Durability Policy

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 ACKs it requires and the tablets which are eligible to send these ACKs.

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 2 implementations supported in this release:

  • semi_sync - This durability policy sets the number of required semi-sync ACKers to 1. It only allows Primary and Replica type servers to acknowledge semi sync. It returns NeutralPromoteRule for Primary and Replica tablet types, MustNotPromoteRule for everything else
  • none (default) - This durability policy does not set any semi-sync configurations. It returns NeutralPromoteRule for Primary and Replica tablet types, MustNotPromoteRule for everything else

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

This configuration should be specified in vtctld, vtctl and vtworker as a flag -durability_policy. It should be specified in vtorc as Durability config.

Currently the durability policies are not used to setup semi-sync in EmergencyReparentShard or PlannedReparentShard. All the RPCs are still using the -enable_semi_sync flag on vttablet to setup semi-sync. This flag is currently being used for promotion rules and to log discrepancies in semi-sync setup. Nonetheless, this flag should be specified correctly for upgrade considerations to future releases when the durability policies will be used to setup semi-sync and -enable_semi_sync is deprecated.

In case you notice any logs that look like the following, please check that your vtctld and vttablet configurations match:

invalid configuration - semi-sync should be setup according to durability policies, but enable_semi_sync is not set

invalid configuration - semi-sync should be setup according to durability policies, but the tablet is not primaryEligible

If -enable_semi_sync is set on the vttablets, then semi_sync durability policy should be used. If semi-sync is not being used then -durability_policy should be set to none.

If the configurations are in order , then you should create an issue here and report it.

If the following log is noticed when all the components are upgraded, then it should also be reported:

invalid configuration - enabling semi sync even though not specified by durability policies. Possibly in the process of upgrading
Durability Policy