Distributed System VI
这一篇介绍域名服务器DNS和内容分发网络CDN。因为跑去参加Oath的Hackson所以旷课了没什么印象…
Domain Name System
DNS的工作是将域名和主机名转换为IP地址。
DNS goals:
- Basically a wide-area distributed database
- Scalability
- Decentralized maintenance
- Robustness
- Global scope
DNS Recodes: (class, name, value, type, ttl)
DB contains tuples called resource records (RRs)
- Classes = Internet (IN), Chaosnet (CH), etc.
- Each class defines value associated with type [A, CNAME, NS, MX]
DNS的层次结构是树形的，每个node存拥有相同后缀的一系列域名。
Zone: contiguous section of name space, an associated set of name servers
Typical Resolution：
- Application calls gethostbyname (RESOLVER)
- Resolver contacts local name server (S1)
- S1 queries root server (S2) for (www.cmu.edu)
- S2 returns NS record for cmu.edu (S3)
- S3 is what the additional information section is for (PREFETCHING)
- S1 queries S3 for www.cmu.edu
- S3 returns A record for www.cmu.edu
Naming
大概是域名的意思，域名和DNS的内容是联合在一起的。利用dig可以查询。反正也不是重点所以失忆感觉实在是太松懈了
Internet Content Delivery
介绍CDN, Content Delivery Networks 内容分发网络。在学习DNS的时候提到，如果有一些域名比较popular怎么办？解决办法就是利用了CDN将它们缓存起来。
DNS based Client Routing
CDN high-level name server chooses appropriate regional POP
- A-record returned is “best” one for the client
- Based on server load, location of client
- Large TTL
CDN low-level name server chooses server in region
- Should try to chose server that has file in cache
- This is where consistent hashing comes in
- Small TTL
这里high level name server是large TTL, low level name server是small TTL。因为client的location不会经常变动，因此cache的内容不会经常变动。而low level存的内容会容易变动，如果某个client崩溃，CDN会快速的设置另一个。
CDN Summary
- Across wide-area Internet: caching is the only way to improve latency
- CDNs move data closer to user
- CDNs maintain consistency
- CDNs balance load
- Use consistent hashes and many other DS techniques
参考资料