Vitess now supports a configurable interface for durability policies. Users can now define, in the interface, which tablets are eligible to be promoted to a PRIMARY instance. They can also specify the number of semi-sync acknowledgements it requires and the tablets which are eligible to send these acknowledgements.

The interface definition looks like:

// Durabler is the interface which is used to get the promotion rules for candidates and the semi sync setup
type Durabler interface {
	promotionRule(*topodatapb.Tablet) promotionrule.CandidatePromotionRule
	semiSyncAckers(*topodatapb.Tablet) int
	isReplicaSemiSync(primary, replica *topodatapb.Tablet) bool
}

There are 3 implementations supported in this release:

• semi_sync - This durability policy sets the number of required semi-sync acknowledgements to 1. It only allows REPLICA type tablets to send semi-sync acknowledgements. It returns NeutralPromoteRule for REPLICA tablet types, MustNotPromoteRule for everything else.
• semi_sync_with_rdonly_ack - This durability policy sets the number of required semi-sync acknowledgements to 1. It allows REPLICA and RDONLY type tablets to acknowledge semi sync. It returns NeutralPromoteRule for REPLICA tablet types, MustNotPromoteRule for everything else.
• none (default) - This durability policy does not set any semi-sync configuration. It returns NeutralPromoteRule for REPLICA tablet types, MustNotPromoteRule for everything else
• cross_cell - This durability policy sets the number of required semi-sync acknowledgements to 1. It only allows REPLICA type tablets from a different cell than the current primary to send semi-sync acknowledgements. It returns NeutralPromoteRule for REPLICA tablet types, MustNotPromoteRule for everything else.
• cross_cell_with_rdonly_ack - This durability policy sets the number of required semi-sync acknowledgements to 1. It only allows REPLICA and RDONLY type tablets from a different cell than the current primary to send semi-sync acknowledgements. It returns NeutralPromoteRule for REPLICA tablet types, MustNotPromoteRule for everything else.

EmergencyReparentShard and PlannedReparentShard will use the durability rules while choosing the correct candidate for promotion.

This configuration must be stored in the topo server in the keyspace record using the command CreateKeyspace or SetKeyspaceDurabilityPolicy.