Golang implements daemon process

With the rapid development of the Internet, applications running on the server are becoming more and more important. However, servers often need to run certain processes for long periods of time, which requires a special process to monitor them and ensure that they are always running. This special process is called a daemon or daemon. In this article, we will discuss how to implement a daemon using Golang.

What is a daemon?

A daemon process is a process that runs in the background and remains running. Typically, a daemon is a service-like program that does not interact with the user but runs when the machine starts and is not stopped until the machine is shut down. Daemons typically monitor specific services or processes and automatically restart them if they stop working.

Why do we need a daemon?

The role of the daemon is to keep the application running, even if an error in the application itself causes it to stop running. When we need to run certain services on the server all the time, especially some services that need to run for a long time, these services may encounter various errors during the running process, and these errors may cause the service to stop running. At this time, we need a mechanism to automatically monitor the running status of the service and automatically restart it when the service stops running.

How to implement the daemon process?

In the Linux environment, we can use systemd or init.d to implement the daemon process. But in this article, we will discuss how to implement a daemon process yourself using Golang.

First, we need to set up the daemon process in the main() function of the program. The following is a sample code to set up the daemon:

package main

import (
    "fmt"
    "os"
    "os/exec"
    "syscall"
)

func main() {
    cmd := exec.Command(os.Args[0], os.Args[1:]...)
    cmd.SysProcAttr = &syscall.SysProcAttr{Setsid: true}

    err := cmd.Start()
    if err != nil {
        fmt.Println("Start error:", err)
        os.Exit(1)
    }

    fmt.Println("Process pid:", cmd.Process.Pid)
    os.Exit(0)
}

Before setting up the daemon, we need to call os.Args to get all the parameters of the application and use os/ One of the commands in the exec module to run it. When running the command, we need to set the Setsid value to true to ensure a new session is created. This makes the application the first process in the new process group and new session.

Once we set up the daemon, we need to define how to handle signals from the operating system. The following is a sample code that captures operating system signals:

package main

import (
    "fmt"
    "os"
    "os/signal"
    "syscall"
)

func main() {
    // daemon code here...

    signalCh := make(chan os.Signal, 1)
    signal.Notify(signalCh, syscall.SIGTERM)
    signal.Notify(signalCh, syscall.SIGQUIT)
    signal.Notify(signalCh, syscall.SIGINT)

    select {
    case signal := <-signalCh:
        fmt.Printf("Received signal: %s
", signal)
    }
}

In this example, we use signals to notify us when to shut down the daemon process. We use the make() function of the OS package to create a signalCh channel to receive the signal, and then use signal.Notify() to register three signals to the channel, respectively are SIGTERM, SIGQUIT and SIGINT. These signals are the ones we need to pay attention to, and when the daemon receives them, it terminates the current process.

To ensure that the daemon meets our expectations, we need a few more steps. First, the daemon needs to change the working directory and flush the file descriptors. The following is a sample code:

package main

import (
    "fmt"
    "os"
    "os/exec"
    "syscall"
)

func main() {
    if os.Getppid() != 1 {
        cmd := exec.Command(os.Args[0], os.Args[1:]...)
        cmd.SysProcAttr = &syscall.SysProcAttr{Setsid: true}

        err := cmd.Start()
        if err != nil {
            fmt.Println("Start error:", err)
            os.Exit(1)
        }

        fmt.Println("Process pid:", cmd.Process.Pid)
        os.Exit(0)
    }

    os.Chdir("/")
    syscall.Umask(0)

    file, err := os.OpenFile("/dev/null", os.O_RDWR, 0)
    if err != nil {
        fmt.Println("file open error:", err)
        os.Exit(1)
    }

    syscall.Dup2(int(file.Fd()), int(os.Stdin.Fd()))
    syscall.Dup2(int(file.Fd()), int(os.Stdout.Fd()))
    syscall.Dup2(int(file.Fd()), int(os.Stderr.Fd()))
    file.Close()

    // daemon code here...
}

In this sample code, we first check whether the current daemon process is running under the init process. If not, create a new daemon. If so, the daemon becomes the first process of the newly created session. Subsequently, the daemon needs to change the working directory and refresh the file descriptors. By using the os.Chdir() function we change the working directory to the root directory, use syscall.Umask() to set the default file permissions, and then use os.OpenFile( ) function opens the /dev/null file as new standard input, output, and error output and copies all file descriptors to the /dev/null file using the syscall.Dup2() function.

Finally, after flushing the file descriptor, we place all daemon related code in the location below.

// daemon code here...

Advantages of Golang daemon

• Golang is very simple to manage the daemon and is suitable for beginners;
• Golang has good concurrency and memory management , which can make the daemon process run more stably under high load;
• Golang can run across platforms, so a daemon process can be developed once and run on multiple platforms.

Summary

In this article, we have learned how to create a daemon process using Golang. By using system calls and signal handling, the Golang daemon can easily ensure that the application remains running and can easily monitor and start some long-running services. In addition, daemons written in Golang have the advantages of cross-platform, powerful memory management, and high concurrency performance, and can be used to develop various applications.

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