Vitess: ignore unhealthy replicas with realtime stats (#136)

* Ignore Vitess replicas without running replication

* Fix typo

* Method rename

* Add to comment

* Copy logic vtgate uses to filter tablets, minus lag+tablet count

* Only check if &TabletRealtimeStats{} is nil, not the HealthError

* Add test for nil realtime stats

* Do not ignore 'serving: false' tablets

* Improve test comments

pkg/vitess/api_client.go

@@ -15,11 +15,25 @@ import (
 
 const defaultTimeout = time.Duration(5) * time.Second
 
+// TabletRealtimeStats represents realtime stats from a running instance of vttablet.
+type TabletRealtimeStats struct {
+	HealthError string `json:"health_error,omitempty"`
+}
+
+// TabletStats represents stats from a running instance of vttablet.
+type TabletStats struct {
+	LastError string               `json:"last_error,omitempty"`
+	Realtime  *TabletRealtimeStats `json:"realtime,omitempty"`
+	Serving   bool                 `json:"serving,omitempty"`
+	Up        bool                 `json:"up,omitempty"`
+}
+
 // Tablet represents information about a running instance of vttablet.
 type Tablet struct {
 	Alias         *topodata.TabletAlias `json:"alias,omitempty"`
 	MysqlHostname string                `json:"mysql_hostname,omitempty"`
 	MysqlPort     int32                 `json:"mysql_port,omitempty"`
+	Stats         *TabletStats          `json:"stats,omitempty"`
 	Type          topodata.TabletType   `json:"type,omitempty"`
 }
 
@@ -36,9 +50,23 @@ func (t Tablet) HasValidCell(validCells []string) bool {
 	return false
 }
 
+// IsServeable returns a bool reflecting if a tablet is eligible to serve traffic based on tablet stats. For
+// backwards-compatibilty tablets are assumed to be healthy if realtime stats is disabled. This method aims
+// to mimic the logic used by vtgate to select tablets for read queries without considering 'serving', minimum
+// tablet count (not important to freno) and replication lag (freno polls its own replication lag)
+func (t Tablet) IsServeable() bool {
+	if t.Stats != nil {
+		return t.Stats.LastError == "" && t.Stats.Realtime != nil
+	}
+	return true
+}
+
 // IsValidReplica returns a bool reflecting if a tablet type is REPLICA
 func (t Tablet) IsValidReplica() bool {
-	return t.Type == topodata.TabletType_REPLICA
+	if t.Type != topodata.TabletType_REPLICA {
+		return false
+	}
+	return t.IsServeable()
 }
 
 var httpClient = http.Client{

pkg/vitess/api_client_test.go

@@ -2,7 +2,6 @@ package vitess
 
 import (
 	"encoding/json"
-	"fmt"
 	"net/http"
 	"net/http/httptest"
 	"testing"
@@ -16,44 +15,68 @@ func TestParseTablets(t *testing.T) {
 	vitessApi := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		switch r.URL.String() {
 		case "/api/keyspace/test/tablets/00", "/api/keyspace/test/tablets/00?cells=cell2":
-			data, _ := json.Marshal([]Tablet{
+			json.NewEncoder(w).Encode([]Tablet{
 				{
-					Alias:         &topodata.TabletAlias{Cell: "cell1"},
-					MysqlHostname: "master",
-					Type:          topodata.TabletType_MASTER,
+					// primary (should be ignored)
+					Alias: &topodata.TabletAlias{Cell: "cell1"},
+					Type:  topodata.TabletType_MASTER,
 				},
 				{
+					// replica without realtime tablet stats enabled (assumed to be healthy)
 					Alias:         &topodata.TabletAlias{Cell: "cell2"},
 					MysqlHostname: "replica1",
 					Type:          topodata.TabletType_REPLICA,
 				},
 				{
+					// replica with healthy realtime tablet stats
 					Alias:         &topodata.TabletAlias{Cell: "cell3"},
 					MysqlHostname: "replica2",
-					Type:          topodata.TabletType_REPLICA,
+					Stats: &TabletStats{
+						Realtime: &TabletRealtimeStats{},
+					},
+					Type: topodata.TabletType_REPLICA,
+				},
+				{
+					// replica with nil realtime stats (should be ignored)
+					Alias:         &topodata.TabletAlias{Cell: "cell1"},
+					MysqlHostname: "replica3",
+					Stats: &TabletStats{
+						Realtime: nil,
+					},
 				},
 				{
+					// replica with realtime tablet stats and 'replication not running' error (should be ignored)
 					Alias:         &topodata.TabletAlias{Cell: "cell2"},
-					MysqlHostname: "spare",
-					Type:          topodata.TabletType_SPARE,
+					MysqlHostname: "replica4",
+					Stats: &TabletStats{
+						LastError: "vttablet error: replication is not running",
+						Realtime: &TabletRealtimeStats{
+							HealthError: "replication is not running",
+						},
+					},
+					Type: topodata.TabletType_REPLICA,
 				},
 				{
-					Alias:         &topodata.TabletAlias{Cell: "cell3"},
-					MysqlHostname: "batch",
-					Type:          topodata.TabletType_BATCH,
+					// spare tablet (should be ignored)
+					Alias: &topodata.TabletAlias{Cell: "cell2"},
+					Type:  topodata.TabletType_SPARE,
 				},
 				{
-					Alias:         &topodata.TabletAlias{Cell: "cell2"},
-					MysqlHostname: "backup",
-					Type:          topodata.TabletType_BACKUP,
+					// batch tablet (should be ignored)
+					Alias: &topodata.TabletAlias{Cell: "cell3"},
+					Type:  topodata.TabletType_BATCH,
 				},
 				{
-					Alias:         &topodata.TabletAlias{Cell: "cell1"},
-					MysqlHostname: "restore",
-					Type:          topodata.TabletType_RESTORE,
+					// backup tablet (should be ignored)
+					Alias: &topodata.TabletAlias{Cell: "cell2"},
+					Type:  topodata.TabletType_BACKUP,
+				},
+				{
+					// restore tablet (should be ignored)
+					Alias: &topodata.TabletAlias{Cell: "cell1"},
+					Type:  topodata.TabletType_RESTORE,
 				},
 			})
-			fmt.Fprint(w, string(data))
 		default:
 			w.WriteHeader(http.StatusNotFound)
 		}