Go by Example

TCP Server

Accept TCP connections, handle clients concurrently, and set deadlines to bound stalled reads.

net.Listen creates a TCP listener, and Accept returns a connection for each client. A connection is a byte stream, so a protocol must define how messages are separated.

This line-based echo server handles each connection in its own goroutine and sets a deadline so a stalled client cannot wait forever.

package main
 
import (
    "bufio"
    "fmt"
    "net"
    "os"
    "time"
)
 
func main() {
    listener, err := net.Listen("tcp", "127.0.0.1:9000")
    if err != nil {
        panic(err)
    }
    defer listener.Close()
 
    for {
        conn, err := listener.Accept()
        if err != nil {
            fmt.Fprintln(os.Stderr, err)
            return
        }
        go func() {
            defer conn.Close()
            if err := conn.SetDeadline(time.Now().Add(30 * time.Second)); err != nil {
                fmt.Fprintln(os.Stderr, err)
                return
            }
            scanner := bufio.NewScanner(conn)
            for scanner.Scan() {
                conn.SetDeadline(time.Now().Add(30 * time.Second))
                if _, err := fmt.Fprintln(conn, scanner.Text()); err != nil {
                    fmt.Fprintln(os.Stderr, err)
                    return
                }
            }
            if err := scanner.Err(); err != nil {
                fmt.Fprintln(os.Stderr, err)
            }
        }()
    }
}

In production

Limit concurrent connections when clients are untrusted, and check read and write errors in real handlers. A deadline is an absolute time; refresh it for long-lived active connections. Scanner's default token buffer is 64 KiB, including any line delimiter, so choose another framing approach for larger messages.

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