SSR Node Groups
Each data node participates in a node group of two data nodes. A cluster with two data nodes has a single node group; a cluster with four data nodes has two node groups, each with two data nodes. Each node group stores different partitions and replicas.
A partition is a portion of all the data stored by the cluster. There are as many cluster partitions as node groups in the cluster. Each node group keeps at least one copy of any partitions assigned to it (that is, at least one replica) available to the cluster.
A replica is a copy of a partition. Each data node in a node group stores a replica of a partition. A replica belongs entirely to a single data node; a node can (and usually does) store several replicas because maintaining two replicas is the fixed setting for the SSR.
Figure 1 shows the data elements of an SSR cluster with four data nodes arranged in two node groups of two nodes each. Nodes 1 and 2 belong to Node Group 1. Nodes 3 and 4 belong to Node Group 2.
Because there are four data nodes, there are four partitions.
The number of replicas is two, to create two copies of each primary partition.
So long as both nodes in one node group are operating, or one node in each node group is operating, the cluster remains viable.
The data stored by the cluster in Figure 1 is divided into four partitions: 0, 1, 2, and 3. Multiple copies of each partition are stored within the same node group. Partitions are stored on alternate node groups:
Partition 0 is stored on Node Group 1. A primary replica is stored on Data Node 1 and a backup replica is stored on Data Node 2.
Partition 1 is stored on the other node group, Node Group 2. The primary replica is on Data Node 3 and its backup replica is on Data Node 4.
Partition 2 is stored on Node Group 1. The placement of its two replicas is reversed from that of Partition 0; the primary replica is stored on Data Node 2 and the backup on Data Node 1.
Partition 3 is stored on Node Group 2, and the placement of its two replicas are reversed from those of partition 1: the primary replica is on Data Node 4 and the backup on Data Node 3.