Append-Only Key-Value Store 🦀

A robust, single-node key-value store built in Rust. It utilizes an append-only log for data persistence and an in-memory hash map for fast retrieval. This project serves as an educational implementation of storage engine basics, demonstrating crash safety and file handling.

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🚀 Features

• Persistent Storage: All data is saved to disk in data/kv.log.
• Fast Reads: An in-memory HashMap index maps keys directly to their file offsets, ensuring O(1) lookup time (excluding disk I/O).
• Crash Recovery: On startup, the store replays the log to rebuild the index, ensuring data survives restarts.
• CLI Interface: Simple command-line tools to interact with the database.
• Append-Only Design: Writes are always appended to the end of the file, optimizing for write performance and simplifying data integrity.

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🛠️ Installation & Usage

Prerequisites

• Rust & Cargo installed.

Build

To compile the project:

cargo build --release

Usage

Run the binary via cargo run (for development) or the built executable.

1. Set a Key-Value Pair

Stores a value associated with a key. If the key exists, it updates the index to point to the new entry (old entries remain in the log but are ignored).

cargo run -- set <KEY> <VALUE>
# Example
cargo run -- set user:101 "Alice Wonderland"

2. Get a Value

Retrieves the value for a given key.

cargo run -- get <KEY>
# Example
cargo run -- get user:101
# Output: Alice Wonderland

3. Remove a Key

Removes a key from the store. This appends a "tombstone" record to the log, effectively hiding the key from future lookups.

cargo run -- rm <KEY>
# Example
cargo run -- rm user:101

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🏗️ Architecture & Implementation

Project Structure

• src/main.rs: The entry point. It uses clap to parse command-line arguments and dispatches commands to the KvStore.
• src/store.rs: The core storage engine.
  • Manages the KvStore struct.
  • Handles file I/O (reading/writing to data/kv.log).
  • Maintains the in-memory index.
• src/utils.rs: A module for utility functions and future extensions (currently a placeholder).

Data Format (data/kv.log)

The storage file is a continuous sequence of binary records. Each record has the following format:

Field	Type	Description
Key Length	u32	Length of the key in bytes (Little Endian).
Value Length	u32	Length of the value in bytes (Little Endian).
Key	[u8]	The key data (UTF-8 string).
Value	[u8]	The value data (UTF-8 string).

How It Works

1. set(key, value):
   • Seeks to the end of the file.
   • Writes the header (key_len, val_len) and then the bodies (key, value).
   • Updates the in-memory index with the key and the start offset of this new record.
2. get(key):
   • Looks up the key in the in-memory index to find the file offset.
   • Seeks to that offset in the file.
   • Reads the lengths and then reads the value directly (skipping the key for performance).
3. load_index() (Startup):
   • Reads the entire log file from beginning to end.
   • Parses every record's key and length.
   • Populates the index map.
   • This ensures the in-memory state matches the persistent log.

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📦 Dependencies

The project relies on a few key crates to ensure robustness and code clarity:

• anyhow: Provides flexible and easy-to-use error handling. It allows us to attach context to errors (e.g., "Failed to open log file") for better debugging.
• clap: A powerful command-line argument parser. We use it with the derive feature to define our CLI structure (Set, Get) directly from Rust structs and enums.
• byteorder: Helpers for reading/writing primitive numbers (like u32) in a specific endianness (Little Endian), ensuring portability of the data file across different systems.

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📚 Why Use This?

• Educational: Perfect for understanding how databases like Bitcask or log-structured merge-trees (LSM) start.
• Simple: Minimal codebase makes it easy to read and modify.
• Safe: Rust's type system and explicit error handling prevent common crashes and data races.