In an Eon Mode database, nodes in a subcluster subscribe to one or more shards in communal storage. Each node handles the data in the shard or shards it subscribes to when processing queries. When a subcluster has at least one node subscribed to each shard, it has shard coverage. It is able to process queries because it has access to all of the data in the database. In a K-safe database, each subcluster has at least two nodes subscribing to each node. If the subcluster loses too many nodes in rapid succession, it may no longer have a node subscribing to each shard. In this case, the subcluster loses shard coverage and cannot process queries.
The database cluster as a whole must have at least one primary node subscribed to each shard in communal storage. This state is called having primary shard coverage. Primary nodes coordinate the maintenance of data in communal storage. If the cluster loses primary nodes to the point that it does not have at least one subscribed to each shard, it loses primary shard coverage. In this case, the database goes into read-only mode because it is unsafe to alter the data in communal storage when one or more shards do not have a primary node subscriber.