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16.2.1. MySQL Cluster Nodes, Node Groups, Replicas, and Partitions

This section discusses the manner in which MySQL Cluster divides and duplicates data for storage.

Central to an understanding of this topic are the following concepts, listed here with brief definitions:

  • (Data) Node: An ndbd process, which stores a replica —that is, a copy of the partition (see below) assigned to the node group of which the node is a member.

    Each data node is usually located on a separate computer. However, it is also possible to host multiple data nodes on a single computer having more than one processor. In such cases, it is feasible to run one instance of ndbd per physical CPU. (Note that a processor with multiple cores is still a single processor.)

    It is common for the terms “node” and “data node” to be used interchangeably when referring to an ndbd process; where mentioned, management nodes (ndb_mgmd processes) and SQL nodes (mysqld processes) are specified as such in this discussion.

  • Node Group: A node group consists of one or more nodes, and stores a partition, or set of replicas (see next item).

    Note: Currently, all node groups in a cluster must have the same number of nodes.

  • Partition: This is a portion of the data stored by the cluster. There are as many cluster partitions as node groups participating in the cluster, and each node group is responible for keeping at least one copy of the partition assigned to it (that is, at least one replica) available to the cluster.

  • Replica: This is a copy of a cluster partition. Each node in a node group stores a replica. Also sometimes known as a partition replica.

The following diagram illustrates a MySQL Cluster with four data nodes, arranged in two node groups of two nodes each. Note that no nodes other than data nodes are shown here, although a working cluster requires an ndb_mgm process for cluster management and at least one SQL node to access the data stored by the cluster.

A MySQL Cluster, with 2 node groups having 2
          nodes each

The data stored by the cluster is divided into two partitions, labeled A and B in the diagram. Each partition is stored — in multiple copies — on a node group. The data making up Partition A is stored on Node A-1, and this data is identical to that stored by Node A-2. The data stored by Nodes B-1 and B-2 is also the same — these two nodes store identical copies of the data making up Partition B.

What this means so far as the continued operation of a MySQL Cluster is this: so long as each node group participating in the cluster has at least one “live” node, the cluster has a complete copy of all data and remains viable. This is illustrated in the next diagram.

Nodes required to keep a 2x2 cluster

In this example, where the cluster consists of two node groups of two nodes each, any combination of at least one node in Node Group A and at least one node in Node Group B is sufficient to keep the cluster “alive” (indicated by arrows in the diagram). However, if both nodes from either node group fail, the remaining two nodes are not sufficient (shown by arrows marked out with an X); in either case, the cluster has lost an entire partition and so can no longer provide access to a complete set of all cluster data.

  Published under the terms of the GNU General Public License Design by Interspire