缓存机制：强缓存与协商缓存

                ┌── 命中 ──▶ 直接用本地副本（不发请求）
需要资源 ─ 强缓存 ┤
                └── 过期 ──▶ 发条件请求 ─ 协商缓存 ┬─ 304 ─▶ 继续用本地副本
                                                └─ 200 ─▶ 用新内容并更新缓存

python3 -c '
from http.server import BaseHTTPRequestHandler, HTTPServer
 
POLICIES = {
    "/app.8f3a2c.js": "public, max-age=31536000, immutable",
    "/index.html":    "no-cache",
    "/api/profile":   "private, max-age=0, must-revalidate",
    "/api/token":     "no-store",
    "/logo.png":      "public, max-age=86400, s-maxage=604800",
}
 
class H(BaseHTTPRequestHandler):
    def do_GET(self):
        cc = POLICIES.get(self.path, "no-store")
        self.send_response(200)
        self.send_header("Cache-Control", cc)
        self.send_header("Content-Length", "0")
        self.end_headers()
    def log_message(self, *a):
        pass
 
HTTPServer(("127.0.0.1", 8000), H).serve_forever()
' &
for _i in $(seq 1 50); do (exec 3<>/dev/tcp/127.0.0.1/8000) 2>/dev/null && break; sleep 0.1; done
 
for p in /app.8f3a2c.js /index.html /api/profile /api/token /logo.png; do
  echo "== $p =="
  curl -si http://127.0.0.1:8000$p | grep -i cache-control
done

echo "cached content v1" > page.html
python3 -m http.server 8000 --bind 127.0.0.1 >/dev/null 2>&1 &
for _i in $(seq 1 50); do (exec 3<>/dev/tcp/127.0.0.1/8000) 2>/dev/null && break; sleep 0.1; done
 
echo "== 第一次请求：200，记下 Last-Modified =="
curl -si http://127.0.0.1:8000/page.html | grep -E "HTTP/|Last-Modified"
 
LM=$(curl -si http://127.0.0.1:8000/page.html | grep -i last-modified | cut -d" " -f2-)
 
echo "== 带 If-Modified-Since 再问：304，没有响应体 =="
curl -s -o /dev/null -w "%{http_code} 下载了 %{size_download} 字节\n" \
  -H "If-Modified-Since: $LM" http://127.0.0.1:8000/page.html
 
echo "== 修改文件后再问：文件变新了，回 200 =="
sleep 1
echo "cached content v2" > page.html
curl -s -o /dev/null -w "%{http_code} 下载了 %{size_download} 字节\n" \
  -H "If-Modified-Since: $LM" http://127.0.0.1:8000/page.html

python3 -c '
from http.server import BaseHTTPRequestHandler, HTTPServer
import hashlib
 
content = {"data": b"article body version 1"}
 
class H(BaseHTTPRequestHandler):
    def do_GET(self):
        if self.path == "/bump":
            content["data"] = b"article body version 2 (edited)"
            self.send_response(200)
            self.send_header("Content-Length", "0")
            self.end_headers()
            return
        body = content["data"]
        etag = "\"" + hashlib.md5(body).hexdigest()[:12] + "\""
        if self.headers.get("If-None-Match") == etag:
            self.send_response(304)     # 304 禁止携带响应体
            self.send_header("ETag", etag)
            self.end_headers()
            return
        self.send_response(200)
        self.send_header("ETag", etag)
        self.send_header("Cache-Control", "no-cache")
        self.send_header("Content-Length", str(len(body)))
        self.end_headers()
        self.wfile.write(body)
    def log_message(self, *a):
        pass
 
HTTPServer(("127.0.0.1", 8000), H).serve_forever()
' &
for _i in $(seq 1 50); do (exec 3<>/dev/tcp/127.0.0.1/8000) 2>/dev/null && break; sleep 0.1; done
 
echo "== 第一次：200，拿到 ETag =="
curl -si http://127.0.0.1:8000/article | grep -E "HTTP/|ETag"
ET=$(curl -si http://127.0.0.1:8000/article | grep -i "^etag" | cut -d" " -f2 | tr -d "\r")
 
echo "== 带 If-None-Match 再问：304 =="
curl -s -o /dev/null -w "%{http_code} 下载 %{size_download} 字节\n" \
  -H "If-None-Match: $ET" http://127.0.0.1:8000/article
 
echo "== 内容更新后：指纹不符，回 200 新内容 =="
curl -s http://127.0.0.1:8000/bump >/dev/null
curl -s -o /dev/null -w "%{http_code} 下载 %{size_download} 字节\n" \
  -H "If-None-Match: $ET" http://127.0.0.1:8000/article

是敏感数据（token、个人隐私）？
├─ 是 → Cache-Control: no-store                     [到此为止]
└─ 否 → 内容会变吗？
    ├─ 文件名带内容 hash（app.8f3a2c.js）
    │      → public, max-age=31536000, immutable
    ├─ 会变，但必须立即生效（HTML 入口、API）
    │      → no-cache（或 max-age=0）+ ETag/Last-Modified
    └─ 会变，可容忍 N 秒延迟（列表页、头像）
           → max-age=N（+ ETag 兜底）
              是否允许 CDN 缓存？ public/s-maxage : private