What is Replication?

Keeping a copy of the same data on multiple machines. Each node that stores the copy is called Replica.

Advantages of Replication

1. Parallel Processing: Master: Handles all write operations, Slaves: Handles all read operations
2. Reliable(Data is served even DB crashes, or some DB goes down under load): Data will be served from other slave
3. Highly available(Even if some of DBs are down, client will get response): Since some slaves will be up.

Types of Replication

Type	Meaning	Digram
Master/Slave or Leader/Follower	Master/Leader: 1 node is designated as master. Client does RW operations with master. Session is stored in master's DB. Then slaves read from it. Slaves: Read Only. slave database gets copies of the data from the master database. All slaves replicate data from most updated slave via replication log. Advantage   1. Parallel Processing: Read(Slaves), Write(Master) is supported paralally   2. Reliable: Since multiple copies of data is maintained   3. Highly availabile: Disadvantage   a. Logic is required to promote slave to master(when master fails)   b. If high number of slaves, more write operations can lead to replication lag.
Master/Master or Active/Active or Leader/Leader	There are 2 or more leader nodes. Client can write to any 1 of leader. Other leaders will act as slave and read all data from present leader.