The majority quorum strategy provides automatic partition tolerance that adapts to cluster size.

Experimental

This strategy is experimental and may change in future releases. Use in production at your own discretion.

This strategy is ideal for clusters that:

• Start with a single node (development or initial deployment)
• Scale up over time
• Want to avoid manual quorum configuration

How It Works

The majority quorum strategy tracks the maximum cluster size ever seen and requires a majority of those nodes to be present for the cluster to operate.

The quorum is calculated as: floor(max_cluster_size / 2) + 1

Example Progression

1 node  → quorum = 1 (can operate)
2 nodes → quorum = 2 (both needed)
3 nodes → quorum = 2 (majority)
5 nodes → quorum = 3 (majority)

Split-Brain Protection

If a 5-node cluster splits into partitions of 3 and 2 nodes:

• The partition with 3 nodes maintains quorum and continues operating
• The partition with 2 nodes loses quorum and enters partition mode (distributed supervisor and all children are terminated)

Configuration

partition_tolerance_strategy: %ProcessHub.Strategy.PartitionTolerance.MajorityQuorum{
  initial_cluster_size: 1,  # Default: 1 (works with single node)
  track_max_size: true      # Default: true (adapts to cluster growth)
}

Intentional Downsizing

Scaling Down Requires Manual Intervention

MajorityQuorum does not automatically handle cluster scale-down. The strategy remembers the maximum cluster size to protect against split-brain scenarios. If you scale down nodes (e.g., from 5 to 3 nodes), the cluster will still require a majority of the original 5 nodes (i.e., 3 nodes) to operate.

For automatic adaptation to gradual scale-down, consider using ProcessHub.Strategy.PartitionTolerance.DynamicQuorum instead.

To reset the expected cluster size after an intentional downsize:

ProcessHub.Strategy.PartitionTolerance.MajorityQuorum.reset_cluster_size(:my_hub, 3)

Options

• initial_cluster_size - The initial expected cluster size. Default: 1. Used as the starting point for quorum calculation until a larger cluster is seen.

• track_max_size - Whether to automatically track and remember the maximum cluster size ever seen. Default: true. If set to false, the quorum will always be calculated based on initial_cluster_size.