Learning Journey: Implementing an Event Loop in Go for a Redis Server

Introduction

When I set out to understand event loops, I faced three challenges: I didn't know how they worked conceptually, how to implement one in Go, or how to integrate one into a Redis implementation. This journey of building an event loop from scratch helped me tackle all three challenges.

Understanding Event Loops

An event loop is a server architecture pattern that continuously processes events from a queue. Its key characteristics include:

• Single-threaded execution core
• Sequential event processing
• Event queueing mechanism
• Continuous monitoring for new events
• Non-blocking I/O operations

In our Redis implementation context, an "event" represents a Redis command received via TCP along with its initiating client connection.

Server Architecture

Our server implementation requires several key components:

type TcpServer struct {
    listener net.Listener
    clients  map[net.Conn]bool
    mu       sync.Mutex
    eventQ   chan Event
}

Each component serves a specific purpose:

• listener: Handles incoming TCP connections
• clients: Tracks active client connections for connection management
• mu: Ensures thread-safe operations on the client map
• eventQ: Serves as our event queue for processing commands

Event Structure

Events in our system are structured to carry all necessary information:

type Event struct {
    conn    net.Conn
    command Command
    result  chan error
}

type Command struct {
    CMD  string
    ARGS []string
}

The result channel deserves special attention. It's specifically designed for error handling:

• Enables asynchronous communication between goroutines
• Allows workers to report errors back to the main handler
• Facilitates proper error logging and handling while maintaining non-blocking operation

Implementation Details

Server Initialization

Our TCP server implementation follows standard Go networking patterns:

func NewTcpServer(address string) (*TcpServer, error) {
    listener, err := net.Listen("tcp", address)
    if err != nil {
        return nil, err
    }

    return &TcpServer{
        listener: listener,
        clients:  make(map[net.Conn]bool),
        eventQ:   make(chan Event),
    }, nil
}

Event Loop Implementation

The event loop's core functionality involves:

1. Continuously monitoring the event queue
2. Processing events sequentially
3. Handling command execution
4. Managing error responses

func (s *TcpServer) eventLoop() {
    for event := range s.eventQ {
        if err := s.processCommand(event); err != nil {
            event.result <- err
        }
    }
}

Connection Handling

The connection handler manages client connections and command processing:

func (s *TcpServer) HandleConnection(conn net.Conn) {
    s.mu.Lock()
    s.clients[conn] = true
    s.mu.Unlock()

    for {
        cmd, args := parseCommand(conn)
        resultChan := make(chan error)

        s.eventQ <- Event{
            conn:    conn,
            command: Command{CMD: cmd, ARGS: args},
            result:  resultChan,
        }

        if err := <-resultChan; err != nil {
            handleError(err)
        }
    }
}

Concurrent Operation

To ensure non-blocking operation, we utilize Go's concurrency features:

func (s *TcpServer) Start() error {
    go s.eventLoop()

    for {
        conn, err := s.listener.Accept()
        if err != nil {
            return err
        }
        go s.HandleConnection(conn)
    }
}

System Flow

The complete flow of our Redis server implementation can be visualized as follows:

1. Server starts and initializes the event loop
2. Client connects via TCP
3. Connection handler creates new event
4. Event is queued in the event loop
5. Event loop processes commands sequentially
6. Results are returned to clients
7. Error handling occurs via dedicated channels

Conclusion

This implementation demonstrates how to build a functional event loop system that maintains Redis's core principles of sequential command processing while leveraging Go's concurrency features for optimal performance. The combination of event loops and goroutines allows us to handle multiple client connections concurrently while ensuring commands are processed in a controlled, sequential manner.

The code structure provides a solid foundation for adding more Redis features while maintaining clean separation of concerns and thread safety. It also serves as a practical example of how theoretical concepts like event loops can be implemented in a real-world application.