OpenSearch cluster fault tolerance

The fault tolerance of an OpenSearch managed database cluster is affected by:

• index replication;
• number of nodes in node groups with Manager and Data roles. Learn more about node groups and their roles in the OpenSearch Node Groups guide;
• node allocation type in node groups.

Index replication

The cluster uses a standard index replication mechanism — for each primary shard, the number of replica shards that you specified during index creation is created. Writing to an index occurs only through primary shards; reading can be performed simultaneously from primary shards and replica shards.

Number of nodes in node groups

The node group with the Manager role contains three nodes. You cannot change the number of nodes in this group or add another node group with the Manager role.

The node group with the Data role consists of one node by default. To increase cluster fault tolerance, you can:

• increase the number of nodes in the group with the Data role in an existing cluster or when creating a cluster. You can add from one to ten nodes to each node group;
• add another node group with the Data role in an existing cluster or when creating a cluster. A cluster can have no more than two node groups with the Data role.

Node allocation type

The node allocation type in a cluster depends on:

• the number of nodes in a group;
• the type of pool where the cluster is located;
• the number of segments in the pool;
• configuration line.

You can view the list of pools and segments in the Servercore Infrastructure table in the Locations: countries, regions, availability zones, pools, and data center addresses guide.

Four node allocation types are available:

• Single-Segment;
• Multi-Segment;
• Single-AZ;
• Multi-AZ.

Single-Segment

Nodes are placed in the same segment of a single-zone pool. Applicable:

• a node group with the Data role that has only one node;
• node groups with multiple nodes, if the cluster is located in a pool with only one segment;
• node groups with multiple nodes in HighFreq configurations.

note

For example, you have created a cluster in the ru-2 pool and used HighFreq line configurations for node groups with Manager and Data roles. All nodes of both groups will be placed in the same pool segment, for example, ru-2c.

Multi-Segment

Nodes are placed in different segments of a single-zone pool. Applicable for node groups with multiple nodes if the cluster is located in a pool with multiple segments.

Segments are located in different racks of the same data center. Nodes are distributed among segments sequentially.

note

For example, you have created a cluster in the ru-1 pool and specified one node in the node group with the Data role. All three nodes of the group with the Manager role will be placed sequentially in the segments of the ru-1a, ru-1b, and ru-1c pool. The node of the group with the Data role will be placed in the ru-1a segment. If you add a second node to the group with the Data role, it will be placed in the ru-1b pool segment.

Single-AZ

Nodes are placed in the same segment of a multi-zone pool. Applicable for a node group with the Data role that has only one node, if the cluster is located in the ru-6 pool.

If you increase the number of nodes in the group, the node allocation type will change to Multi-AZ.

note

For example, you have created a cluster in the ru-1 pool and specified one node in the node group with the Data role. The node will be placed in one pool segment, for example, ru-6a.

Multi-AZ

Nodes are placed in different segments of a multi-zone pool. Applicable for node groups with multiple nodes if the cluster is located in the ru-6 pool.

Each pool segment is located in a separate data center. Nodes are distributed among segments sequentially.

note

For example, you have created a cluster in the ru-6 pool and specified one node in the node group with the Data role. All three nodes of the group with the Manager role will be placed sequentially in the segments of the ru-6a, ru-6b, and ru-6c pool. The node of the group with the Data role will be placed in the ru-6a segment. If you add a second node to the group with the Data role, it will be placed in the ru-6b pool segment.

Increase the number of nodes in a group with the Data role

You can add no more than ten nodes to a node group with the Data role.

1. In the Dashboard, on the top menu, click Products and select Managed Databases.
2. Open the Active tab.
3. Open the database cluster page → Settings tab.
4. In the menu of the node group with the Data role, select Change configuration.
5. Specify the number of nodes in the group.
6. Click Save.

Add a node group with the Data role

The total number of node groups with the Data role in a cluster cannot exceed two.

1. In the Dashboard, on the top menu, click Products and select Managed Databases.

2. Open the Active tab.

3. Open the database cluster page → Settings tab.

4. In the Cluster block, click Add node group.

5. Optional: change the node group name.

6. Select a node group with the Data role.

7. Select a configuration line.

8. If you selected a line with custom configurations, specify the number of vCPUs, RAM size, and disk size.

9. If you selected a line with fixed configurations, select a configuration.

10. Specify the number of nodes in the group.

11. Optional: to make the group nodes accessible from the internet, connect a public IP address to them:

    • if you selected an existing private subnet, enable the Public access toggle. The private subnet must meet the requirements;
    • if you are creating a new private subnet, connect a public IP address after adding the node group.

    IP addresses are automatically assigned to all nodes in the group.

12. Click Save.