MySQL Cluster Nodes, Node Groups, Replicas, and Partitions
1518
• Global Checkpoint (GCP).
A GCP occurs every few seconds, when transactions for all nodes
are synchronized and the redo-log is flushed to disk.
17.1.2. 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 should be located on a separate computer. While it is also possible to host multiple
ndbd
processes on a single computer, such a configuration is not supported.
It is common for the terms “node” and “data node” to be used interchangeably when referring to
an
ndbd
process; where mentioned, management (MGM) 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 partitions, or sets of
replicas (see next item).
The number of node groups in a MySQL Cluster is not directly configurable; it is function of the
number of data nodes and of the number of replicas (
NoOfReplicas
configuration parameter), as
shown here:
[
number_of_node_groups
] =
number_of_data_nodes
/
NoOfReplicas
Thus, a MySQL Cluster with 4 data nodes has 4 node groups if
NoOfReplicas
is set to 1 in the
config.ini
file, 2 node groups if
NoOfReplicas
is set to 2, and 1 node group if
NoOfReplicas
is set to 4. Replicas are discussed later in this section; for more information about
NoOfReplicas
,
see
Section 17.3.2.5, “Defining MySQL Cluster Data Nodes”
.
Note
All node groups in a MySQL Cluster must have the same number of data
nodes.
• Partition.
This is a portion of the data stored by the cluster. There are as many cluster partitions
as nodes participating in the cluster. Each node is responsible for keeping at least one copy of any
partitions assigned to it (that is, at least one replica) available to the cluster.
A replica belongs entirely to a single node; a node can (and usually does) store several replicas.
• 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 number of replicas is equal to the number of nodes per
node group.
The following diagram illustrates a MySQL Cluster with four data nodes, arranged in two node groups
of two nodes each; nodes 1 and 2 belong to node group 0, and nodes 3 and 4 belong to node group
1. Note that only data (
ndbd
) 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,
these have been omitted in the figure for clarity.
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