BigTable: in-memory cache for /totals endpoint

Change-Id: I48199951b42ab8d57bb12fef6b6b1e0da9f3392f

commit-id:13588560

event_database/cloud_functions/totals.go

@@ -25,6 +25,11 @@ type totalsResult struct {
 	DailyTotals  map[string]map[string]int
 }
 
+// warmCache keeps some data around between invocations, so that we don't have
+// to do a full table scan with each request.
+// https://cloud.google.com/functions/docs/bestpractices/tips#use_global_variables_to_reuse_objects_in_future_invocations
+var warmTotalsCache = map[string]map[string]map[string]int{}
+
 // derive the result index relevant to a row.
 func makeGroupKey(keySegments int, rowKey string) string {
 	var countBy string
@@ -68,6 +73,13 @@ func createCountsOfInterval(tbl *bigtable.Table, ctx context.Context, prefix str
 	// there will be a query for each previous day, plus today
 	intervalsWG.Add(numPrevDays + 1)
 
+	// create the unique identifier for this query, for cache
+	cachePrefix := prefix
+	if prefix == "" {
+		cachePrefix = "*"
+	}
+	cachePrefix = fmt.Sprintf("%v-%v", cachePrefix, keySegments)
+
 	for daysAgo := 0; daysAgo <= numPrevDays; daysAgo++ {
 		go func(tbl *bigtable.Table, ctx context.Context, prefix string, daysAgo int) {
 			// start is the SOD, end is EOD
@@ -86,6 +98,35 @@ func createCountsOfInterval(tbl *bigtable.Table, ctx context.Context, prefix str
 			start := time.Date(year, month, day, 0, 0, 0, 0, loc)
 			end := time.Date(year, month, day, 23, 59, 59, maxNano, loc)
 
+			dateStr := start.Format("2006-01-02")
+
+			mu.Lock()
+			// initialize the map for this date in the result set
+			results[dateStr] = map[string]int{"*": 0}
+			// check to see if there is cache data for this date/query
+			if dateCache, ok := warmTotalsCache[dateStr]; ok {
+				// have a cache for this date
+
+				if val, ok := dateCache[cachePrefix]; ok {
+					// have a cache for this query
+					if daysAgo >= 1 {
+						// only use the cache for yesterday and older
+						results[dateStr] = val
+						mu.Unlock()
+						intervalsWG.Done()
+						return
+					}
+				} else {
+					// no cache for this query
+					warmTotalsCache[dateStr][cachePrefix] = map[string]int{}
+				}
+			} else {
+				// no cache for this date, initialize the map
+				warmTotalsCache[dateStr] = map[string]map[string]int{}
+				warmTotalsCache[dateStr][cachePrefix] = map[string]int{}
+			}
+			mu.Unlock()
+
 			var result []bigtable.Row
 			var fetchErr error
 
@@ -96,12 +137,6 @@ func createCountsOfInterval(tbl *bigtable.Table, ctx context.Context, prefix str
 				log.Fatalf("fetchRowsInInterval returned an error: %v", fetchErr)
 			}
 
-			dateStr := start.Format("2006-01-02")
-			mu.Lock()
-			// initialize the map for this date in the result set
-			if results[dateStr] == nil {
-				results[dateStr] = map[string]int{"*": 0}
-			}
 			// iterate through the rows and increment the count
 			for _, row := range result {
 				countBy := makeGroupKey(keySegments, row.Key())
@@ -114,10 +149,14 @@ func createCountsOfInterval(tbl *bigtable.Table, ctx context.Context, prefix str
 				// add this key to the set
 				seenKeySet[countBy] = true
 			}
-			mu.Unlock()
+
+			// set the result in the cache
+			warmTotalsCache[dateStr][cachePrefix] = results[dateStr]
 		}(tbl, ctx, prefix, daysAgo)
 	}
 
+	intervalsWG.Wait()
+
 	// ensure each date object has the same keys:
 	for _, v := range results {
 		for key := range seenKeySet {
@@ -127,7 +166,6 @@ func createCountsOfInterval(tbl *bigtable.Table, ctx context.Context, prefix str
 			}
 		}
 	}
-	intervalsWG.Wait()
 
 	return results, nil
 }