weaseldb/design.md

# WeaselDB Development Guide

## Project Summary

WeaselDB is a high-performance write-side database component designed for systems where reading and writing are decoupled. The system focuses exclusively on handling transactional commits with optimistic concurrency control, while readers are expected to maintain their own queryable representations by subscribing to change streams.

## Quick Start

### Build System
- Use CMake with C++20 standard
- Primary build commands:
  - `mkdir -p build && cd build`
  - `cmake .. -DCMAKE_BUILD_TYPE=Release`
  - `ninja` or `make -j$(nproc)`

### Testing and Development Workflow
- **Run all tests**: `ninja test` or `ctest`
- **Individual test targets**:
  - `./test_arena_allocator` - Arena allocator unit tests
  - `./test_commit_request` - JSON parsing and validation tests
- **Benchmarking**:
  - `./bench_arena_allocator` - Memory allocation performance
  - `./bench_commit_request` - JSON parsing performance
  - `./bench_parser_comparison` - Compare against nlohmann::json and RapidJSON
- **Debug tools**: `./debug_arena` - Analyze arena allocator behavior

### Code Style and Conventions
- **C++ Style**: Modern C++20 with RAII and move semantics
- **Memory Management**: Prefer arena allocation over standard allocators
- **String Handling**: Use `std::string_view` for zero-copy operations
- **Error Handling**: Return error codes or use exceptions appropriately
- **Naming**: snake_case for variables/functions, PascalCase for classes
- **Performance**: Always consider allocation patterns and cache locality

### Dependencies and External Libraries
- **weaseljson**: Must be installed system-wide (high-performance JSON parser)
- **simdutf**: Fetched automatically (SIMD base64 encoding/decoding)
- **toml11**: Fetched automatically (TOML configuration parsing)
- **doctest**: Fetched automatically (testing framework)
- **nanobench**: Fetched automatically (benchmarking library)
- **gperf**: System requirement for perfect hash generation

## Architecture Overview

### Core Components

#### 1. **Arena Allocator** (`src/arena_allocator.hpp`)
- **Ultra-fast memory allocator** (~1ns per allocation vs ~20-270ns for malloc)
- **Lazy initialization** with geometric block growth (doubling strategy)
- **Intrusive linked list design** for minimal memory overhead
- **Memory-efficient reset** that keeps the first block and frees others
- **STL-compatible interface** via `ArenaStlAllocator`

Key features:
- O(1) amortized allocation
- Proper alignment handling for all types
- Move semantics for efficient transfers
- Requires trivially destructible types only

#### 2. **Commit Request Data Model** (`src/commit_request.hpp`)
- **Format-agnostic data structure** for representing transactional commits
- **Arena-backed string storage** with efficient memory management
- **Move-only semantics** for optimal performance
- **Builder pattern** for constructing commit requests
- **Zero-copy string views** pointing to arena-allocated memory

#### 3. **JSON Commit Request Parser** (`src/json_commit_request_parser.{hpp,cpp}`)
- **High-performance JSON parser** using `weaseljson` library
- **Streaming parser support** for incremental parsing of network data
- **gperf-optimized token recognition** for fast JSON key parsing
- **Base64 decoding** using SIMD-accelerated simdutf
- **Comprehensive validation** of transaction structure

Parser capabilities:
- One-shot parsing for complete JSON
- Streaming parsing for network protocols
- Parse state management with error recovery
- Memory-efficient string views backed by arena storage
- Perfect hash table lookup for JSON keys using gperf

#### 4. **Parser Interface** (`src/commit_request_parser.hpp`)
- **Abstract base class** for commit request parsers
- **Format-agnostic parsing interface** supporting multiple serialization formats
- **Streaming and one-shot parsing modes**
- **Standardized error handling** across parser implementations

#### 5. **Configuration System** (`src/config.{hpp,cpp}`)
- **TOML-based configuration** using `toml11` library
- **Structured configuration** with server, commit, and subscription sections
- **Default fallback values** for all configuration options
- **Type-safe parsing** with validation and bounds checking
- **Comprehensive validation** with meaningful error messages

See `config.md` for complete configuration documentation.

#### 6. **JSON Token Optimization** (`src/json_tokens.gperf`, `src/json_token_enum.hpp`)
- **Perfect hash table** generated by gperf for O(1) JSON key lookup
- **Compile-time token enumeration** for type-safe key identification
- **Minimal perfect hash** reduces memory overhead and improves cache locality
- **Build-time code generation** ensures optimal performance

#### 7. **Server** (`src/server.{hpp,cpp}`)
- **High-performance multi-threaded networking** using epoll with thread pools
- **Factory pattern construction** via `Server::create()` ensures proper shared_ptr semantics
- **Safe shutdown mechanism** with async-signal-safe shutdown() method
- **Connection ownership management** with automatic cleanup on server destruction
- **Pluggable protocol handlers** via ConnectionHandler interface

Key features:
- Multi-threaded architecture: separate accept and network thread pools
- EPOLL_EXCLUSIVE load balancing across accept threads
- Connection lifecycle safety with weak_ptr references
- Graceful shutdown with proper resource cleanup
- RAII-based connection management with unique_ptr ownership

#### 8. **Connection** (`src/connection.{hpp,cpp}`)
- **Efficient per-connection state management** with arena-based memory allocation
- **Safe ownership transfer** between server threads and protocol handlers
- **Automatic cleanup** on connection closure or server shutdown
- **Handler interface isolation** - only exposes necessary methods to protocol handlers

Key features:
- Arena allocator per connection for efficient memory management
- **Request/Response arena lifecycle**: Arena resets after each complete request/response cycle
- Weak reference to server for safe cleanup after server destruction
- Private networking details accessible only to Server via friend relationship
- Public handler interface: appendMessage(), closeAfterSend(), getArena(), getId()
- Thread-safe ownership transfer with Server::releaseBackToServer()

#### 9. **ConnectionHandler Interface** (`src/connection_handler.hpp`)
- **Abstract protocol interface** decoupling networking from application logic
- **Ownership transfer support** allowing handlers to take connections for async processing
- **Streaming data processing** with partial message handling
- **Connection lifecycle hooks** for initialization and cleanup

Key features:
- process_data() with unique_ptr<Connection>& for ownership transfer
- on_connection_established/closed() hooks for protocol state management
- Zero-copy data processing with arena allocator integration
- Thread-safe ownership transfer via Server::releaseBackToServer()

### Data Model

#### Transaction Structure
```
CommitRequest {
  - request_id: Optional unique identifier
  - leader_id: Expected leader for consistency
  - read_version: Snapshot version for preconditions
  - preconditions[]: Optimistic concurrency checks
    - point_read: Single key existence/content validation
    - range_read: Range-based consistency validation
  - operations[]: Ordered mutation operations
    - write: Set key-value pair
    - delete: Remove single key
    - range_delete: Remove key range
}
```

#### Memory Management
- **Arena-based allocation** ensures efficient bulk memory management per connection
- **String views** eliminate unnecessary copying of JSON data
- **Zero-copy design** for binary data handling
- **RAII-based connection lifecycle** with automatic cleanup on destruction
- **Safe ownership transfer** between server threads and protocol handlers
- **Weak reference safety** prevents crashes when connections outlive server

Connection Ownership Model:
1. **Creation**: Accept threads create connections, transfer to epoll as raw pointers
2. **Processing**: Network threads claim ownership by wrapping in unique_ptr
3. **Handler Transfer**: Handlers can take ownership for async processing via unique_ptr.release()
4. **Return Path**: Handlers use Server::releaseBackToServer() to return connections
5. **Safety**: All transfers use weak_ptr to server for safe cleanup
6. **Cleanup**: RAII ensures proper resource cleanup in all scenarios

Arena Memory Lifecycle:
1. **Request Processing**: Handler uses `conn->getArena()` to allocate memory for parsing request data
2. **Response Generation**: Handler uses arena for temporary response construction (headers, JSON, etc.)
3. **Response Queuing**: Handler calls `conn->appendMessage()` which copies data to arena-backed message queue
4. **Response Writing**: Server writes all queued messages to socket via `writeBytes()`
5. **Arena Reset**: After successful write completion, arena resets to reclaim all memory from the request/response cycle

This design assumes request/response pairs (HTTP-like protocols) but works for any protocol where there's a clear completion point for memory reclamation.

### API Design

The system implements a RESTful API with three core endpoints:

1. **GET /v1/version**: Retrieve current committed version and leader
2. **POST /v1/commit**: Submit transactional operations
3. **GET /v1/subscribe**: Stream committed transactions (implied)
4. **GET /v1/status**: Check commit status by request_id (implied)

### Performance Characteristics

#### Memory Allocation
- **~1ns allocation time** vs standard allocators
- **Bulk deallocation** eliminates individual free() calls
- **Optimized geometric growth** uses current block size for doubling strategy
- **Alignment-aware** allocation prevents performance penalties

#### JSON Parsing
- **Streaming parser** handles large payloads efficiently
- **Incremental processing** suitable for network protocols
- **Arena storage** eliminates string allocation overhead
- **SIMD-accelerated base64 decoding** using simdutf for maximum performance
- **Perfect hash table** provides O(1) JSON key lookup via gperf
- **Zero hash collisions** for known JSON tokens eliminates branching

### Design Principles

1. **Performance-first**: Every component optimized for high throughput
2. **Memory efficiency**: Arena allocation eliminates fragmentation
3. **Zero-copy**: Minimize data copying throughout pipeline
4. **Streaming-ready**: Support incremental processing
5. **Type safety**: Compile-time validation where possible
6. **Resource management**: RAII and move semantics throughout

### Testing & Benchmarking

The project includes comprehensive testing infrastructure:
- **Unit tests** using doctest framework
- **Performance benchmarks** using nanobench
- **Memory allocation benchmarks** for arena performance
- **JSON parsing validation** for correctness

Build targets:
- `test_arena_allocator`: Arena allocator functionality tests
- `test_commit_request`: JSON parsing and validation tests
- Main server executable (compiled from `src/main.cpp`)
- `bench_arena_allocator`: Arena allocator performance benchmarks
- `bench_commit_request`: JSON parsing performance benchmarks
- `bench_parser_comparison`: Comparison benchmarks vs nlohmann::json and RapidJSON
- `debug_arena`: Debug tool for arena allocator analysis

### Dependencies

- **weaseljson**: High-performance streaming JSON parser
- **simdutf**: SIMD-accelerated UTF-8 validation and base64 encoding/decoding
- **toml11**: TOML configuration file parsing
- **doctest**: Lightweight testing framework
- **nanobench**: Micro-benchmarking library
- **gperf**: Perfect hash function generator for JSON token optimization
- **nlohmann::json**: Reference JSON parser for benchmarking comparisons
- **RapidJSON**: High-performance JSON parser for benchmarking comparisons

### Future Considerations

This write-side component is designed to integrate with:
- **Leader election** systems for distributed consensus
- **Replication** mechanisms for fault tolerance
- **Read-side systems** that consume the transaction stream
- **Monitoring** systems for operational visibility

The modular design allows each component to be optimized independently while maintaining clear interfaces for system integration.

## Development Guidelines

### Important Implementation Details
- **Server Creation**: Always use `Server::create()` factory method - direct construction is impossible
- **Connection Ownership**: Use unique_ptr semantics for safe ownership transfer between components
- **Arena Allocator Pattern**: Always use `ArenaAllocator` for temporary allocations within request processing
- **String View Usage**: Prefer `std::string_view` over `std::string` when pointing to arena-allocated memory
- **Ownership Transfer**: Use `Server::releaseBackToServer()` for returning connections to server from handlers
- **JSON Token Lookup**: Use the gperf-generated perfect hash table in `json_tokens.hpp` for O(1) key recognition
- **Base64 Handling**: Always use simdutf for base64 encoding/decoding for performance
- **Error Propagation**: Use structured error types that can be efficiently returned up the call stack
- **Thread Safety**: Connection ownership transfers are designed to be thread-safe with proper RAII cleanup

### File Organization
- **Core Headers**: `src/` contains all primary implementation files
- **Tests**: `tests/` contains doctest-based unit tests
- **Benchmarks**: `benchmarks/` contains nanobench performance tests
- **Tools**: `tools/` contains debugging and analysis utilities
- **Build-Generated**: `build/` contains CMake-generated files including `json_tokens.cpp`

### Adding New Protocol Handlers
- Inherit from `ConnectionHandler` in `src/connection_handler.hpp`
- Implement `process_data()` with proper ownership semantics
- Use connection's arena allocator for temporary allocations: `conn->getArena()`
- Handle partial messages and streaming protocols appropriately
- Use `Server::releaseBackToServer()` if taking ownership for async processing
- Add corresponding test cases and integration tests
- Consider performance implications of ownership transfers

### Adding New Parsers
- Inherit from `CommitRequestParser` in `src/commit_request_parser.hpp`
- Implement both streaming and one-shot parsing modes
- Use arena allocation for all temporary string storage
- Add corresponding test cases in `tests/`
- Add benchmark comparisons in `benchmarks/`

### Performance Considerations
- **Memory**: Arena allocation eliminates fragmentation - use it for all request-scoped data
- **CPU**: Perfect hashing and SIMD operations are critical paths - avoid alternatives
- **I/O**: Streaming parser design supports incremental network data processing
- **Cache**: String views avoid copying, keeping data cache-friendly

### Configuration Management
- All configuration is TOML-based using `config.toml`
- Comprehensive documentation available in `config.md`
- Type-safe parsing with validation and bounds checking
- Always validate configuration values and provide meaningful errors

### Testing Strategy
- **Unit tests** validate individual component correctness
- **Benchmarks** ensure performance characteristics are maintained
- **Debug tools** help analyze memory usage patterns
- Always run both tests and benchmarks before submitting changes

### Build System Details
- CMake generates gperf hash tables at build time
- Ninja is preferred over make for faster incremental builds
- Release builds include debug symbols for profiling
- All external dependencies except weaseljson are auto-fetched

## Common Patterns

### Server Creation Pattern
```cpp
// Server must be created via factory method
auto server = Server::create(config, handler);

// Never create on stack or with make_shared (won't compile):
// Server server(config, handler);  // Compiler error - constructor private
// auto server = std::make_shared<Server>(config, handler);  // Compiler error
```

### ConnectionHandler Implementation Patterns

#### Simple Synchronous Handler
```cpp
class HttpHandler : public ConnectionHandler {
public:
  void process_data(std::string_view data, std::unique_ptr<Connection>& conn_ptr) override {
    // Parse HTTP request using connection's arena
    ArenaAllocator& arena = conn_ptr->getArena();

    // Generate response
    conn_ptr->appendMessage("HTTP/1.1 200 OK\r\n\r\nHello World");

    // Server retains ownership
  }
};
```

#### Async Handler with Ownership Transfer
```cpp
class AsyncHandler : public ConnectionHandler {
public:
  void process_data(std::string_view data, std::unique_ptr<Connection>& conn_ptr) override {
    // Take ownership for async processing
    auto connection = std::move(conn_ptr); // conn_ptr is now null

    work_queue.push([connection = std::move(connection)](std::string_view data) mutable {
      // Process asynchronously
      connection->appendMessage("Async response");

      // Return ownership to server when done
      Server::releaseBackToServer(std::move(connection));
    });
  }
};
```

### Arena-Based String Handling
```cpp
// Preferred: Zero-copy string view with arena allocation
std::string_view process_json_key(const char* data, ArenaAllocator& arena);

// Avoid: Unnecessary string copies
std::string process_json_key(const char* data);
```

### Error Handling Pattern
```cpp
enum class ParseResult { Success, InvalidJson, MissingField };
ParseResult parse_commit_request(const char* json, CommitRequest& out);
```

### Builder Pattern Usage
```cpp
CommitRequest request = CommitRequestBuilder(arena)
    .request_id("example-id")
    .leader_id("leader-123")
    .read_version(42)
    .build();
```

### Safe Connection Ownership Transfer
```cpp
// In handler - take ownership for background processing
Connection* raw_conn = conn_ptr.release();

// Process on worker thread
background_processor.submit([raw_conn]() {
  // Do work...
  raw_conn->appendMessage("Background result");

  // Return to server safely (handles server destruction)
  Server::releaseBackToServer(std::unique_ptr<Connection>(raw_conn));
});
```