CDN, Load Balancer, Reverse Proxy — edge-এর স্থাপত্য

~৯০ লাইনে পুরো edge-miniature — stdlib only:

#!/usr/bin/env python3
"""edge_lab.py — tiny reverse proxy + RR-LB over 3 backends w/ health eviction"""
import http.server, socketserver, threading, time, urllib.request

BACKENDS = [{'port': p, 'healthy': True} for p in (9001, 9002, 9003)]
rr_index = {'next': 0}


class Backend(http.server.BaseHTTPRequestHandler):
    def do_GET(self):
        me = self.server.backend_port
        body = f'hello from backend :{me} (path={self.path})\n'.encode()
        self.send_response(200)
        self.send_header('Content-Type', 'text/plain; charset=utf-8')
        self.send_header('Content-Length', str(len(body)))
        self.end_headers()
        self.wfile.write(body)

    def log_message(self, *a): pass


class Proxy(http.server.BaseHTTPRequestHandler):
    def do_GET(self):
        for _ in range(len(BACKENDS)):            # সর্বোচ্চ pool-size retry
            healthy = [b for b in BACKENDS if b['healthy']]
            if not healthy:
                self.send_error(503, 'all backends unhealthy')
                return
            b = healthy[rr_index['next'] % len(healthy)]
            rr_index['next'] += 1
            try:
                r = urllib.request.urlopen(
                    f'http://127.0.0.1:{b["port"]}{self.path}', timeout=2)
                body = r.read()
                self.send_response(r.status)
                self.send_header('X-Upstream', f'127.0.0.1:{b["port"]}')
                self.send_header('X-Round-Robin-Index', str(rr_index['next'] - 1))
                self.send_header('Content-Type', r.headers['Content-Type'])
                self.send_header('Content-Length', str(len(body)))
                self.end_headers()
                self.wfile.write(body)
                return
            except Exception:
                b['healthy'] = False              # passive eviction!
                print(f'[lb] backend :{b["port"]} FAILED → evicted')
        self.send_error(502, 'exhausted retries')

    def log_message(self, *a): pass


def spawn_backend(port):
    socketserver.TCPServer.allow_reuse_address = True
    srv = socketserver.ThreadingTCPServer(('127.0.0.1', port), Backend)
    srv.backend_port = port
    threading.Thread(target=srv.serve_forever, daemon=True).start()


def recovery_loop():
    while True:
        for b in BACKENDS:
            if b['healthy']:
                continue
            try:
                urllib.request.urlopen(
                    f'http://127.0.0.1:{b["port"]}/healthz', timeout=1)
                b['healthy'] = True
                print(f'[lb] backend :{b["port"]} recovered → admitted')
            except Exception:
                pass
        time.sleep(1)


if __name__ == '__main__':
    for b in BACKENDS:
        spawn_backend(b['port'])
    threading.Thread(target=recovery_loop, daemon=True).start()
    print('[lb] proxy :8000 → backends 9001-9003')
    ProxySrv = socketserver.ThreadingTCPServer
    ProxySrv(('127.0.0.1', 8000), Proxy).serve_forever()

Drill-session (দ্বিতীয় terminal):

for i in 1 2 3; do curl -si localhost:8000/x | grep -i x-upstream; done
# X-Upstream: 127.0.0.1:9001
# X-Upstream: 127.0.0.1:9002
# X-Upstream: 127.0.0.1:9003          ← perfect rotation

kill -9 $(lsof -ti :9002)             # এক backend মেরে ফেলুন
for i in 1 2 3 4; do curl -si localhost:8000/x | grep -i 'x-upstream\|HTTP'; done
# X-Upstream: 127.0.0.1:9001 ... 9003 ... 9001 ... 9003
#                                      ← 9002 গায়েব, বাকিরা seamless

নিজে বাড়ান: (১) least-connections mode — in-flight counter রেখে; (২) consistent-hash mode — hash(client_ip) % ring দিয়ে affinity demo (kill করলে শুধু ওই arc-এর keys remap); (৩) X-Cache simulation — proxy-তে dict-cache + max-age expiry বসিয়ে HIT/MISS header; (৪) active-health-check interval config + hysteresis thresholds।