{"action":"opened","issue":{"url":"https://api.github.com/repos/cxqdmm/blog/issues/3","repository_url":"https://api.github.com/repos/cxqdmm/blog","labels_url":"https://api.github.com/repos/cxqdmm/blog/issues/3/labels{/name}","comments_url":"https://api.github.com/repos/cxqdmm/blog/issues/3/comments","events_url":"https://api.github.com/repos/cxqdmm/blog/issues/3/events","html_url":"https://github.com/cxqdmm/blog/issues/3","id":382520309,"node_id":"MDU6SXNzdWUzODI1MjAzMDk=","number":3,"title":"CND","user":{"login":"cxqdmm","id":8651068,"node_id":"MDQ6VXNlcjg2NTEwNjg=","avatar_url":"https://avatars0.githubusercontent.com/u/8651068?v=4","gravatar_id":"","url":"https://api.github.com/users/cxqdmm","html_url":"https://github.com/cxqdmm","followers_url":"https://api.github.com/users/cxqdmm/followers","following_url":"https://api.github.com/users/cxqdmm/following{/other_user}","gists_url":"https://api.github.com/users/cxqdmm/gists{/gist_id}","starred_url":"https://api.github.com/users/cxqdmm/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/cxqdmm/subscriptions","organizations_url":"https://api.github.com/users/cxqdmm/orgs","repos_url":"https://api.github.com/users/cxqdmm/repos","events_url":"https://api.github.com/users/cxqdmm/events{/privacy}","received_events_url":"https://api.github.com/users/cxqdmm/received_events","type":"User","site_admin":false},"labels":[],"state":"open","locked":false,"assignee":null,"assignees":[],"milestone":null,"comments":0,"created_at":"2018-11-20T06:33:38Z","updated_at":"2018-11-20T06:33:38Z","closed_at":null,"author_association":"OWNER","body":"> CDN基础架构\r\n![1](https://user-images.githubusercontent.com/8651068/48755561-b3481b00-ecd0-11e8-9126-0aec6c6a7152.png)\r\n\r\n- 当用户点击网站页面上的内容URL，经过本地DNS系统解析，DNS系统会最终将域名的解析权交给CNAME指向的CDN专用DNS服务器\r\n- CDN的DNS服务器将CDN的全局负载均衡设备IP地址返回用户\r\n- 用户向CDN的全局负载均衡设备发起内容URL访问请求\r\n- CDN全局负载均衡设备根据用户IP地址，以及用户请求的内容URL，选择一台用户所属区域的区域负载均衡设备，告诉用户向这台设备发起请求\r\n- 区域负载均衡设备会为用户选择一台合适的缓存服务器提供服务，选择的依据包括：根据用户IP地址，判断哪一台服务器距用户最近；根据用户所请求的URL中携带的内容名称，判断哪一台服务器上有用户所需内容；查询各个服务器当前的负载情况，判断哪一台服务器尚有服务能力。基于以上这些条件的综合分析之后，区域负载均衡设备会向全局负载均衡设备返回一台缓存服务器的IP地址\r\n-  全局负载均衡设备把服务器的IP地址返回给用户\r\n- 用户向缓存服务器发起请求，缓存服务器响应用户请求，将用户所需内容传送到用户终端。如果这台缓存服务器上并没有用户想要的内容，而区域均衡设备依然将它分配给了用户，那么这台服务器就要向它的上一级缓存服务器请求内容，直至追溯到网站的源服务器将内容拉到本地\r\n"}}
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