=======
print("Hi")
>>>>>>> feature
```
Decision: Which greeting do you want?

**Both are needed:**
```
<<<<<<< HEAD
timeout: 5000
=======
debug: true
>>>>>>> feature
```
Decision: Keep both (add comma)!

**Deletion conflict:**
```
<<<<<<< HEAD
# Function deleted on your branch
=======
def old_function():
    pass
>>>>>>> feature
```
Decision: Delete or keep the function?

## Common Mistakes to Avoid

❌ **Forgetting to remove conflict markers**
```json
<<<<<<< HEAD    ← Don't leave these in!
    "timeout": 5000,
    "debug": true
>>>>>>> add-debug    ← Don't leave these in!
```
This breaks your code! Always remove ALL markers.

❌ **Committing without staging**
```bash
git commit    # Error! You didn't add the file
```
Always `git add` the resolved file first!

❌ **Keeping only one side when both are needed**
If you delete one setting, you lose that work!

❌ **Breaking syntax**
```json
"timeout": 5000    ← Missing comma!
"debug": true
```
Always verify your file is valid after resolving!

❌ **Not testing the result**
Always check that your resolved code works!

## Aborting a Merge

Changed your mind? You can abort the merge anytime before committing:

```bash
git merge --abort
```

This returns your repository to the state before you started the merge. No harm done!

## Key Commands

```bash
# Discover changes before merging
git diff branch1 branch2

# Attempt a merge
git merge <branch-name>

# Check which files have conflicts
git status

# Abort the merge and start over
git merge --abort

# After resolving conflicts:
git add <resolved-file>
git commit

# View conflicts in a different style
git diff --ours         # Your changes
git diff --theirs       # Their changes
git diff --base         # Original version
```

## Pro Tips

💡 **Use `git diff` first**
Always compare branches before merging:
```bash
git diff main..feature-branch
```

💡 **Prevent conflicts**
- Pull changes frequently
- Communicate with your team about who's working on what
- Keep branches short-lived and merge often

💡 **Make conflicts easier**
- Work on different files when possible
- Make small, focused commits
- If editing the same file, coordinate with teammates

💡 **When stuck**
- Read the conflict markers carefully
- Look at `git log` to understand what each side changed
- Use `git diff` to see the changes
- Ask a teammate to review your resolution
- Use a merge tool: `git mergetool`

## Merge Tools

Git supports visual merge tools that make resolving conflicts easier:

```bash
# Configure a merge tool (one-time setup)
git config --global merge.tool vscode  # or meld, kdiff3, etc.

# Use the merge tool during a conflict
git mergetool
```

This opens a visual interface showing both versions side-by-side.

## Real-World Scenario

This exercise simulates a common real-world situation:

**Scenario:** Two developers working on the same file
- Alice adds a timeout configuration
- Bob adds debug mode configuration
- Both push their changes
- When Bob tries to merge, he gets a conflict
- Bob resolves it by keeping both changes
- Everyone's work is preserved!

This happens all the time in team development. Conflicts are normal!

## What You've Learned

After completing this module, you understand:

- ✅ Merge conflicts happen when the same lines are changed differently
- ✅ `git diff` helps you discover changes before merging
- ✅ Conflict markers show both versions
- ✅ You decide what the final code should look like
- ✅ Remove all markers before committing
- ✅ Test your resolution to ensure it works
- ✅ Conflicts are normal and easy to resolve with practice

## Next Steps

Ready to continue? The next module covers **cherry-picking** - selectively applying specific commits from one branch to another.

To start over:
```bash
.\reset.ps1
.\setup.ps1
```

**Need help?** Review the steps above, or run `git status` to see what Git suggests!