A quorum is the minimum number of nodes that a distributed cluster has to obtain in order to be allowed to perform an operation. This can be used to enforce consistent operation in a distributed system.

Quorum size

You must configure this distribution strategy and specify its minimum quorum size:

config :swarm,
  distribution_strategy: Swarm.Distribution.StaticQuorumRing,
  static_quorum_size: 5

It defines the minimum number of nodes that must be connected in the cluster to allow process registration and distribution.

If there are fewer nodes currently available than the quorum size, any calls to Swarm.register_name/5 will return {:error, :no_node_available} until enough nodes have started.

You can configure the :kernel application to wait for cluster formation before starting your application during node start up. The sync_nodes_optional configuration specifies which nodes to attempt to connect to within the sync_nodes_timeout window, defined in milliseconds, before continuing with startup. There is also a sync_nodes_mandatory setting which can be used to enforce all nodes are connected within the timeout window or else the node terminates.

config :kernel,
  sync_nodes_optional: [:"node1@192.168.1.1", :"node2@192.168.1.2"],
  sync_nodes_timeout: 60_000

The sync_nodes_timeout can be configured as :infinity to wait indefinitely for all nodes to connect. All involved nodes must have the same value for sync_nodes_timeout.

Example

In a 9 node cluster you would configure the :static_quorum_size as 5. During a network split of 4 and 5 nodes, processes on the side with 5 nodes will continue running, whereas processes on the other 4 nodes will be stopped.

Be aware that in the running 5 node cluster, no more failures can be handled because the remaining cluster size would be less than the required 5 node minimum. All running processes would be stopped in the case of another single node failure.