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Bitcask - Log-structured storage engine designed for fast key-value data. Implemented in CPP

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# Bitcask

A C++17 implementation of the [Bitcask](https://riak.com/assets/bitcask-intro.pdf) log-structured key-value storage engine, built as a shared library.

Bitcask provides low-latency reads and writes by keeping all keys in memory (via a hash index) and appending every write to an append-only log file. When a log file exceeds a configurable size threshold it is rotated into an immutable "stable" file and a new active file is created.

## Features

- **O(1) reads** - key lookup goes through an in-memory index directly to the on-disk value offset
- **Append-only writes** - no in-place updates; every write is a sequential append
- **Synchronous and asynchronous Put** - `Put` blocks until committed; `PutAsync` returns a `std::future<void>`
- **Automatic file rotation** - active file rotates when it exceeds `Setting::maxFileSize`
- **Crash recovery** - on restart, all existing `.db` files are scanned and the in-memory index is rebuilt
- **CRC32C integrity** - every record is checksummed on write and verified on read
- **Thread-safe** - concurrent reads and writes via `std::shared_mutex` and `libcuckoo::cuckoohash_map`

## Architecture

```
┌──────────────────────────────────────────────────────────┐
│  Public API  (include/bitcask/)                          │
│  Bitcask ──► BitcaskImpl  (pimpl)                        │
└────────────────────────┬─────────────────────────────────┘
                         │
           ┌─────────────▼─────────────┐
           │  In-memory index          │
           │  RecordMap  key → Hint    │
           │  ActiveMap  key → Value   │
           └─────────────┬─────────────┘
                         │
           ┌─────────────▼─────────────┐
           │  Storage layer            │
           │  ActiveFile  (write/read) │
           │  StableFile  (read-only)  │
           └───────────────────────────┘
```

**Write path:** `Put` enqueues a write → `CommitWorker` (background thread) drains the queue → appends a record to the active file → updates `RecordMap` and `ActiveMap` → rotates active file if it exceeds `maxFileSize`.

**Read path:** look up key in `RecordMap` to get a `Hint` (file id, offset, size) → if the hint points to the active file, serve from `ActiveMap` (in-memory) → otherwise seek into the corresponding `StableFile` and read the bytes at the stored offset.

## Requirements

| Tool | Version |
|------|---------|
| C++ compiler | C++17 or later |
| CMake | 3.16 or later |

Dependencies are fetched automatically via CMake `FetchContent`:

| Library | Purpose |
|---------|---------|
| [google/crc32c](https://github.com/google/crc32c) | CRC32C checksum |
| [google/googletest](https://github.com/google/googletest) v1.17.0 | Unit testing |
| [efficient/libcuckoo](https://github.com/efficient/libcuckoo) | Concurrent hash maps |

## Build

```bash
# Configure (fetches dependencies automatically)
cmake -B build

# Build
cmake --build build

# Or build with parallel jobs
cmake --build build -- -j$(nproc)
```

The output shared library is `build/libbitcask_cpp.dylib` (macOS) or `build/libbitcask_cpp.so` (Linux).

## Testing

```bash
# Run all tests
cd build && ctest

# Run a specific test suite
./build/test/record
./build/test/active_map
./build/test/record_map
./build/test/init_from_existed_db
./build/test/bitcask
./build/test/bitcask_benchmark

# Run a single test case
./build/test/bitcask --gtest_filter="BitCaskTest.OverwriteKeyReturnsLatestValue"
```

| Suite | What it covers |
|-------|---------------|
| `record` | On-disk record serialisation and CRC32C verification |
| `active_map` | In-memory key-value cache including concurrent access |
| `record_map` | In-memory key→Hint index including concurrent access |
| `init_from_existed_db` | DB recovery and data persistence across restarts |
| `bitcask` | Public API (Put, PutAsync, Get, Delete) and file rotation |
| `bitcask_benchmark` | Throughput with 100 000 keys and file rotation enabled |

## Usage

```cpp
#include <bitcask/Bitcask.hpp>
#include <bitcask/Logger.hpp>
#include <iostream>

int main() {
    // Optional: enable logging
    bitcask::logger::init(
        bitcask::logger::LOG_LEVEL_FROM_INFO,
        [](const std::string &msg) { std::cout << msg << "\n"; });

// Open (or create) a database
    bitcask::Bitcask *db = bitcask::Bitcask::Create("/tmp/mydb/");

// Synchronous write - blocks until committed to disk
    db->Put("hello", "world");

// Asynchronous write - returns immediately with a future
    auto f = db->PutAsync("key2", "value2");
    f.get(); // wait for commit if needed

// Read
    auto val = db->Get("hello");
    if (val) {
        std::cout << *val << "\n"; // "world"
    }

delete db;
}
```

### Custom settings

```cpp
bitcask::Setting setting;
setting.dbPath = "/tmp/mydb/"; 
setting.maxFileSize = 64 * 1024 * 1024; // rotate every 64 MB (default: 1 GB)

bitcask::Bitcask *db = bitcask::Bitcask::Create(setting);
```

### Linking (CMake consumer)

```cmake
target_link_libraries(my_app PRIVATE bitcask_cpp)
target_include_directories(my_app PRIVATE path/to/bitcask-cpp/include)
```

## Limitations

- **`Delete` is not yet implemented** - `Delete()` returns `false` (TBD).
- **No compaction** - stable files are never merged; disk usage grows monotonically until compaction is implemented.
- All keys must fit in memory (standard Bitcask constraint).

## Benchmarks

Benchmarks are in `benchmark/bitcask_benchmark.cpp` and use [Google Benchmark](https://github.com/google/benchmark) (bundled via crc32c's FetchContent, v1.9.2).

**Machine:** Apple M1 Pro (virtual), 6 cores, 12 MB L2 cache  
**Build:** CMake Release mode, Apple Clang, macOS

```
cmake -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build --target bitcask_benchmark
./build/benchmark/bitcask_benchmark
```

### Results

| Benchmark | Value size | Wall time / op | Throughput |
|-----------|-----------|---------------|------------|
| `Put` (sync) | 16 B | ~36 µs | ~110 K ops/s |
| `Put` (sync) | 128 B | ~39 µs | ~105 K ops/s |
| `Put` (sync) | 1024 B | ~46 µs | ~86 K ops/s |
| `PutAsync` | 16 B | ~12 µs | ~578 K ops/s |
| `PutAsync` | 128 B | ~16 µs | ~497 K ops/s |
| `PutAsync` | 1024 B | ~13 µs | ~496 K ops/s |
| `Get` - active file (hot cache) | 16 B | ~2.2 µs | ~482 K ops/s |
| `Get` - active file (hot cache) | 128 B | ~2.4 µs | ~458 K ops/s |
| `Get` - active file (hot cache) | 1024 B | ~2.4 µs | ~449 K ops/s |
| `Get` - stable file (disk) | 16 B | ~2.9 µs | ~363 K ops/s |
| `Get` - stable file (disk) | 128 B | ~3.9 µs | ~269 K ops/s |
| `Get` - stable file (disk) | 1024 B | ~3.9 µs | ~270 K ops/s |
| Mixed 80 % reads / 20 % writes | 64 B | ~11 µs | ~238 K ops/s |

**Key observations:**
- `PutAsync` is **4–5× faster** than synchronous `Put` - the background commit worker amortises fsync overhead across batches.
- `Get` from the active file (served from `libcuckoo` in-memory map) takes ~2 µs regardless of value size, demonstrating O(1) hot-cache reads.
- Stable-file reads add only ~1–2 µs over the hot-cache path, thanks to precomputed `Hint` offsets avoiding index scans.

## License

This project is open source. See the repository for licence details.

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