Databases & System Design•Part 5: Real-Time Architecture, Replication & Scale (51 - 60)
Database Failover & Leader Election (Raft):
In Simple Words: When Primary fails heartbeats, replicas run Raft leader election. The replica with the latest log offset is promoted to new Primary.
bash
# PostgreSQL failover trigger using pg_ctl or patroni:
patroni topology cluster_name
# Output:
# + Cluster: app-db +---------+
# | Member | Host | Role | State | TL | Lag in MB |
# +----------+---------+--------+---------+----+-----------+
# | pg-node1 | 10.0.1 | Leader | running | 2 | |
# | pg-node2 | 10.0.2 | Sync | running | 2 | 0 |