# Distributed Systems, Networking, TCP/IP, RPC (21,22)

## Distributed Systems

### Centralized vs. Distributed Systems

![centralized](/files/-Lr6l_LWEDAeR3eaXmTA)

Centralized System 中，绝大部分功能都在一台物理机器上完成。最开始连客户端也在同一台机器上，后来逐渐演化成 C/S 模型。

![Distributed](/files/-Lr6lwkASTWB5BMFiK5h)

Distributed System 中，不同计算机通过合作的方式完成一项任务。早期以同机房下的多台机器组成的集群形式为主，后来逐渐演化为 peer-to-peer/wide-spread collaboration。

### Motivation/Issue

#### Why do we want distributed systems?

* 造许多简单的计算机与造一台复杂计算机相比，前者成本低且难度小
* 容易逐渐扩容和缩容
* 用户可以对部分机器拥有完全控制权
* 方便更多的用户通过网络合作

#### The promise of distributed systems

* Higher availability：一台机器挂了，用另一台
* Better durability：复制数据
* More security：更小的粒度上进行安全控制

理想很丰满，现实很骨感。现实是：

* Worse availability：availability 有赖于所有正在运行的机器，任何服务挂了都有可能导致服务不可用。Lamport 曾说：“a distributed system is one where I can't do work because some machine I've never heard of isn't working!”
* Worse reliability：机器崩溃可能导致数据丢失
* Worse security：世界上任何人都可以入侵到系统中

除此之外，随着机器数量增加，正确、合理地调度所有机器 (coordination) 来共同完成任务变得异常困难。

> What would be easy in a centralized system becomes a lot more difficult

#### Goals & Requirements

如果只用一个词概括 Distributed System 的目标，那就是 **Transparency**。

> The ability of the system to mask its complexity behind a simple interface

这里的 transparency 又包括：

* Location：无需知道资源的位置
* Migration：资源在用户无感知的情况下可能被合理移动
* Replication：无需知道资源有多少份副本
* Concurrency：无需关心有多少用户正在使用资源
* Parallelism：系统可以通过将大任务拆分成小任务，再通过并行的方式加速任务运行
* Fault Tolerance：系统中出现少量问题用户无感知

## Networking

### Protocol

Distributed system 中，分布在不同机器上的不同进程间需要通过某种方式来交流，我们可以用人类之间交流使用的不同的语言来类比。在计算机范畴中，我们将这些方式称为 protocol。Protocol 包括两部分：

* Syntax：信息的识别和构成，包括格式、信息展示的顺序等等
* Semantics：信息的含义

对应语言，Syntax 就是语法，Semantics 就是语义。通常， protocol 可以使用状态机 (state machine) 来表示。

### *剩余话题概括*

*这里忽略网络部分的剩余讨论，该部分内容与网络原理、分布式系统等课程内容有交叉，话题包括：*

* Client/Server vs. Peer-to-Peer
* Network Protocols
  * Broadcast (Aloha network)
  * Carrier Sense, Multiple Access/Collision Detection (CSMA/CD)
  * Point-to-point
  * The Internet Protocol (IP)
    * Address Subnets
    * Address Ranges
    * Hierarchical Networking
    * Routing/Routing Tables
    * DNS
* Network Layering
* TCP/IP
  * ordering
  * reliable delivery (exactly once)
  * congestion avoidance
  * sequence number
* Sockets
* Distributed Decision Making
  * General's Paradox
  * [2PC](/open-courses/mit-6.824/2pc-and-3pc.md)
  * Byzantine General's Problem
* RPC & Microkernel operating systems

## 参考

lecture note [21](https://people.eecs.berkeley.edu/~kubitron/courses/cs162-S15/sp15/static/lectures/21.pdf), [22](https://people.eecs.berkeley.edu/~kubitron/courses/cs162-S15/sp15/static/lectures/22.pdf)


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