- Master database generally only supports write operations.
- A slave database gets copies of the data from the master database and only supports read operations
- The number of slave databases in a system is usually larger than the number of master databases since most applications require a much higher ratio of reads to writes.
The web server maintains the credentials for both the master and slave databases. It is responsible for routing write requests to master and read requests to slave
Advantages
Better performance
- Allows more queries to be processed in parallel
- Because of distribution of write and read responsibilities between master and slave
Reliability
- Since data is replicated across multiple locations, data is still preserved even if one of the DB servers is destroyed by a natural disaster
High availability
- Even if one of the DB servers is offline, you can access data stored in another DB server. This way your app is highly available.
What happens when one DB server goes offline?
When the slave DB goes offline
- All read operations are directed to the master DB temporarily
- In case of multiple slave databases, read operations are redirected to other healthy slave databases.
When the master DB goes offline
- A slave DB will be promoted to be the new master
- A new slave DB will replace the old one