Single-primary

Single-primary is the most common operating mode in MySQL Group Replication, where only one node in the cluster — the primary — accepts write operations. The other nodes (secondaries) are read-only (read_only=ON, super_read_only=ON) and receive changes through the group’s synchronous replication.

How it works #

The parameter group_replication_single_primary_mode=ON enables this mode. The primary is the only node with read_only=OFF. If the primary is stopped or becomes unreachable, the cluster triggers an automatic election and one of the secondaries becomes the new primary within seconds.

Why it’s used #

Single-primary mode avoids the concurrent write conflicts typical of multi-primary setups. In production, most MySQL clusters use this mode because it’s more predictable: applications write to a single endpoint, replication is linear, and debugging is simpler.

What can go wrong #

When the primary is stopped for maintenance, the cluster performs an automatic failover. During those seconds, active connections may be dropped and in-flight transactions may fail. It’s a brief disruption but it must be communicated. The practical rule: in a maintenance intervention on a single-primary cluster, secondaries are touched first, the primary last.