1.2. GlossaryΒΆ

Galera Arbitrator

If the expense of adding, for example, a third datacenter is too high, you can use the Galera arbitrator. An arbitrator is a member of the cluster which participates in voting, but not in actual replication.

See Galera Arbitrator for more details.

Galera Replication Plugin

Galera Replication Plugin is a general purpose replication plugin for any transactional system. It can be used to create a synchronous multi-master replication solution to achieve high availability and scale-out.

See Galera Replication Plugin for more details.

Global Transaction ID

To keep the state identical on all nodes, the wsrep API uses global transaction IDs (GTID), which are used to both:

  • Identify the state change
  • Identify the state itself by the ID of the last state change

The GTID consists of:

  • A state UUID, which uniquely identifies the state and the sequence of changes it undergoes
  • An ordinal sequence number (seqno, a 64-bit signed integer) to denote the position of the change in the sequence

See wsrep API for more details.

Incremental State Transfer (IST)

In incremental state transfer, a node only receives the missing write sets and catch up with the group by replaying them. See also the definition for State Snapshot Transfer (SST).

See Incremental State Transfer (IST) for more details.

Primary Component

In addition to single node failures, the cluster may be split into several components due to network failure. In such a situation, only one of the components can continue to modify the database state to avoid history divergence. This component is called the Primary Component (PC).

See Primary Component for more details.

Rolling Schema Upgrade

The rolling schema upgrade is a DDL processing method, where the DDL will only be processed locally at the node. The node is desynchronized from the cluster for the duration of the DDL processing in a way that it does not block the rest of the nodes. When the DDL processing is complete, the node applies the delayed replication events and synchronizes back with the cluster.

See Rolling Schema Upgrade for more details.

State Snapshot Transfer (SST)

State Snapshot Transfer refers to a full data copy from one cluster node (donor) to the joining node (joiner). See also the definition for Incremental State Transfer (IST).

See State Snapshot Transfer (SST) for more details.

Total Order Isolation

By default, DDL statements are processed by using the Total Order Isolation (TOI) method. In TOI, the query is replicated to the nodes in a statement form before executing on master. The query waits for all preceding transactions to commit and then gets executed in isolation on all nodes simultaneously.

See Total Order Isolation for more details.

Writeset Cache (GCache)

Galera stores write sets in a special cache called Writeset Cache (GCache). In short, GCache is a memory allocator for write sets and its primary purpose is to minimize the write set footprint on the RAM.

See Writeset Cache (GCache) for more details.

wsrep API

The wsrep API is a generic replication plugin interface for databases. The API defines a set of application callbacks and replication plugin calls.

See wsrep API for more details.