(mysql.info) mysql-cluster-nodes-groups

 
 15.2.1 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).
 
      * 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 viable
 
 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.
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