orc: cross_cell durability

Signed-off-by: Sugu Sougoumarane <ssougou@gmail.com>

config/orchestrator/default.json

@@ -1,5 +1,6 @@
 {
   "Debug": true,
+  "Durability": "cross_cell",
   "MySQLTopologyUser": "orc_client_user",
   "MySQLTopologyPassword": "orc_client_user_password",
   "MySQLReplicaUser": "vt_repl",

go/vt/orchestrator/inst/durability.go

@@ -23,6 +23,8 @@ import (
 	topodatapb "vitess.io/vitess/go/vt/proto/topodata"
 )
 
+//=======================================================================
+
 var (
 	durabilityPolicies  = make(map[string]durabler)
 	curDurabilityPolicy durabler
@@ -31,6 +33,7 @@ var (
 func init() {
 	registerDurability("none", &durabilityNone{})
 	registerDurability("semi_sync", &durabilitySemiSync{})
+	registerDurability("cross_cell", &durabilityCrossCell{})
 }
 
 type durabler interface {
@@ -46,6 +49,8 @@ func registerDurability(name string, d durabler) {
 	durabilityPolicies[name] = d
 }
 
+//=======================================================================
+
 func SetDurabilityPolicy(name string) error {
 	curDurabilityPolicy = durabilityPolicies[name]
 	if curDurabilityPolicy == nil {
@@ -85,6 +90,8 @@ func ReplicaSemiSyncFromTablet(master, replica *topodatapb.Tablet) bool {
 	return curDurabilityPolicy.replicaSemiSync(master, replica)
 }
 
+//=======================================================================
+
 type durabilityNone struct{}
 
 func (d *durabilityNone) promotionRule(tablet *topodatapb.Tablet) CandidatePromotionRule {
@@ -103,6 +110,8 @@ func (d *durabilityNone) replicaSemiSync(master, replica *topodatapb.Tablet) boo
 	return false
 }
 
+//=======================================================================
+
 type durabilitySemiSync struct{}
 
 func (d *durabilitySemiSync) promotionRule(tablet *topodatapb.Tablet) CandidatePromotionRule {
@@ -124,3 +133,31 @@ func (d *durabilitySemiSync) replicaSemiSync(master, replica *topodatapb.Tablet)
 	}
 	return false
 }
+
+//=======================================================================
+
+type durabilityCrossCell struct{}
+
+func (d *durabilityCrossCell) promotionRule(tablet *topodatapb.Tablet) CandidatePromotionRule {
+	switch tablet.Type {
+	case topodatapb.TabletType_MASTER, topodatapb.TabletType_REPLICA:
+		return NeutralPromoteRule
+	}
+	return MustNotPromoteRule
+}
+
+func (d *durabilityCrossCell) masterSemiSync(instanceKey InstanceKey) int {
+	return 1
+}
+
+func (d *durabilityCrossCell) replicaSemiSync(master, replica *topodatapb.Tablet) bool {
+	// Prevent panics.
+	if master.Alias == nil || replica.Alias == nil {
+		return false
+	}
+	switch replica.Type {
+	case topodatapb.TabletType_MASTER, topodatapb.TabletType_REPLICA:
+		return master.Alias.Cell != replica.Alias.Cell
+	}
+	return false
+}