Python server programming: how daemons are implemented

Python is a very popular programming language, especially widely used in network programming. Server-side programming is also one of them. In server-side programming, daemon processes are a common concept. This article introduces how to implement daemon processes in Python server programming.

What is a daemon process

In the process of running the server-side program, in order to ensure the long-term stable operation of the program, we need to convert it into a daemon process. A daemon is a process that runs continuously in the background. This process does not occupy the console or login session, and can be automatically restarted to ensure that the program runs for a long time.

In Linux systems, daemon processes are usually implemented through the fork() system call. The specific implementation process is as follows:

1. Call fork() to create a child process.
2. Call setsid() in the child process to create a new session.
3. Call fork() again in the child process to prevent the child process from becoming the lead process of the terminal session.
4. Close all file descriptors.
5. Redirect standard input, output, and error output to /dev/null.

In Python, we can create a daemon process by overriding the run() function of the parent process. The specific implementation is as follows:

import os
import sys

class Daemon:
    def __init__(self, pidfile, stdin='/dev/null', stdout='/dev/null', stderr='/dev/null'):
        self.stdin = stdin
        self.stdout = stdout
        self.stderr = stderr
        self.pidfile = pidfile

    def daemonize(self):
        if os.path.exists(self.pidfile):
            raise RuntimeError('PID file %s already exists' % self.pidfile)

        # First fork (detaches from parent)
        try:
            if os.fork() > 0:
                sys.exit(0)
        except OSError as e:
            raise RuntimeError('fork #1 failed: %d (%s)' % (e.errno, e.strerror))

        os.chdir('/')
        os.umask(0)
        os.setsid()

        # Second fork (relinquish session leadership)
        try:
            if os.fork() > 0:
                sys.exit(0)
        except OSError as e:
            raise RuntimeError('fork #2 failed: %d (%s)' % (e.errno, e.strerror))

        # Flush I/O buffers
        sys.stdout.flush()
        sys.stderr.flush()

        # Redirect standard file descriptors
        with open(self.stdin, 'rb', 0) as f:
            os.dup2(f.fileno(), sys.stdin.fileno())
        with open(self.stdout, 'ab', 0) as f:
            os.dup2(f.fileno(), sys.stdout.fileno())
        with open(self.stderr, 'ab', 0) as f:
            os.dup2(f.fileno(), sys.stderr.fileno())

        # Write pidfile
        with open(self.pidfile, 'w') as f:
            print(os.getpid(), file=f)

    def start(self):
        self.daemonize()
        self.run()

    def stop(self):
        if os.path.exists(self.pidfile):
            with open(self.pidfile) as f:
                os.kill(int(f.read()), signal.SIGTERM)
            os.remove(self.pidfile)

    def restart(self):
        self.stop()
        self.start()

    def run(self):
        raise NotImplementedError

In the above code, we create a Python daemon process by implementing a class named Daemon. Among them, the self.daemonize() method implements the creation process of the daemon process, the self.start() method starts the daemon process, the self.stop() method stops the daemon process, the self.restart() method restarts the daemon process, and self. The run() method needs to be rewritten according to the actual situation.

You need to pay attention to the following issues when using Python to implement a daemon process:

1. The file descriptors of the parent process and the child process are independent of each other, and there may be a failure to open the file. To avoid this from happening, you should use absolute paths for the files you want to open.
2. The file stream cannot be accessed through sys.stdout and sys.stderr in the daemon process. Therefore, these file streams need to be redirected to files to facilitate viewing logs.
3. Be sure to pay attention to signal processing in the daemon process, because the daemon process runs for a long time and may die due to memory leaks and other problems. It needs to be restarted in time when abnormal conditions occur.
4. Terminal-related operations cannot be performed in the daemon process, such as obtaining the width and height of the terminal.

Summary

This article introduces how to implement daemon processes in Python server programming. Creating a Python daemon is relatively simple, but its stability and error handling require more attention. Through the introduction of this article, readers can learn how to use Python to create a stable and reliable daemon process to ensure the long-term stable operation of Python server-side programs.

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