docker-time-machine

🐳 Docker Time Machine (DTM)

Track your Docker image evolution through git history. Find bloat, optimize builds, and understand how your images changed over time.

⚡ Fast Analysis with Layer Caching

DTM leverages Docker's intelligent layer caching to analyze your image history blazingly fast. Unchanged layers are reused across builds, so you only pay the build cost for layers that actually changed. Analyzing 20+ commits typically takes just minutes, not hours.

What It Does

DTM walks through your git history, builds the Docker image at each commit, and records key metrics:

• Image size — Total size in MB
• Layer count — Number of layers in the image
• Layer sizes — Individual layer sizes for detailed analysis
• Build time — How long each build took (indicative only)

It then generates reports showing trends, comparing layers across commits, and highlighting significant changes.

Features

• 📊 Track image size changes across commits
• 📦 Layer-by-layer comparison — see which layers changed between commits
• 📈 Stacked layer visualization — see how each layer contributes to total image size over time
• ⚡ Fast builds — leverages Docker layer caching for rapid analysis
• 🔍 Find bloat — automatically identifies the commit with the biggest size increase
• ✅ Find optimizations — identifies commits that reduced image size
• 📈 Multiple output formats — table, JSON, CSV, Markdown, interactive HTML charts

Installation

From Source

git clone https://github.com/jtodic/docker-time-machine.git
cd docker-time-machine
go mod download
make build
make install

Prerequisites

• Go 1.24+
• Docker (running daemon)
• Git repository with a Dockerfile

Quick Start

# Navigate to your project with a Dockerfile
cd your-project

# Analyze last 20 commits (default)
dtm analyze

# Generate interactive HTML report
dtm analyze --format chart

Usage Examples

Basic Analysis

# Analyze current repository
dtm analyze

# Analyze specific number of commits
dtm analyze --max-commits 10

# Analyze all commits (no limit)
dtm analyze --max-commits 0

Output Formats

# Table output (default) - prints to terminal
dtm analyze

# Interactive HTML charts
dtm analyze --format chart

# JSON for programmatic processing
dtm analyze --format json --output results.json

# CSV for spreadsheets
dtm analyze --format csv --output results.csv

# Markdown for documentation
dtm analyze --format markdown --output DOCKER_HISTORY.md

Filtering Commits

# Analyze specific branch
dtm analyze --branch develop

# Analyze commits within date range
dtm analyze --since 2024-01-01 --until 2024-06-30

# Analyze a different repository
dtm analyze --repo /path/to/other/project

# Custom Dockerfile location
dtm analyze --dockerfile build/Dockerfile.prod

Handling Build Failures

# Skip commits that fail to build and continue
dtm analyze --skip-failed

# Verbose output to see what's happening
dtm analyze --verbose

Combined Examples

# Full analysis of develop branch with HTML report
dtm analyze --branch develop --format chart --output develop-report.html

# Last 50 commits of 2024, skip failures, JSON output
dtm analyze --max-commits 50 --since 2024-01-01 --skip-failed --format json -o analysis.json

# Quick check of last 5 commits with verbose output
dtm analyze -n 5 -v

Output Examples

Table Output

📊 Docker Image Evolution Report
=================================
 COMMIT     DATE         AUTHOR       SIZE (MB)  DIFF    LAYERS  TIME (S)  MESSAGE
 c5c65db7   2025-11-27   Josko        6.84               3       2.7       test
 35287b4b   2025-02-14   Natanael     6.84       +0.0    3       2.5       remove template
 66dcff23   2025-02-14   Natanael     6.84       +0.0    3       2.5       Revert "add loongarch64"

⚠️  Biggest size increase: a1b2c3d4
   Size increased by: 12.50 MB
   Message: Add node_modules to image

✅ Biggest size reduction: e5f6g7h8
   Size reduced by: 8.20 MB
   Message: Switch to alpine base

📦 Layer Size Comparison Across Commits:
-----------------------------------------
 LAYER                                     c5c65db7  35287b4b  66dcff23
 COPY file:e53e22235bc8d5dab61245a70...    0.00      0.00      0.00
 apk add --no-cache lua5.3 lua-files...    1.55      1.55      1.55
 ADD file:b308dfeecaa300a430b4e65e31...    5.29      5.29      5.29

JSON Output

{
  "results": [
    {
      "commit_hash": "c5c65db7...",
      "commit_message": "test",
      "author": "Josko",
      "date": "2025-11-27T10:30:00Z",
      "image_size": 7172874,
      "build_time_seconds": 2.7,
      "layer_count": 3,
      "layers": [
        {"created_by": "ADD file:b308...", "size_mb": 5.29},
        {"created_by": "apk add --no-cache lua5.3...", "size_mb": 1.55}
      ]
    }
  ],
  "layer_comparison": [
    {
      "layer_command": "apk add --no-cache lua5.3...",
      "size_by_commit": {
        "c5c65db7": 1.55,
        "35287b4b": 1.55,
        "66dcff23": -1
      }
    }
  ],
  "commit_order": ["c5c65db7", "35287b4b", "66dcff23"]
}

HTML Chart Output

Generates an interactive report with:

• 📈 Image size trend over time — line chart showing total image size evolution
• 📊 Image size by layer — stacked bar chart showing how each layer contributes to total size across commits
• ⏱️ Build time analysis — bar chart with note that times are indicative only
• 📦 Layer size comparison table — detailed table comparing layer sizes across all commits

Command Reference

dtm analyze [flags]

Flags:
  -r, --repo string        Path to git repository (default ".")
  -d, --dockerfile string  Path to Dockerfile relative to repo root (default "Dockerfile")
  -f, --format string      Output format: table, json, csv, chart, markdown (default "table")
  -n, --max-commits int    Maximum commits to analyze, 0 = all (default 20)
  -b, --branch string      Git branch to analyze (default: current branch)
  -o, --output string      Output file path (default: stdout, or timestamped file for chart)
      --since string       Analyze commits since date (YYYY-MM-DD)
      --until string       Analyze commits until date (YYYY-MM-DD)
      --skip-failed        Skip commits that fail to build
  -v, --verbose            Verbose output
  -h, --help               Help for analyze

How It Works

1. Reads git history — Gets commits from the specified branch
2. Checks out each commit — Temporarily switches to each commit
3. Builds the Docker image — Using docker build (layer cache makes this fast!)
4. Records metrics — Image size, layer info, build time
5. Cleans up — Removes temporary images
6. Restores branch — Returns to original branch/commit
7. Generates report — In your chosen format

Notes & Limitations

• Build times are indicative only — They depend on Docker's layer cache state and system load
• Commits are analyzed by git parent chain — Not chronological order by date
• Temporary images are cleaned up — Named dtm-<commit-hash>
• Layer matching uses command string — Layers are compared by their Dockerfile instruction

Use Cases

Finding Image Bloat

# Analyze history and look for the "Biggest size increase" in output
dtm analyze --max-commits 50

# Or export to JSON and process programmatically
dtm analyze --format json | jq '.results | max_by(.size_diff)'

Monitoring Image Size Over Time

# Generate weekly reports
dtm analyze --since $(date -d '7 days ago' +%Y-%m-%d) --format chart -o weekly-report.html

Comparing Branches

# Analyze main branch
dtm analyze --branch main --format json -o main.json

# Analyze feature branch  
dtm analyze --branch feature/new-build --format json -o feature.json

License

MIT License - see LICENSE file.

Contributing

Contributions welcome! Please open an issue or submit a pull request.