Object storage format

Object storage format is a structured method for storing and managing data, particularly in Git, where it determines how data is organized, stored, and retrieved. Different storage formats impact performance, scalability, and compatibility, making the choice of format critical for system design.

Basic Concepts of Object Storage

Object storage treats data as independent units (objects), each containing the data itself, metadata, and a unique identifier. In Git, the core object storage consists of four types: blob, tree, commit, and tag. Here’s a brief overview:

• blob: Stores file content without filenames or permissions.
• tree: Acts like a directory, recording filenames, permissions, and corresponding blobs or subtrees.
• commit: Points to a tree object and includes author, commit message, and parent commit(s).
• tag: Provides a human-readable name for a specific object (usually a commit).

Storage Structure of Git Objects

Git uses SHA-1 hashes (or optionally SHA-256) as unique identifiers for objects. Objects are stored in the .git/objects directory, with the path derived from the hash: the first two characters form the directory name, and the remaining characters form the filename. For example, an object with the hash a1b2c3... is stored at a1/b2c3....

Objects are stored in a compressed format and can be decompressed using zlib. Below is an example code snippet for reading a Git object:

const fs = require('fs');
const zlib = require('zlib');

function readGitObject(hash) {
  const dir = hash.substring(0, 2);
  const file = hash.substring(2);
  const path = `.git/objects/${dir}/${file}`;
  
  const compressed = fs.readFileSync(path);
  return new Promise((resolve) => {
    zlib.inflate(compressed, (err, data) => {
      if (err) throw err;
      resolve(data.toString());
    });
  });
}

// Example: Reading a blob object
readGitObject('a1b2c3...').then(console.log);

Optimization Techniques for Object Storage

Git employs several techniques to optimize object storage, including:

Packfiles

Git packs multiple objects into a single file to reduce storage space and I/O operations. Packfiles (.pack) and their index files (.idx) are typically located in .git/objects/pack. Packfiles use delta compression, storing only the differences between objects.

Refs and Symbolic Refs

Refs are pointers to objects, such as branches (refs/heads/) and tags (refs/tags/). Symbolic refs (e.g., HEAD) are indirect refs that point to another ref. Here’s an example of resolving HEAD:

function resolveHead() {
  const headPath = '.git/HEAD';
  const headContent = fs.readFileSync(headPath, 'utf-8').trim();
  
  if (headContent.startsWith('ref: ')) {
    const refPath = headContent.substring(5);
    return fs.readFileSync(`.git/${refPath}`, 'utf-8').trim();
  }
  return headContent; // Directly returns the commit hash (detached HEAD state)
}

console.log(resolveHead()); // Outputs the commit hash pointed to by HEAD

Custom Object Storage Formats

In some scenarios, custom object storage formats may be needed. For example, extending Git to support large file storage (e.g., Git LFS). Below is a simple implementation of custom storage:

class CustomStorage {
  constructor() {
    this.objects = new Map(); // Simulated storage
  }

  store(hash, data) {
    this.objects.set(hash, data);
  }

  retrieve(hash) {
    return this.objects.get(hash);
  }
}

const storage = new CustomStorage();
storage.store('abc123', 'Hello, Git!');
console.log(storage.retrieve('abc123')); // Output: Hello, Git!

Performance Considerations for Object Storage

The performance of object storage is influenced by the following factors:

1. Hashing Algorithm: SHA-1 is faster than SHA-256 but less secure.
2. Compression Level: Higher compression reduces storage but increases CPU overhead.
3. Caching Mechanisms: Git uses memory caches (e.g., core.deltaBaseCacheLimit) to speed up object access.

Here’s a simple cache implementation example:

class ObjectCache {
  constructor(maxSize) {
    this.cache = new Map();
    this.maxSize = maxSize;
  }

  get(hash) {
    if (this.cache.has(hash)) {
      const value = this.cache.get(hash);
      // Refresh cache order
      this.cache.delete(hash);
      this.cache.set(hash, value);
      return value;
    }
    return null;
  }

  set(hash, value) {
    if (this.cache.size >= this.maxSize) {
      const oldest = this.cache.keys().next().value;
      this.cache.delete(oldest);
    }
    this.cache.set(hash, value);
  }
}

const cache = new ObjectCache(100);
cache.set('abc123', 'Cached data');
console.log(cache.get('abc123')); // Output: Cached data

Extended Applications of Object Storage

Object storage is not limited to Git and can be used for:

• Version Control Systems: Like Mercurial, which uses a similar object model.
• Distributed Databases: Such as IPFS, which employs content-addressable storage.
• Static Website Hosting: Like GitHub Pages, which relies on Git object storage.

Below is an example simulating IPFS storage:

class IPFSStorage {
  constructor() {
    this.blocks = new Map();
  }

  async put(block) {
    const hash = await crypto.subtle.digest('SHA-256', block);
    const hashHex = Array.from(new Uint8Array(hash))
      .map(b => b.toString(16).padStart(2, '0'))
      .join('');
    this.blocks.set(hashHex, block);
    return hashHex;
  }

  async get(hash) {
    return this.blocks.get(hash);
  }
}

const ipfs = new IPFSStorage();
const data = new TextEncoder().encode('IPFS rocks!');
ipfs.put(data).then(hash => console.log(`Stored with hash: ${hash}`));