Adding alias support for elastic mapping caching in Segments API

If alias is used for index name on elastic server, Segments API couldn't
get mapping information for that segment. If such case occurs, db_elastic.go
tries to get available aliases for requested index as a fallback
and use latest index (alphabetically) to get mapping information.

Beam/go/model/db_elastic.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"math"
+	"sort"
 	"strconv"
 	"time"
 
@@ -503,10 +504,27 @@ func (eDB *ElasticDB) cacheFieldMapping(index string) (map[string]string, error)
 		return nil, errors.Wrap(err, fmt.Sprintf("unable to get field mappings for index: %s", index))
 	}
 
+	// read mapping info of index
 	root, ok := result[index].(map[string]interface{})
 	if !ok {
-		return nil, fmt.Errorf("invalid index provided, no mapping data available: %s", index)
+		// there's no such index, but there might be an alias
+		aliases, err := eDB.Client.Aliases().Index(index).Do(eDB.Context)
+		if err != nil {
+			return nil, errors.Wrap(err, fmt.Sprintf("unable to get aliases for index: %s", index))
+		}
+
+		indices := aliases.IndicesByAlias(index)
+		if len(indices) == 0 {
+			return nil, fmt.Errorf("invalid index provided, no mapping data available: %s", index)
+		}
+
+		sort.Strings(indices)
+		root, ok = result[indices[len(indices)-1]].(map[string]interface{})
+		if !ok {
+			return nil, fmt.Errorf("invalid alias provided, no mapping data available: %s", index)
+		}
 	}
+
 	mappings, ok := root["mappings"].(map[string]interface{})
 	if !ok {
 		return nil, fmt.Errorf("\"mappings\" field not present within field mapping response")