weaselab/weaseldb
2
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

Update documentation with new networking model

Browse Source
This commit is contained in:
Andrew Noyes
2025-09-14 19:03:56 -04:00
parent 0389fd2c9f
commit f39149d516
3 changed files with 65 additions and 66 deletions
Download Patch File Download Diff File Expand all files Collapse all files

118
design.md
View File

@@ -259,37 +259,37 @@ CommitRequest {
> **Note**: Call `conn->reset()` periodically to reclaim arena memory. Best practice is after all outgoing bytes have been written.
#### Threading Model and EPOLLONESHOT
#### Threading Model and Server-Owned Connections
**EPOLLONESHOT Design Rationale:**
WeaselDB uses `EPOLLONESHOT` for all connection file descriptors to enable safe multi-threaded ownership transfer without complex synchronization:
**Server-Owned Connection Design:**
WeaselDB uses a server-owned connection model where the server retains ownership of all connections while providing safe concurrent access to handlers:
**Key Benefits:**
1. **Automatic fd disarming** - When epoll triggers an event, the fd is automatically removed from epoll monitoring
1. **Race-free ownership transfer** - Handlers can safely take connection ownership and move to other threads
1. **Zero-coordination async processing** - No manual synchronization needed between network threads and handler threads
1. **Simplified ownership** - Server always owns connections, eliminating complex ownership transfers
1. **Safe concurrent access** - Connection mutexes synchronize access between I/O threads and handlers
1. **WeakRef pattern** - Handlers use WeakRef<Connection> for safe async processing without ownership
**Threading Flow:**
1. **Event Trigger**: Network thread gets epoll event → connection auto-disarmed via ONESHOT
1. **Safe Transfer**: Handler can take ownership (`std::move(conn_ptr)`) with no epoll interference
1. **Async Processing**: Connection processed on handler thread while epoll cannot trigger spurious events
1. **Return & Re-arm**: Internal server method re-arms fd with `epoll_ctl(EPOLL_CTL_MOD)` via `Server::release_back_to_server()`
1. **Event Trigger**: Network thread gets epoll event and processes data
1. **Handler Invocation**: Handler receives Connection& reference - server retains ownership
1. **Async Processing**: Handler obtains WeakRef<Connection> for safe background processing
1. **Connection Cleanup**: Server manages connection lifecycle including file descriptor operations
**Performance Trade-off:**
**Performance Benefits:**
- **Cost**: One `epoll_ctl(MOD)` syscall per connection return (~100-200ns)
- **Benefit**: Eliminates complex thread synchronization and prevents race conditions
- **Alternative cost**: Manual `EPOLL_CTL_DEL`/`ADD` + locking would be significantly higher
- **Reduced syscalls**: Eliminates epoll_ctl(MOD) calls needed for ownership transfer
- **Simplified synchronization**: Connection mutexes provide clear concurrent access patterns
- **Memory efficiency**: No unique_ptr overhead for ownership management
**Without EPOLLONESHOT risks:**
**Safe Async Processing:**
- Multiple threads processing same fd simultaneously
- Use-after-move when network thread accesses transferred connection
- Complex synchronization between epoll events and ownership transfers
- WeakRef<Connection> prevents use-after-free in background threads
- Connection mutex ensures thread-safe access to connection state
- Server handles all file descriptor management automatically
This design enables the async handler pattern where connections can be safely moved between threads for background processing while maintaining high performance and thread safety.
This design provides high performance concurrent processing while maintaining thread safety through clear ownership boundaries and synchronization primitives.
### API Endpoints
@@ -317,13 +317,13 @@ See [style.md](style.md) for comprehensive C++ coding standards and conventions.
- **Server Creation**: Always use `Server::create()` factory method - direct construction is impossible
- **Connection Creation**: Only the Server can create connections - no public constructor or factory method
- **Connection Ownership**: Use unique_ptr semantics for safe ownership transfer between components
- **Connection Ownership**: Server retains ownership, handlers use Connection& references
- **Arena Allocator Pattern**: Always use `Arena` 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::release_back_to_server()` for returning connections to server from handlers
- **Async Processing**: Use `conn.get_weak_ref()` for safe background processing without ownership
- **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
- **Thread Safety**: Connection ownership transfers are designed to be thread-safe with proper RAII cleanup
- **Thread Safety**: Connection mutexes provide safe concurrent access between threads
### Project Structure
@@ -338,12 +338,12 @@ See [style.md](style.md) for comprehensive C++ coding standards and conventions.
#### Adding New Protocol Handlers
1. Inherit from `ConnectionHandler` in `src/connection_handler.hpp`
1. Implement `on_data_arrived()` with proper ownership semantics
1. Use connection's arena allocator for temporary allocations: `conn->get_arena()`
1. Implement `on_data_arrived()` using Connection& reference parameter
1. Use connection's arena allocator for temporary allocations: `conn.get_arena()`
1. Handle partial messages and streaming protocols appropriately
1. Use `Server::release_back_to_server()` if taking ownership for async processing
1. Use `conn.get_weak_ref()` for safe async processing without ownership transfer
1. Add corresponding test cases and integration tests
1. Consider performance implications of ownership transfers
1. Consider performance implications of concurrent access patterns
#### Adding New Parsers
@@ -395,33 +395,32 @@ Only Server can create connections (using private constructor via friend access)
```cpp
class HttpHandler : public ConnectionHandler {
public:
void on_data_arrived(std::string_view data, std::unique_ptr<Connection>& conn_ptr) override {
void on_data_arrived(std::string_view data, Connection& conn) override {
// Parse HTTP request using connection's arena
Arena& arena = conn_ptr->get_arena();
Arena& arena = conn.get_arena();
// Generate response
conn_ptr->append_message("HTTP/1.1 200 OK\r\n\r\nHello World");
conn.append_message("HTTP/1.1 200 OK\r\n\r\nHello World");
// Server retains ownership
}
};
```
#### Async Handler with Ownership Transfer
#### Async Handler with WeakRef
```cpp
class AsyncHandler : public ConnectionHandler {
public:
void on_data_arrived(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
void on_data_arrived(std::string_view data, Connection& conn) override {
// Get weak reference for async processing
auto weak_conn = conn.get_weak_ref();
work_queue.push([connection = std::move(connection)](std::string_view data) mutable {
// Process asynchronously
connection->append_message("Async response");
// Return ownership to server when done
Server::release_back_to_server(std::move(connection));
work_queue.push([weak_conn, data = std::string(data)]() {
// Process asynchronously - connection may be closed
if (auto conn_ref = weak_conn.lock()) {
conn_ref->append_message("Async response");
}
});
}
};
@@ -442,21 +441,20 @@ public:
delete static_cast<MyProtocolData*>(conn.user_data);
}
void on_data_arrived(std::string_view data,
std::unique_ptr<Connection> &conn_ptr) override {
void on_data_arrived(std::string_view data, Connection& conn) override {
// Process data and maybe store some results in the user_data
auto* proto_data = static_cast<MyProtocolData*>(conn_ptr->user_data);
auto* proto_data = static_cast<MyProtocolData*>(conn.user_data);
proto_data->process(data);
}
void on_batch_complete(std::span<std::unique_ptr<Connection>> batch) override {
void on_batch_complete(std::span<Connection *const> batch) override {
// Process a batch of connections
for (auto& conn_ptr : batch) {
if (conn_ptr) {
auto* proto_data = static_cast<MyProtocolData*>(conn_ptr->user_data);
for (auto* conn : batch) {
if (conn) {
auto* proto_data = static_cast<MyProtocolData*>(conn->user_data);
if (proto_data->is_ready()) {
// This connection is ready for the next stage, move it to the pipeline
pipeline_.push(std::move(conn_ptr));
// This connection is ready for the next stage, get weak ref for pipeline
pipeline_.push(conn->get_weak_ref());
}
}
}
@@ -477,12 +475,12 @@ public:
conn.append_message("y\n");
}
void on_write_progress(std::unique_ptr<Connection> &conn) override {
if (conn->outgoing_bytes_queued() == 0) {
void on_write_progress(Connection &conn) override {
if (conn.outgoing_bytes_queued() == 0) {
// Don't use an unbounded amount of memory
conn->reset();
conn.reset();
// Write "y\n" repeatedly
conn->append_message("y\n");
conn.append_message("y\n");
}
}
};
@@ -500,19 +498,19 @@ std::string_view process_json_key(const char* data, Arena& arena);
std::string process_json_key(const char* data);
```
#### Safe Connection Ownership Transfer
#### Safe Async Connection Processing
```cpp
// In handler - take ownership for background processing
Connection* raw_conn = conn_ptr.release();
// In handler - get weak reference for background processing
auto weak_conn = conn.get_weak_ref();
// Process on worker thread
background_processor.submit([raw_conn]() {
background_processor.submit([weak_conn]() {
// Do work...
raw_conn->append_message("Background result");
// Return to server safely (handles server destruction)
Server::release_back_to_server(std::unique_ptr<Connection>(raw_conn));
if (auto conn_ref = weak_conn.lock()) {
conn_ref->append_message("Background result");
}
// Connection automatically cleaned up by server
});
```

2
threading_performance_report.md
View File

@@ -69,7 +69,7 @@ I/O Threads (8) → HttpHandler::on_batch_complete() → Commit Pipeline
| Stage 3: Release (connection return)
| (optimized futex wake)
| ↓
└─────────────────────── Server::release_back_to_server()
└─────────────────────── Connection returned to server pool
```
## Test Configuration

11
todo.md
View File

@@ -4,10 +4,11 @@
### Core Database Features
- [ ] Design commit pipeline architecture with three-stage processing
- [ ] Stage 1: Version assignment and precondition validation thread
- [ ] Stage 2: Transaction persistence and subscriber streaming thread
- [ ] Stage 3: Connection return to server thread
- [ ] Design commit pipeline architecture with four-stage processing
- [ ] Stage 0: Sequence assignment and request validation
- [ ] Stage 1: Precondition resolution and conflict detection
- [ ] Stage 2: Transaction persistence and subscriber streaming
- [ ] Stage 3: Response generation and connection cleanup
- [ ] Use ThreadPipeline for inter-stage communication
- [ ] Design persistence interface for pluggable storage backends (S3, local disk)
- [ ] Integrate https://git.weaselab.dev/weaselab/conflict-set for optimistic concurrency control
@@ -84,6 +85,6 @@
- [x] Built streaming JSON parser for commit requests with high-performance parsing
- [x] Implemented HTTP server with multi-threaded networking using multiple epoll instances
- [x] Created threading model with pipeline for serial request processing for optimistic concurrency control
- [x] Designed connection ownership transfer system to enable the serial processing model
- [x] Implemented server-owned connection model with WeakRef pattern for safe concurrent access
- [x] Implemented arena-per-connection memory model for clean memory lifetime management
- [x] Built TOML configuration system for server settings