Incremental Build Rules

An incremental build system guide that saves time by only building changed packages in local development environments.

Overview#

Level-based Parallel Build#

Incremental build optimizes performance by building packages at the same priority level in parallel.

레벨 [1]: shared/* (4개) ─────────┬─ 병렬 실행
                                  ↓ 레벨 1 완료
레벨 [3]: package/* (8개) ────────┬─ 병렬 실행
                                  ↓ 레벨 3 완료
레벨 [4]: feature/common/* (6개) ─┬─ 병렬 실행
                                  ↓ 레벨 4 완료
레벨 [5]: feature/application/*  ─┬─ 병렬 실행
                                  ↓ 레벨 5 완료
레벨 [7]: app/* (1개) ────────────┴─ 빌드

Parallel concurrency control:

# Set concurrent build count (default: 4)
BUILD_CONCURRENCY=6 melos run build:incremental

# Limit to 1 in low-resource environments
BUILD_CONCURRENCY=1 melos run build:incremental:low-resources

Melos Commands#

Basic Commands#

# Incremental build of changed packages + reverse dependencies only
melos run build:incremental

# Check changed packages without building
melos run build:incremental:dry-run

# Check build status
melos run build:incremental:status

Advanced Commands#

CI Cache Synchronization#

Syncs CI build cache to local, enabling accurate incremental builds even after squash merges.

Usage Scenarios#

PR 브랜치:  A → B → C  (last-commit.sha = C)
                ↓ squash merge
main:       X → D      (D는 새로운 SHA, C는 orphan)

After squash merge, when pulling main locally, the stored commit SHA (C) no longer exists so Git diff may fail. In this case, hash-based fallback automatically activates.

Commands#

# Sync CI cache to local
melos run build:sync-from-ci

# Preview sync (without downloading)
melos run build:sync-from-ci:dry-run

Prerequisites#

• GitHub CLI (gh) installation required: brew install gh
• GitHub authentication required: gh auth login

How It Works#

1. Download build-metadata from GitHub Actions artifacts
2. Copy hash files to .build-state/ directory
3. Enables accurate change detection in next incremental build

Change Detection Modes#

Auto Mode (Default)#

# Automatically select optimal mode
melos run build:incremental

Automatically selects Git-based inside Git repo, hash-based outside.

Squash merge handling: Automatically falls back to hash-based detection when stored commits are orphaned.

Git Mode#

# Git diff-based change detection
scripts/local_incremental_build.sh --mode=git

• Compare last build commit with current HEAD
• Include working directory changes
• Detect both staged/unstaged files

Time/Hash Mode#

# File hash-based change detection
scripts/local_incremental_build.sh --mode=time

• Compare source file SHA-256 hashes
• Auto-exclude generated files (.g.dart, .freezed.dart, etc.)
• Works without Git

Environment Variables#

Usage Examples#

# Build without delete option
BUILD_RUNNER_OPTS= " "   melos run build:incremental

# Build in low-resources mode
BUILD_RUNNER_OPTS= " --low-resources-mode -d "   melos run build:incremental

Build Priority#

Incremental build builds in dependency order:

Cache Structure#

Storage Location#

.build-state/
├── last-commit.sha        # 마지막 빌드 커밋 SHA
├── last-build.timestamp   # 마지막 빌드 시간
├── package-feature_console_console_sales_analysis.hash
├── package-package_core.hash
└── ...

Cache File Format#

• File명: package-{Path_슬래when를_언더스코어로}.hash
• 내용: 소스 File들of the SHA-256 해when

Cache Invalidation Conditions#

1. Source file modification (.dart files)
2. pubspec.yaml modification
3. Using --force option
4. Deleting .build-state/ directory

Troubleshooting#

Cache Issues#

# Full cache reset
melos run build:incremental:reset

# Force rebuild
melos run build:incremental:force

Change Detection Failure#

# Check with detailed logs
melos run build:incremental:verbose

# Direct script execution
scripts/detect_local_changes.sh --verbose --mode=time

Insufficient Memory#

# Use low-resources mode
melos run build:incremental:low-resources

# Or set via environment variables
BUILD_RUNNER_OPTS= " --low-resources-mode "   melos run build:incremental

Build Specific Package Only#

# Direct specification instead of incremental build
melos exec --scope=console_sales_analysis -- dart run build_runner build -d

Script Structure#

Local Scripts#

CI Scripts#

Script Relationships#

┌─────────────────────────────────────────────────┐
│ melos run build:incremental                      │
└───────────────────┬─────────────────────────────┘
                    ▼
┌─────────────────────────────────────────────────┐
│ scripts/local_incremental_build.sh               │
│ - 옵션 파싱 (--dry-run, --force, --low-resources)│
│ - 변경 감지 호출                                  │
│ - CI 스크립트 재사용                              │
└───────────────────┬─────────────────────────────┘
         ┌──────────┴──────────┐
         ▼                     ▼
┌─────────────────┐  ┌─────────────────────────────┐
│ detect_local_   │  │ .github/scripts/            │
│ changes.sh      │  │ smart_incremental_build.sh  │
│ (변경 감지)      │  │ (실제 빌드 실행)             │
└─────────────────┘  └─────────────────────────────┘

Squash Merge Handling#

Problem#

When squash merging a PR, original commits are replaced with a new single commit. The commit stored in .build-state/last-commit.sha becomes orphaned, causing Git diff to fail.

Solution#

1. Auto-fallback: Automatically switches to content hash-based detection on Git diff failure
2. CI sync: Download CI cache with build:sync-from-ci for improved accuracy

Recommended Workflow#

# After squash merge
git checkout main
git pull

# Option 1: Use auto-fallback (hash-based)
melos run build:incremental

# Option 2: Build after CI cache sync (more accurate)
melos run build:sync-from-ci
melos run build:incremental

Notes#

• .build-state/ directory is included in .gitignore
• CI uses a separate cache strategy (4-layer cache)
• Reverse dependency analysis enabled when jq is installed
• CI build metadata is stored as artifacts for 7 days