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Redis cluster fault tolerance

By default, a Redis Cloud Database cluster consists of one master node. To ensure cluster fault tolerance, you need to add replicas to it. In multi-node clusters, one node will be the master node, and the others will be replicas.

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For example, if you have created a three-node cluster, the cluster will have one master node and two replicas. If you add more nodes, they will be replicas.

The node allocation type in the cluster depends on whether the cluster has replicas, the type of the pool where the cluster is located, and the number of segments in the pool.

Master node

By default, a cluster consists of one main node — a master node. When connecting to the master node, all read and write operations are available.

All data changes on the master node are duplicated on the replicas. The replication process does not affect the performance of the replicas and the master node.

Replicas

Replicas are full copies of the master node. They are available for data reading only.

Having replicas in a cluster ensures its fault tolerance — if the master node stops working, its role will be transferred to one of the replicas, and the cluster will continue to operate normally. When the master node is restored, it will take over the role of a replica. Note that the master node address will change.

For a cluster with replicas, an SLA applies — we guarantee 99.95% write availability and 99.99% read availability.

If a cluster has no replicas, it will be unavailable until the master node is restored. Data will not be lost during this process.

We recommend creating fault-tolerant clusters with replicas or adding replicas to existing clusters.

Node placement

The type of node placement in a cluster depends on the number of replicas in the cluster, the type of pool where the cluster is located, and the number of segments in the pool:

  • Single-Segment — in one segment of a single-AZ pool. Applicable to:

    • clusters without replicas;
    • clusters with replicas that are located in pools with only one segment;
    • clusters with replicas in HighFreq and Dedicated configuration lines;
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For example, you created a three-node cluster (one master node and two replicas) in the HighFreq configuration line in the ru-2 pool. All nodes will be placed in one segment of the ru-2c pool.

  • Multi-Segment — in different segments of a single-AZ pool. Applicable to clusters with replicas that are located in pools with multiple segments. Segments are located in different racks of the same data center. Nodes are distributed across segments sequentially;
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For example, you created a four-node cluster (one master node and three replicas) in the ru-1 pool. The first three nodes will be placed sequentially in segments of the ru-1a, ru-1b, and ru-1c pools. The fourth node will be placed in the ru-1a segment. If you add a fifth node, it will be placed in the ru-1b pool segment.

  • Single-AZ — in one segment of a multi-AZ pool. Applicable to clusters without replicas in the ru-6 pool. If you add a replica to the cluster, the node placement type will change to Multi-AZ;

  • Multi-AZ — in different segments of a multi-AZ pool. Applicable to clusters with replicas in the ru-6 pool. Each pool segment is located in a separate data center. Nodes are distributed across segments sequentially.

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For example, you created a four-node cluster (one master node and three replicas) in the ru-6 pool. The first three nodes will be placed sequentially in segments of the ru-6a, ru-6b, and ru-6c pools. The fourth node will be placed in the ru-6a segment. If you add a fifth node, it will be placed in the ru-6b pool segment.

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

Change the number of replicas

You can increase or decrease the number of replicas. While you are changing the number of replicas, the cluster continues to run.

  1. In the Dashboard, on the top menu, click Products and select Managed Databases.
  2. Open the Active tab.
  3. Open the cluster page → Settings tab.
  4. Click Change configuration.
  5. In the Fault tolerance block, in the Number of nodes field, enter the new number of replicas. If there are no free addresses in the subnet to which the cluster is connected, you cannot add a replica, as each new replica occupies a new address in the subnet.
  6. Click Save.