st=>start: 浏览器请求
e=>end: 浏览器渲染
op1=>operation: 有缓存
op2=>operation: 从缓存读取(from cache)
op3=>operation: �带If-None-Match向web服务器请求
op4=>operation: 带If-Modified-Since向服务器请求
op5=>operation: 向服务器请求
op6=>operation: 从缓存读取
op7=>operation: 请求响应
cond1=>condition: Cache-Control是否过期?
cond2=>condition: 判断是否有 Last-Modified
cond3=>condition: 判断是否有 Etag?
cond4=>condition: 服务器判断是200?
st->op1->cond1
cond1(no)->op2
op2->e
cond1(yes)->cond3
cond3(no)->cond2
cond2(no)->op5
op5->op7
op7->e
cond3(yes)->op3
op3->cond4
cond4(no)->op6
cond4(yes)->op7
op6->e
op4->cond4
cond2(yes)->op4
op7=>e