Primary Component (PC)

In a Galera cluster, not all nodes are equal at all times: only those forming the Primary Component are authorised to process writes. The PC is the subset of nodes that has achieved quorum — a majority of the cluster’s total votes — and can therefore continue operating safely. Nodes excluded from the PC enter non-Primary state and stop accepting DML statements.

How it works #

Galera uses a membership protocol built on wsrep (Write-Set Replication). Each node exchanges heartbeats and votes with the others. When a network partition or a crash isolates a group of nodes, each group independently calculates whether it holds more than half of the total cluster votes.

• The group with more than half the votes becomes (or remains) the Primary Component.
• The minority group transitions to non-Primary state: client connections receive an error and no writes are accepted.

This mechanism is the main safeguard against split-brain: two partitions cannot both believe they are the PC and silently diverge.

Operational context #

The critical edge case is a two-node cluster: if one node goes down, the survivor holds exactly 50% of the votes and cannot reach quorum. It enters non-Primary state even if it is fully healthy. The standard solution is to add a third node or a lightweight arbitrator (garbd) so that one side can always exceed 50%.

In maintenance or emergency bootstrap scenarios, it is possible to manually force PC formation:

SET GLOBAL wsrep_provider_options='pc.bootstrap=YES';

This must be done carefully: if another partition is still active and holds more recent data, promoting the wrong node risks losing committed transactions.