Stacked Diffs Workflow

Stacked Diffs Workflow

Purpose

Stacked diffs (also known as stacked PRs or stacked branches) is a code review workflow pioneered at Meta (Facebook) that enables:

• Breaking large changes into small, reviewable chunks
• Parallel code review of dependent changes
• Faster iteration without waiting for reviews
• Cleaner git history with logical commits
• Reduced context switching for reviewers

Meta engineers use stacked diffs extensively with their internal tools, and this practice has spread to companies like Uber, Airbnb, and many others.

Features

Stacked Diffs vs Traditional PRs

Traditional PR Workflow

Main ─────────────────────────────────────────────►
       \
        └── Feature Branch (1000 lines) ──────────►
                        │
                        └── Single large PR
                            - Hard to review
                            - Long review time
                            - All-or-nothing merge

Stacked Diffs Workflow

Main ────────────────────────────────────────────────►
       \
        └── PR 1: Add data model (100 lines)
             \
              └── PR 2: Add API endpoint (150 lines)
                   \
                    └── PR 3: Add frontend (200 lines)
                         \
                          └── PR 4: Add tests (150 lines)

Each PR:
- Small and focused
- Can be reviewed in parallel
- Merged independently (in order)

Tools

Graphite (Recommended)

# Install Graphite CLI
npm install -g @withgraphite/graphite-cli

# Authenticate with GitHub
gt auth

# Create first branch in stack
gt branch create feature/add-data-model
# ... make changes ...
git add . && gt commit -m "Add user data model"

# Create dependent branch
gt branch create feature/add-api
# ... make changes ...
git add . && gt commit -m "Add user API endpoints"

# Create another dependent branch
gt branch create feature/add-frontend
# ... make changes ...
git add . && gt commit -m "Add user frontend components"

# View your stack
gt log

# Submit all PRs in stack
gt stack submit

# After PR 1 is approved and merged, restack remaining
gt stack restack

# Sync with remote
gt repo sync

ghstack (GitHub)

# Install ghstack
pip install ghstack

# Configure
ghstack config set github.com

# Create stack from commits
# First, make commits on a local branch
git checkout -b feature/user-system
git commit -m "Add user data model"
git commit -m "Add user API"
git commit -m "Add user frontend"

# Submit stack to GitHub
ghstack submit

# Update a commit in the middle
git rebase -i HEAD~3
# Edit the commit
ghstack submit

# After reviews, land the stack
ghstack land

git-branchless

# Install git-branchless
cargo install git-branchless

# Initialize
git branchless init

# Create commits (each becomes a "branch")
git commit -m "Add data model"
git commit -m "Add API"
git commit -m "Add frontend"

# View stack
git smartlog

# Submit to GitHub
git branchless submit

# Rebase and update
git branchless reword HEAD~2  # Edit middle commit
git branchless submit         # Update all PRs

Sapling (Meta's Tool)

# Install Sapling
brew install sapling

# Clone repo
sl clone https://github.com/org/repo

# Create commits in stack
sl commit -m "Add data model"
sl commit -m "Add API"
sl commit -m "Add frontend"

# View stack
sl smartlog

# Submit for review
sl pr submit

# Amend a commit in the middle
sl goto <commit-hash>
# make changes
sl amend

# Rebase dependent commits
sl rebase -d <base>

Workflow Patterns

Creating a Stack

# Start from main
git checkout main && git pull

# Create first logical change
gt branch create feat/step-1-models
# Make changes for data models
git add . && gt commit -m "feat: add user and post models"

# Create second logical change (builds on first)
gt branch create feat/step-2-api
# Make changes for API layer
git add . && gt commit -m "feat: add REST API for users"

# Create third logical change (builds on second)
gt branch create feat/step-3-frontend
# Make changes for frontend
git add . && gt commit -m "feat: add user management UI"

# Submit entire stack
gt stack submit

# Result: 3 PRs created, each targeting the previous

Updating Middle Commits

# Scenario: Reviewer requests changes to PR 2 (API layer)

# Check out the branch for PR 2
gt branch checkout feat/step-2-api

# Make the requested changes
# ... edit files ...

# Amend the commit
git add . && git commit --amend

# Restack all dependent branches
gt stack restack

# Update all PRs
gt stack submit

Handling Merge Conflicts

# When PR 1 is merged and conflicts arise

# Sync with remote
gt repo sync

# Restack your branches on top of main
gt stack restack --onto main

# If conflicts occur, resolve them
# ... resolve conflicts ...
git add . && git rebase --continue

# Update PRs
gt stack submit

Merging a Stack

# Option 1: Merge from bottom up
# After PR 1 approved:
gh pr merge 123 --squash  # PR 1
gt repo sync
gt stack restack

# After PR 2 approved:
gh pr merge 124 --squash  # PR 2
gt repo sync
gt stack restack

# Continue until all merged

# Option 2: Use Graphite's merge queue
gt stack merge  # Merges all approved PRs in order

Best Practices

1. Keep PRs Small and Focused

## Good Stack Structure

PR 1: Database migrations and models
- Add User table migration
- Add Post table migration
- Add UserPost junction table

PR 2: Repository layer
- Add UserRepository with CRUD
- Add PostRepository with CRUD

PR 3: Service layer
- Add UserService with business logic
- Add PostService with business logic

PR 4: API controllers
- Add /users endpoints
- Add /posts endpoints

PR 5: Integration tests
- Add API integration tests
- Add E2E user flow tests

2. Write Clear Stack Descriptions

## PR Description Template for Stacks

### Stack Overview (in first PR)
This stack implements user authentication:
1. **PR 1** (this PR): Database schema for users
2. PR 2: Authentication service
3. PR 3: Login/Register endpoints
4. PR 4: JWT middleware
5. PR 5: Frontend auth components

### This PR
Adds the database schema and migrations for the user authentication system.

### Dependencies
- Base: `main`
- Next in stack: PR #124

### Testing
- [ ] Unit tests pass
- [ ] Migration tested locally

### Review Notes
Please review the schema design first. Later PRs depend on this structure.

3. Coordinate with Reviewers

// PR naming convention for stacks
const prNamingConvention = {
  format: '[Stack: <stack-name>] <n>/<total>: <description>',
  examples: [
    '[Stack: user-auth] 1/5: Add user database schema',
    '[Stack: user-auth] 2/5: Add authentication service',
    '[Stack: user-auth] 3/5: Add login endpoints',
    '[Stack: user-auth] 4/5: Add JWT middleware',
    '[Stack: user-auth] 5/5: Add frontend components'
  ]
};

4. Handle Reviews Efficiently

# Respond to review comments efficiently

# If change affects only one PR
gt branch checkout feat/affected-branch
# make changes
git add . && gt commit --amend
gt stack submit

# If change affects multiple PRs
# Start from the earliest affected PR
gt branch checkout feat/earliest-affected
# make changes
git add . && gt commit --amend
gt stack restack  # Propagates changes up the stack
gt stack submit

Anti-Patterns

Git Commands for Manual Stacking

When tools aren't available, you can manage stacks manually:

# Create a stack manually
git checkout main
git checkout -b stack/feature-1
# make changes, commit

git checkout -b stack/feature-2
# make changes, commit

git checkout -b stack/feature-3
# make changes, commit

# Create PRs targeting each other
gh pr create --base main --head stack/feature-1
gh pr create --base stack/feature-1 --head stack/feature-2
gh pr create --base stack/feature-2 --head stack/feature-3

# After feature-1 merges, rebase the rest
git checkout stack/feature-2
git rebase main
git push --force-with-lease

git checkout stack/feature-3
git rebase stack/feature-2
git push --force-with-lease

# Update PR targets
gh pr edit 124 --base main  # feature-2 now targets main

Interactive Rebase for Stack Edits

# Edit a commit in the middle of a stack
git rebase -i main

# In the editor, change 'pick' to 'edit' for the commit to modify
# pick abc123 Add data model
# edit def456 Add API endpoints  <-- edit this one
# pick ghi789 Add frontend

# Make your changes
git add . && git commit --amend

# Continue rebase
git rebase --continue

# Force push all branches
git push --force-with-lease origin stack/feature-1
git push --force-with-lease origin stack/feature-2
git push --force-with-lease origin stack/feature-3

IDE Integration

VS Code with Graphite

// .vscode/settings.json
{
  "graphite.enabled": true,
  "graphite.showStackInStatusBar": true,
  "git.branchPrefix": "stack/"
}

JetBrains IDEs

# Use terminal integration
# Add to .zshrc or .bashrc
alias gs='gt stack'
alias gb='gt branch'
alias gc='gt commit'

Use Cases

1. Large Feature Development

## Stack: E-commerce Checkout Redesign

1. **PR 1**: Cart data model updates
   - Add discount fields
   - Add shipping options

2. **PR 2**: Cart service layer
   - Discount calculation logic
   - Shipping cost estimation

3. **PR 3**: Checkout API endpoints
   - POST /checkout/initiate
   - POST /checkout/complete

4. **PR 4**: Payment integration
   - Stripe checkout session
   - Webhook handlers

5. **PR 5**: Checkout UI components
   - Cart summary
   - Payment form
   - Order confirmation

6. **PR 6**: E2E tests
   - Full checkout flow tests

2. Refactoring Safely

## Stack: Extract Service from Monolith

1. **PR 1**: Add new service interface
   - Define contracts
   - Add feature flag

2. **PR 2**: Implement new service
   - Core logic extraction
   - Unit tests

3. **PR 3**: Add service client
   - HTTP client implementation
   - Circuit breaker

4. **PR 4**: Migrate callers (batch 1)
   - Update user service
   - Update order service

5. **PR 5**: Migrate callers (batch 2)
   - Update remaining services

6. **PR 6**: Remove old code
   - Delete deprecated methods
   - Remove feature flag

3. Cross-Team Collaboration

# When multiple developers work on a stack

# Developer A creates base
gt branch create feature/base
git add . && gt commit -m "feat: add base infrastructure"
gt stack submit

# Developer B branches from A's work
gt branch checkout feature/base
gt branch create feature/extension
git add . && gt commit -m "feat: extend with new capability"
gt stack submit

# When A updates their PR
gt repo sync
gt stack restack  # B's changes are rebased automatically

Metrics and Benefits

Before Stacked Diffs

After Stacked Diffs

Related Skills

• methodology/finishing-development-branch - Branch completion
• methodology/requesting-code-review - Code review best practices
• devops/github-actions - CI/CD for stacks
• devops/feature-flags - Trunk-based development

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