Galera Node State Machine

Different Galera layers have different state machines, depending on their semantics. The top layer that represents “node” state has the following FSM:

1. Node establishes connection to a primary component.
2. Node succeeds in state transfer request. It starts to cache the writesets.
3. Node receives state snapshot. Now it has all cluster data and can start applying the cached writesets. Flow control is turned on to ensure eventual slave queue decrease.
4. Node completes catching up with the cluster (slave queue is empty). Flow control is turned on to keep slave queue empty. In MySQL wsrep_ready status variable is set to 1 and the node is allowed to process transactions.
5. Node receives state transfer request. Flow control is relaxed as for JOINER. The node caches the writesets it can't apply.
6. Node completes state transfer to joiner.

For clarity the following transitions were omitted from the graph:

• At any time, cluster configuration change event can send the node to PRIMARY or OPEN state, e.g. SYNCED → OPEN when a node loses connection to primary component due to network partition.
• If the node does not need a state transfer (e.g. node restart in an idle cluster) it goes straight PRIMARY → JOINED.