Design of master-slave replication system

copy （replica）： Each node that stores the complete data set of the database . When there are multiple copies , Problems inevitably arise ： How to ensure that all data falls on all copies ？

Each direction DB All writes of must be propagated to all copies , Otherwise, the copy will be inconsistent . The most common solution is master based replication （ Also known as Take the initiative / Passive or active / From copying ）, Pictured -1.

1.1 Master slave replication principle

1. Specify a replica as the primary replica or primary node . When the client DB Writing time , The request must be sent to the master copy , The primary replica writes new data to its local storage
2. Other copies are called slave copies or read-only copies , From the node , Hot standby （hot-standby）[^i]. Whenever the master node writes new data to the local storage , Data changes will also be sent to all followers , be called Copy log （replication log） Record or Change flow （change stream）. After each change log is obtained from the replica, it is applied locally , And strictly keep the same write order as the primary copy
3. The client from DB When reading data , Queries can be executed on master or slave nodes . But only the primary node can accept write requests . Client angle , Slave nodes are read-only ）

[^i]: Different pairs heat （hot）, temperature （warm）, cold （cold） Backup servers have different definitions . Such as PostgreSQL in , Hot standby （hot standby） It refers to the copy that can accept the client's read request . and Warm preparation （warm standby） Just follow the leader , But does not process any queries from the client .

This replication pattern is a built-in feature of many relational databases , Such as PostgreSQL（9.0 Version start ）、MySQL、Oracle Data Guard and SQL Server Of AlwaysOn Availability Groups. And some non relational databases such as MongoDB、RethinkDB and Espresso It also supports master-slave replication . Master-slave replication is not limited to DB： image Kafka、RabbitMQ Such distributed message brokers also use .