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Leaders and Followers

Single Leader Replication


Each node storing a copy of the database is called a replica. A database system may have many replicas. With replication, these replica nodes are in sync, and they remain consistent over time.

We use various terms to refer them, such as Leader and Followers, Master and Slave, and Primary and Secondary. Replicas are sometimes called Hot Standbys.

Single Leader Replication

In single-leader replication, one of the replicas is designated the leader (also known as master or primary). When clients want to write to the database, they must send their requests to the leader, which first writes the new data to its local storage.

The other replicas are known as followers (read replicas, slaves, secondaries, or hot standbys). Whenever the leader writes new data to its local storage, it also sends the data change to all of its followers as part of a replication log or change stream. Each follower takes the log from the leader and updates its local copy of the data‐ base accordingly, by applying all writes in the same order as they were processed on the leader.

When a client wants to read from the database, it can query either the leader or any of the followers. However, writes are only accepted on the leader (the followers are read-only from the client’s point of view).

Single Leader Replication

This mode of replication is a built-in feature of many relational databases, such as PostgreSQL (since version 9.0), MySQL, Oracle Data Guard, and SQL Server’s AlwaysOn Availability Groups. It is also used in some non-relational databases, including MongoDB, RethinkDB, and Espresso. Finally, leader-based replication is not restricted to only databases: distributed message brokers such as Kafka and RabbitMQ highly available queues also use it. Some network filesystems and replicated block devices such as DRBD are similar.