反向代理与负载均衡的原理与实现

题目描述
反向代理和负载均衡是现代后端架构中的核心组件。反向代理作为服务器的网关，接收客户端请求并转发到后端服务；负载均衡则负责将流量合理分配到多个服务器实例。理解其工作原理对设计高可用系统至关重要。

核心原理

1. 反向代理：位于客户端和后端服务器之间，隐藏真实服务器信息，提供统一入口
2. 负载均衡：通过特定算法将请求分发到多个服务器，提高系统吞吐量和可靠性

实现细节

第一步：反向代理基础实现

# 简易反向代理示例
import http.server
import socketserver
import requests

class ReverseProxyHandler(http.server.BaseHTTPRequestHandler):
    backend_servers = ['http://localhost:8001', 'http://localhost:8002']
    
    def do_GET(self):
        # 选择后端服务器（简单轮询）
        backend = self.backend_servers.pop(0)
        self.backend_servers.append(backend)
        
        # 转发请求
        resp = requests.get(f"{backend}{self.path}")
        
        # 返回响应给客户端
        self.send_response(resp.status_code)
        for key, value in resp.headers.items():
            self.send_header(key, value)
        self.end_headers()
        self.wfile.write(resp.content)

第二步：负载均衡算法演进

1. 轮询（Round Robin）：

class RoundRobinBalancer:
    def __init__(self, servers):
        self.servers = servers
        self.index = 0
    
    def get_server(self):
        server = self.servers[self.index]
        self.index = (self.index + 1) % len(self.servers)
        return server

2. 加权轮询（Weighted Round Robin）：

class WeightedRoundRobinBalancer:
    def __init__(self, server_weights):
        self.servers = []
        for server, weight in server_weights.items():
            self.servers.extend([server] * weight)
        self.index = 0
    
    def get_server(self):
        server = self.servers[self.index]
        self.index = (self.index + 1) % len(self.servers)
        return server

3. 最少连接（Least Connections）：

class LeastConnectionsBalancer:
    def __init__(self, servers):
        self.server_connections = {server: 0 for server in servers}
    
    def get_server(self):
        return min(self.server_connections.items(), key=lambda x: x[1])[0]
    
    def release_connection(self, server):
        if server in self.server_connections:
            self.server_connections[server] -= 1

第三步：健康检查机制

import threading
import time

class HealthChecker:
    def __init__(self, servers, check_interval=30):
        self.servers = servers
        self.healthy_servers = set(servers)
        self.check_interval = check_interval
    
    def check_server(self, server):
        try:
            resp = requests.get(f"{server}/health", timeout=5)
            return resp.status_code == 200
        except:
            return False
    
    def start_checking(self):
        def check_loop():
            while True:
                for server in self.servers:
                    is_healthy = self.check_server(server)
                    if is_healthy and server not in self.healthy_servers:
                        self.healthy_servers.add(server)
                    elif not is_healthy and server in self.healthy_servers:
                        self.healthy_servers.remove(server)
                time.sleep(self.check_interval)
        
        thread = threading.Thread(target=check_loop)
        thread.daemon = True
        thread.start()

第四步：会话保持（Session Affinity）

class SessionAwareBalancer:
    def __init__(self, servers):
        self.servers = servers
        self.session_map = {}  # session_id -> server
    
    def get_server(self, session_id=None):
        if session_id and session_id in self.session_map:
            return self.session_map[session_id]
        
        server = self.servers[len(self.session_map) % len(self.servers)]
        if session_id:
            self.session_map[session_id] = server
        return server

第五步：完整实现示例

class AdvancedLoadBalancer:
    def __init__(self, servers):
        self.servers = servers
        self.balancer = LeastConnectionsBalancer(servers)
        self.health_checker = HealthChecker(servers)
        self.session_balancer = SessionAwareBalancer(servers)
        self.health_checker.start_checking()
    
    def handle_request(self, request):
        # 检查会话粘性
        session_id = request.cookies.get('session_id')
        if session_id:
            server = self.session_balancer.get_server(session_id)
        else:
            server = self.balancer.get_server()
        
        # 转发请求
        self.balancer.server_connections[server] += 1
        try:
            return self.forward_request(server, request)
        finally:
            self.balancer.release_connection(server)

关键技术点

1. 连接池管理：复用后端连接减少开销
2. 超时控制：设置连接和读取超时避免阻塞
3. 故障转移：自动检测失败请求并重试其他服务器
4. 动态配置：支持运行时添加/移除后端服务器

实际应用场景

• Nginx：基于事件驱动的反向代理实现
• HAProxy：专业的TCP/HTTP负载均衡器
• 云服务：AWS ALB、GCP Load Balancer等托管服务

这种架构通过分散请求压力和提高容错能力，显著提升系统可用性和性能。