Add database_routing_tag for InfluxDB output plugin

Allows user to configure a tag that, if it exists, will be used as
the database name for the metric.

X-Github-Closes #397

outputs/influxdb/influxdb.go

@@ -17,6 +17,7 @@ type InfluxDB struct {
 	// URL is only for backwards compatability
 	URL          string
 	URLs         []string `toml:"urls"`
+	DBRoutingTag string   `toml:"database_routing_tag"`
 	Username     string
 	Password     string
 	Database     string
@@ -49,6 +50,10 @@ var sampleConfig = `
   # user_agent = "telegraf"
   # Set UDP payload size, defaults to InfluxDB UDP Client default (512 bytes)
   # udp_payload = 512
+
+  # Route metrics to an InfluxDB database based on value of a tag
+  # note: if this tag has many unique values, write performance may suffer
+  # database_routing_tag = "mytag"
 `
 
 func (i *InfluxDB) Connect() error {
@@ -129,27 +134,53 @@ func (i *InfluxDB) Description() string {
 // Choose a random server in the cluster to write to until a successful write
 // occurs, logging each unsuccessful. If all servers fail, return error.
 func (i *InfluxDB) Write(points []*client.Point) error {
-	bp, _ := client.NewBatchPoints(client.BatchPointsConfig{
+	var bps map[string]client.BatchPoints = make(map[string]client.BatchPoints)
+
+	// Default database
+	bps["___default"], _ = client.NewBatchPoints(client.BatchPointsConfig{
 		Database:  i.Database,
 		Precision: i.Precision,
 	})
 
 	for _, point := range points {
-		bp.AddPoint(point)
+		if i.DBRoutingTag != "" {
+			if h, ok := point.Tags()[i.DBRoutingTag]; ok {
+				// Create a new batch point config for this tag value
+				if _, ok := bps[h]; !ok {
+					bps[h], _ = client.NewBatchPoints(client.BatchPointsConfig{
+						Database:  h,
+						Precision: i.Precision,
+					})
+				}
+				bps[h].AddPoint(point)
+			}
+		}
+
+		// Nothing found in overrides, lets push this into the default bucket
+		bps["___default"].AddPoint(point)
 	}
 
 	// This will get set to nil if a successful write occurs
 	err := errors.New("Could not write to any InfluxDB server in cluster")
 
+	for k, bp := range bps {
 		p := rand.Perm(len(i.conns))
 		for _, n := range p {
 			if e := i.conns[n].Write(bp); e != nil {
 				log.Println("ERROR: " + e.Error())
+
+				// Stop trying immediately if the error is for a missing database
+				// and we are trying a database routing tag
+				if k != "___default" && strings.HasPrefix(e.Error(), "database not found") {
+					err = nil
+					break
+				}
 			} else {
 				err = nil
 				break
 			}
 		}
+	}
 	return err
 }