Innovative Software Technology-Beyond `git commit`: Unpacking Git’s Internals by Building It in Go

For many developers, Git is a daily tool, yet its underlying mechanisms often remain a mystery. We use git add, git commit, and navigate branches without truly understanding the magic happening behind the scenes. This was my experience until I embarked on a journey to reconstruct Git from its fundamental components in Go, without relying on any external Git libraries. It was a revelation.

The goal wasn’t to create a production-ready Git client, but to dismantle and reassemble it, piece by piece, to grasp its core principles. This intensive process, focused on SHA-256 hashing, intricate tree structures, and commit graphs, transformed years of surface-level usage into a profound comprehension of what makes Git tick.

My Learning Expedition

My understanding wasn’t born from a single guide but pieced together through a combination of resources and significant trial and error:

CodeCrafters’ “Build Your Own Git” challenge: Provided the essential framework and motivated the hands-on implementation.
YouTube explorations: Countless searches for “how git works internally” yielded sporadic but valuable insights.
“Building Git” book: Offered clarifications on specific object formats, even if not the primary instructional source for my approach.

Ultimately, the most potent learning method was the iterative cycle of breaking things, deciphering error messages, and debugging for extended periods. This hands-on struggle cemented my knowledge far more effectively than passive consumption.

The Motivation: Why Rebuild Git?

My daily interaction with Git felt like using a black box. I could operate it, but the “why” behind its efficiency remained elusive. I yearned for answers to questions like:

Why are new commits so lightweight?
How does Git manage automatic file deduplication?
What exactly is a “tree object”?
Why is branching an incredibly fast operation?

The best way to truly comprehend a complex system, I discovered, is to build it yourself. This project was my answer to these fundamental questions.

Introducing Go-Git: A Core Implementation

My Go-Git project meticulously implements Git’s core functionalities, enabling operations like:

go-git init                    # Initialize repository
go-git config                  # Set user identity
go-git add <files...>          # Stage files
go-git commit -m "message"     # Create commit
go-git log                     # View history

It handles content-addressable storage, the staging area (index), tree objects, commit history, and even zlib compression. This covers the essential mechanics of code management, excluding advanced features like branches, merges, and remote interactions.

Git’s Foundational Concepts: The Pillars

1. Content-Addressable Storage: The Hash as Identity

At Git’s heart is the concept of content-addressable storage. Every piece of data—be it a file (blob), a directory listing (tree), or a commit—is stored and referenced by its SHA-256 hash. The object’s hash IS its unique identifier and its address:

.git/objects/ab/c123def456...
            ↑↑  ↑↑↑↑↑↑↑
            │   └─ Rest of hash (filename)
            └───── First 2 chars (subdirectory)

This design is remarkably elegant. Identical content always produces the same hash, leading to automatic deduplication. If the same README.md appears in a hundred commits, it only occupies disk space once. No separate indexing system is needed; the hash directly points to the content.

2. The Three Trees: Git’s Layered Approach

Git maintains your project’s state across three distinct “trees”:

Working Directory  →  Staging Area  →  Repository
   (your files)        (.git/index)     (.git/objects)

go-git add transitions files from your working directory to the staging area.
go-git commit captures the current state of the staging area, creating a snapshot in the repository.

The staging area, deceptively simple, is essentially a list mapping file paths to their respective content hashes:

100644 abc123... README.md
100644 def456... src/main.go

When you commit, this entire staging area is itself hashed and stored as a tree object, representing the directory structure at that commit.

3. Tree Objects: The Directory Structure Revealed

Files are stored as blobs, while directories are represented by trees. Consider a simple project:

project/
  README.md
  src/
    main.go
    lib/
      helper.go

Git translates this into a hierarchical structure of objects:

Commit (abc123)
    ↓
Root Tree (def456)
├─ blob: README.md (hash: abc123)
└─ tree: src/ (hash: def456)
      ├─ blob: main.go (hash: ghi789)
      └─ tree: lib/ (hash: jkl012)
            └─ blob: helper.go (hash: mno345)

The crucial insight here is that tree objects don’t contain their children; they reference them by hash. This indirection is the cornerstone of Git’s efficiency. If a directory remains unchanged across commits, its tree object’s hash also remains the same, allowing it to be reused without re-storing any data. This is why commits are cheap and branching is fast.

The challenging aspect: Building these tree objects requires a bottom-up approach. You cannot compute a parent tree’s hash until you have the hashes of all its child blobs and subtrees. For instance, the hash of src/ depends on the hash of src/lib/, and the root tree’s hash depends on src/. The order of operations is absolutely critical, a detail that consumed a significant portion of my implementation time.

Navigating Implementation Hurdles

Building Go-Git was rife with challenges, each offering a deep dive into Git’s intricacies:

Tree Construction Order

My initial attempt to build trees top-down—starting from the root—quickly failed because child tree hashes were unavailable. The solution involved a reverse-depth sort:

sort.Slice(dirs, func(i, j int) bool {
    return strings.Count(dirs[i], "/") > strings.Count(dirs[j], "/")
})

This allowed for the construction of deepest trees first, linking them to their parents as their hashes became available.

Binary vs. Hexadecimal Hash Encoding

A subtle but critical error involved storing hashes in tree objects. Git expects 32 binary bytes, not a 64-character hexadecimal string. My initial bug:

content += entry.BlobHash  // Wrong! This is a 64-char hex string

The correction:

hashBytes, _ := hex.DecodeString(entry.BlobHash)
content = append(content, hashBytes...)  // 32 binary bytes

This oversight resulted in trees that were twice their intended size and completely disrupted tree traversal. Debugging this subtle issue was a significant learning curve.

Excluding the `.git/` Directory During Staging

When performing a go-git add ., it’s imperative to prevent the Git repository’s internal files from being added to the index. A naive check for .git in the path proved inadequate, as it would incorrectly exclude legitimate files like my.git.file.

The correct method involved using filepath.SkipDir during directory traversal:

if d.Name() == ".git" && d.IsDir() {
    return filepath.SkipDir
}

This ensures that the internal Git directory is properly ignored.

The Anatomy of Git Objects

Every object in Git adheres to a consistent format:

<type> <size><content>

This structure is then compressed using zlib and stored at .git/objects/<hash[:2]>/<hash[2:]>.

Blob Object: Represents a file.
```
blob 13Hello, World!
    
```
Hashed, it becomes a0b1c2d3..., stored at .git/objects/a0/b1c2d3...
Tree Object: Represents a directory listing, containing references to blobs and other trees.
```
tree 740644 README.md<32-byte-hash>040000 src<32-byte-hash>
    
```
Modes:
- 100644: Standard file
- 040000: Directory (tree object)
Commit Object: Represents a snapshot of the repository at a specific point in time.
```
commit <size>
tree abc123...
parent 789xyz...
author Uthman <email> timestamp
committer Uthman <email> timestamp

Initial commit
    
```
Commits form a directed acyclic graph (DAG). The go-git log command simply traverses this chain backward from HEAD to display history.

Profound Learnings from the Build

This project crystallized several fundamental truths about Git:

The Elegance of Content-Addressable Storage: The hash as a unique address is a brilliant design choice, enabling effortless deduplication, integrity verification, and highly efficient storage.
Trees as Graphs, Not Nested Structures: Understanding that tree objects merely reference their children by hash—rather than containing them—was a pivotal realization. This indirection underpins Git’s efficiency, allowing multiple commits to share identical directory structures without redundant storage.
The Necessity of Bottom-Up Construction: The strict requirement to build trees from the deepest elements upwards, due to hash dependencies, highlights the meticulous ordering inherent in Git’s object model.
The Imperative of Compression: Without zlib compression, the .git/objects/ directory would be three to four times larger, underscoring the critical role compression plays in Git’s disk space management.
The Nuances of Binary Formats: Handling null bytes () and raw binary hash data demands careful programming, a trade-off for the increased efficiency over easier-to-parse text formats.

Limitations of a Learning Project

It’s important to note that this Go-Git implementation, while powerful for learning, is not a production-grade tool. Its deliberate limitations include:

No support for multiple branches (only main exists).
Absence of merge operations.
Lack of diff/status commands.
No remote operations (push, pull, fetch).
No .gitignore file processing.
Objects are stored as separate files, without Git’s optimized “packed objects.”
The index is a plain text file, unlike Git’s binary format.

These omissions were intentional, allowing the project to intensely focus on Git’s core object model, staging, and commit mechanisms without the added complexity of advanced features.

Experience Go-Git Yourself

To truly grasp these concepts, I encourage you to interact with the Go-Git project:

git clone https://github.com/codetesla51/go-git.git
cd go-git
./install.sh

# Or build manually:
go build -buildvcs=false -o go-git
ln -s $(pwd)/go-git ~/.local/bin/go-git

Then, explore its functionality:

mkdir my-project
cd my-project
go-git init
go-git config

echo "Hello World" > README.md
go-git add README.md
go-git commit -m "Initial commit"
go-git log

For an even deeper understanding, deliberately introduce errors. Change the hash function to MD5, or remove zlib compression, and observe how Git’s fundamental assumptions about immutability and content-addressing cascade into system-wide failures. This hands-on experimentation is invaluable.

Concluding Thoughts

Rebuilding Git from first principles was an unparalleled educational experience. It transformed abstract concepts into concrete understanding: why commits are efficient (simple pointers to trees), how deduplication functions (content-addressable storage), and the speed of branching (just shifting a pointer). If you genuinely seek to master a tool, the most effective path is often to build it yourself.

This project was built entirely in Go, without the use of any existing Git libraries. All hashing, compression, and object storage mechanisms are custom implementations, a testament to the power of understanding fundamentals.

Discover more of my work at devuthman.vercel.app or explore the Go-Git repository on GitHub.