Notes from Tiger’s Lecture

How to communicate between two users?

Client Server model

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User 1 <== connection ==> User 2

(How to know where is the other guy? TCP/IP. But how to know the IP address? ===> server. If there is no server,  we can use MAC. Why use IP instead of MAC? MAC地址可以修改， 同时没有搭配的协议栈. If there is no server or IP, use P2P.)

How to connect

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A  <==> B 

mail model: short connection
phone model: persistent connection （每个 router 都记录 A-B 的路径，不需要每次都传递具体通信的 meta 信息）

Could we build WeChat without Server

How to save traffic

  1. user persistent connection
  2. P2P 不稳定 & 法律监管 & 带宽环境变好
  3. 批量压缩
  4. web socket in http

pull push model 消息延迟加载，先推送新消息的计数，等打开 app 才刷新消息的具体内容，如果消息多的话还可以压缩以便进一笔节省流量

What data need to be save in client

  1. message
  2. account
  3. contacts

Sync between two clients (mobile/desktop)

timestamp，但是用户的时间可能是错的，所以用逻辑时钟/版本号
user pull version or server push

How to build API

Scenario

  1. send message
  2. account
  3. read message
  4. 红包
  5. log in/out
  6. 朋友圈
  7. location

=> chat, account, feed

client ==> 顶层业务 API ==> 逻辑 API / DATA API （可并行串行，微信用并行） ==> DATA

How to communicate between services?

common gate way interface

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API
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a large container determines which module should API go to

缺点：耦合，单点故障，连带效应（故障/升级）
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Micro service
底下的服务每个都提供一个 http 接口，每个都是独立的，在 master 那里注册一下，多个 master。每个都是单独的进程

How to support QQ message

How to support group chat

异步 RPC
disruptor

设计广播/feed模式

假设群内有 u1 u2 u3，群发就是 u1-u2,u1-u3,…,etc，而不是 u1-channel （逻辑复杂），微信群设人数上限

How to configure API

How to store data

  1. message
  2. account
  3. contacts

Should we save them together

What is the key

  1. message, NoSQL
  2. account, SQL
  3. contacts, SQL

Why do we need memory

在内存里进行复杂的 join 等操作

Is one data center enough

容灾

How to monitor the number of registered users

MySQL count

How to monitor the number of online users

log,上线了就+1离线了就-1，注意掉线的情况

How to notify the numbers to team

What are the steps for log

分布式 log

  1. log统计代码
  2. 保存在本地或者发送到 log server 上 （每台机器上有个进程专门处理 lock，加减等操作）