S3 Mirror 🪞

S3 Mirror is a production-ready Python utility for synchronizing buckets and objects between S3-compatible endpoints. Built on boto3, it provides enterprise-grade reliability with comprehensive logging, parallelized transfers, and automation-friendly operation.

---

Table of Contents

• Overview
• Key Features
• Prerequisites
• Installation
• Configuration
• Usage
• Logging
• Safety Considerations
• Continuous Integration
• Project Structure
• Contributing
• License

---

Overview

Motivation: While MinIO's mc client has served as a capable mirroring tool, recent upstream changes and deprecation of essential features created concerns about long-term reliability and availability. S3 Mirror addresses this gap by providing:

• Complete independence from proprietary tooling ecosystems
• Foundation on boto3, the industry-standard AWS SDK for Python
• Full transparency and auditability of synchronization operations
• Universal S3 compatibility across AWS, MinIO, Ceph, Backblaze B2, Wasabi, and other providers

This tool is designed for infrastructure engineers and DevOps teams requiring dependable, scriptable S3 replication without vendor lock-in.

---

Key Features

✅ Multi-Endpoint Synchronization – Mirror buckets and objects between any S3-compatible services
✅ Performance Optimization – Configurable parallelization with multipart upload support
✅ True Mirroring – Optional deletion of extraneous destination objects for exact replication
✅ Flexible Configuration – YAML/JSON config files with CLI flag overrides
✅ Production Logging – Multiple logging modes including cron-friendly file output with silent console operation
✅ Automation Ready – Idempotent design for reliable scheduled execution
✅ CI/CD Validated – Automated linting and formatting across Python 3.10–3.13
✅ Dependency Management – Automated security updates via Dependabot

---

Prerequisites

• Python 3.10 or higher (tested through 3.13)
• S3 Credentials: AWS access keys or IAM credentials for both source and destination endpoints
• Network Access: Connectivity to both S3 endpoints (including proxy/firewall configuration if required)

---

Installation

Clone the repository and set up the Python environment:

git clone https://github.com/soakes/s3mirror.git
cd s3mirror

# Create and activate virtual environment
python -m venv .venv
source .venv/bin/activate  # Windows: .venv\Scripts\activate

# Install dependencies
pip install -r requirements.txt

---

Configuration

S3 Mirror uses YAML or JSON configuration files to define connection parameters and synchronization behavior. Create a config file based on the template below:

source:
  endpoint_url: "https://s3.source.example.com"
  aws_access_key_id: "SOURCE_ACCESS_KEY"
  aws_secret_access_key: "SOURCE_SECRET_KEY"
  region_name: "us-east-1"
  verify_ssl: false

destination:
  endpoint_url: "https://s3.destination.example.com"
  aws_access_key_id: "DEST_ACCESS_KEY"
  aws_secret_access_key: "DEST_SECRET_KEY"
  region_name: "us-east-1"
  verify_ssl: false

performance:
  max_workers: 20                # Parallel transfer threads
  multipart_threshold: 8388608   # 8 MB - files larger trigger multipart upload
  multipart_chunksize: 8388608   # 8 MB - chunk size for multipart uploads
  max_concurrency: 10            # Concurrent S3 operations per thread
  max_pool_connections: 50       # HTTP connection pool size

sync:
  delete_extraneous: true        # Remove objects in destination not present in source
  exclude_buckets: []            # Bucket names to skip during mirroring

Configuration Parameters

Source/Destination Blocks:

• endpoint_url: S3-compatible API endpoint URL
• aws_access_key_id / aws_secret_access_key: Authentication credentials
• region_name: AWS region identifier (required even for non-AWS endpoints)
• verify_ssl: SSL certificate verification (disable for self-signed certificates)

Performance Tuning:

• Adjust max_workers based on available CPU cores and network bandwidth
• Set multipart_threshold and multipart_chunksize according to typical object sizes
• Increase max_pool_connections for high-throughput scenarios

Sync Behavior:

• delete_extraneous: Enable true mirroring by removing destination-only objects
• exclude_buckets: Skip specific buckets (useful for test/temporary buckets)

---

Usage

Basic Operation

Execute a synchronization using your configuration file:

./s3mirror.py --config config.yaml

Command-Line Options

# Silent mode (console shows errors only)
./s3mirror.py --config config.yaml --quiet

# Log to file with silent console (ideal for cron jobs)
./s3mirror.py --config config.yaml --log-file /var/log/s3mirror.log

# Debug mode with verbose output
./s3mirror.py --config config.yaml --debug

# Disable deletion of extraneous objects
./s3mirror.py --config config.yaml --no-delete

Cron Automation Example

Add to your crontab for scheduled synchronization:

# Run daily at 2:00 AM with file logging
0 2 * * * /path/to/s3mirror/.venv/bin/python /path/to/s3mirror/s3mirror.py --config /path/to/config.yaml --log-file /var/log/s3mirror.log --quiet

---

Logging

S3 Mirror provides multiple logging modes tailored to different operational contexts:

Mode	Console Output	File Output	Use Case
Normal	Human-readable progress messages	None	Interactive execution
Debug	Colorized [LEVEL] messages with details	None	Troubleshooting
File Log	Errors only	Full DEBUG with timestamps	Production automation
Quiet	None (unless errors occur)	None	Minimal output scenarios

Recommendation: For production cron jobs, use --log-file with --quiet to maintain detailed logs while preventing unnecessary console output.

---

Safety Considerations

⚠️ Deletion Behavior: When delete_extraneous: true, S3 Mirror removes objects from the destination that do not exist in the source. This ensures perfect replication but requires careful consideration.

Best Practices:

• Test in non-production environments first to validate configuration
• Enable deletion only when true mirroring is required (vs. one-way copying)
• Use exclude_buckets to protect specific buckets from synchronization
• Review logs regularly to identify unexpected deletions or errors
• Maintain backup copies of critical data before enabling deletion

To disable deletion while still copying new/changed objects:

./s3mirror.py --config config.yaml --no-delete

Or set delete_extraneous: false in the configuration file.

---

Continuous Integration

Every commit and pull request is automatically validated through GitHub Actions across Python 3.10 through 3.13:

Code Quality Workflows

Linting (lint.yml):

• Pylint: Static code analysis for code quality and standards compliance
• Cross-version testing: Validates compatibility across all supported Python versions

Formatting (format.yml):

• Black: Code formatting verification (PEP 8 conformance)
• isort: Import statement organization
• Consistent style enforcement across the entire codebase

Dependency Management

Dependabot (dependabot.yml):

• Automated dependency updates for security patches and version bumps
• Weekly scanning of Python packages and GitHub Actions
• Auto-merge workflow (dependabot-auto-merge.yml) for patch and minor updates

The CI pipeline ensures code quality, security, and cross-version compatibility, providing confidence for production deployment.

---

Project Structure

s3mirror/
├── .github/
│   ├── dependabot.yml                    # Dependabot configuration
│   └── workflows/
│       ├── dependabot-auto-merge.yml     # Auto-merge for dependency updates
│       ├── format.yml                    # Code formatting checks (Black, isort)
│       └── lint.yml                      # Linting workflow (Pylint)
├── .pylintrc                             # Pylint configuration and standards
├── LICENSE                               # MIT License
├── README.md                             # This documentation
├── requirements.txt                      # Python dependencies (boto3, PyYAML, etc.)
└── s3mirror.py                           # Main synchronization script

---

Contributing

Contributions are welcome and appreciated. To contribute:

1. Fork the repository on GitHub
2. Create a feature branch (git checkout -b feature/your-feature)
3. Implement your changes with appropriate tests
4. Ensure CI passes (run pylint, black, and isort locally)
5. Submit a pull request with a clear description of changes

Local Development

Run code quality checks before committing:

# Format code
black s3mirror.py

# Sort imports
isort s3mirror.py

# Run linter
pylint s3mirror.py

Areas for contribution:

• Bug fixes and reliability improvements
• Performance optimizations
• Enhanced error handling and recovery
• Documentation improvements
• Additional S3-compatible endpoint testing

Please open an issue before starting work on major features to discuss implementation approach.

---

License

This project is licensed under the MIT License. See the LICENSE file for complete details.

---

Developed by Simon Oakes
Infrastructure Engineer | Open Source Contributor
© 2025

---