负载均衡主动检查与被动检查

负载均衡主要是为了提高应用的可用性和性能，在实际应用中有两种不同的检查方式：主动检查和被动检查。本文将详细介绍这两种检查方式以及它们的实现原理。

1. 主动检查

主动检查是指应用自身定时发送请求去检查其它应用的状态。通常这种检查方式会通过HTTP请求或者TCP请求实现，负载均衡器会根据应用返回的信息来判断其可用性。如果该应用返回的信息正常，则认为该应用可用，否则认为该应用不可用。

下面是一个基于HTTP的负载均衡主动检查的示例代码：

import requests
import time

class AppHealthChecker:
    def __init__(self, app_url):
        self.app_url = app_url

    def check(self):
        try:
            response = requests.get(self.app_url)
            status_code = response.status_code
            content = response.content
            if status_code != 200:
                return False
            if "OK" not in content:
                return False
        except requests.exceptions.RequestException:
            return False
        return True

class LoadBalancer:
    def __init__(self, app_urls):
        self.app_urls = app_urls
        self.health_check_interval = 5
        self.failed_apps = []
        self.failed_time = {}

    def load_balance(self):
        available_apps = []
        for app_url in self.app_urls:
            checker = AppHealthChecker(app_url)
            if checker.check():
                available_apps.append(app_url)
            else:
                self.failed_apps.append(app_url)
                self.failed_time[app_url] = time.time()
        if len(available_apps) == 0:
            return None
        return available_apps[0]

    def check_failed_apps(self):
        for app_url in self.failed_apps:
            failed_time = self.failed_time[app_url]
            if time.time() - failed_time > self.health_check_interval:
                checker = AppHealthChecker(app_url)
                if checker.check():
                    self.failed_apps.remove(app_url)
            
load_balancer = LoadBalancer(["http://app1.com", "http://app2.com", "http://app3.com"])
while True:
    app_url = load_balancer.load_balance()
    if app_url is None:
        print("All apps are down!")
    else:
        print("The selected app URL is", app_url)
    load_balancer.check_failed_apps()
    time.sleep(load_balancer.health_check_interval)

在上面的代码中，我们实