Galera Cluster Deployment Variants

MariaDB Galera Cluster is flexible and can be deployed in several different topologies to meet various business needs, from high availability within a single data center to geographically distributed disaster recovery. The primary deployment patterns are designed for Local Area Networks (LAN) and Wide Area Networks (WAN).

Standard LAN Cluster (Single Data Center)

This is the most common deployment pattern for achieving high availability and read scaling within a single data center.

Topology

The cluster consists of an odd number of nodes (typically 3 or 5) located in the same data center, connected by a high-speed, low-latency network.

Purpose

Purpose	Description
High Availability	The cluster can survive the failure of one or more nodes (depending on its size) without interruption of service.
Read Scalability	Application read traffic can be distributed across all nodes in the cluster.
Maintenance	Nodes can be taken offline for maintenance (e.g., software upgrades) without causing downtime.

Key Consideration

While this provides excellent protection against server failure, the entire cluster is vulnerable to a full data center outage.

Wide Area Network (WAN) Cluster (Multi-Data Center)

This pattern is designed for disaster recovery and providing lower latency for a geographically distributed user base.

Topology

The cluster nodes are distributed across two or more physical locations (data centers), connected by a WAN link. To maintain quorum, it is essential to have an odd number of nodes and an odd number of locations. A typical setup involves three data centers with one or more nodes in each.

Purpose:

Aspect	Description
Disaster Recovery	If one data center experiences a complete outage, the cluster can maintain quorum and continue to operate from the remaining locations.
Reduced Latency	Client applications can be directed to the topologically closest node, reducing network latency for read operations.

Key Considerations:

Consideration	Description
Network Latency	WAN replication is sensitive to network latency. The round-trip time between the most distant nodes will set the baseline for transaction commit latency. High latency affects writes.
Network Stability	The WAN link must be stable and reliable. Frequent network partitions can lead to nodes being evicted from the cluster, impacting stability.
Segments	Nodes within the same location can be configured into a segment using the gmcast.segment parameter, optimizing communication by using the fast local network for replication.

Two-Node Cluster with a Galera Arbitrator

This is a special deployment variant used to achieve high availability with only two full data nodes.

Topology

The cluster consists of two MariaDB Galera nodes and one lightweight Galera Arbitrator (garbd) process, which runs on a third, separate machine.

Purpose:

Cost-Effective High Availability: It provides automatic failover for a two-node cluster without the resource cost of a third full database server.

How it Works

The Galera Arbitrator acts as a voting member for quorum calculations but does not store any data or handle any client traffic. This creates a 3-member cluster from a voting perspective. If one of the data nodes fails, the remaining data node and the arbitrator still form a majority (2 out of 3), allowing the cluster to maintain quorum and stay online.

Key Consideration

If the node running the arbitrator fails, the cluster reverts to a standard 2-node setup with no automatic failover capability. Therefore, the arbitrator itself should be monitored.

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