short lived cache for /recent function

Change-Id: I762791b352161dc20b14739e492a3dee86a0f047

event_database/cloud_functions/recent.go

@@ -12,27 +12,70 @@ import (
 	"sort"
 	"strconv"
 	"strings"
+	"time"
 
 	"cloud.google.com/go/bigtable"
 )
 
+// 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 warmCache = map[string]map[string]string{}
+var lastCacheReset = time.Now()
+
 // query for last of each rowKey prefix
 func getLatestOfEachEmitterAddress(tbl *bigtable.Table, ctx context.Context, prefix string, keySegments int) map[string]string {
+	// get cache data for query
+	cachePrefix := prefix
+	if prefix == "" {
+		cachePrefix = "*"
+	}
+
+	var rowSet bigtable.RowSet
+	rowSet = bigtable.PrefixRange(prefix)
+	now := time.Now()
+	oneHourAgo := now.Add(-time.Duration(1) * time.Hour)
+	if oneHourAgo.Before(lastCacheReset) {
+		// cache is less than one hour old, use it
+		if cached, ok := warmCache[cachePrefix]; ok {
+			// use the highest possible sequence number as the range end.
+			maxSeq := "9999999999999999"
+			rowSets := bigtable.RowRangeList{}
+			for k, v := range cached {
+				start := fmt.Sprintf("%v:%v", k, v)
+				end := fmt.Sprintf("%v:%v", k, maxSeq)
+				rowSets = append(rowSets, bigtable.NewRange(start, end))
+			}
+			if len(rowSets) >= 1 {
+				rowSet = rowSets
+			}
+		}
+	} else {
+		// cache is more than hour old, don't use it, reset it
+		warmCache = map[string]map[string]string{}
+		lastCacheReset = now
+	}
 
 	mostRecentByKeySegment := map[string]string{}
-	err := tbl.ReadRows(ctx, bigtable.PrefixRange(prefix), func(row bigtable.Row) bool {
+	err := tbl.ReadRows(ctx, rowSet, func(row bigtable.Row) bool {
 
 		keyParts := strings.Split(row.Key(), ":")
 		groupByKey := strings.Join(keyParts[:2], ":")
-		mostRecentByKeySegment[groupByKey] = row.Key()
+		mostRecentByKeySegment[groupByKey] = keyParts[2]
 
 		return true
 		// TODO - add filter to only return rows created within the last 30(?) days
-	}, bigtable.RowFilter(bigtable.StripValueFilter()))
+	}, bigtable.RowFilter(
+		bigtable.ChainFilters(
+			bigtable.CellsPerRowLimitFilter(1),
+			bigtable.StripValueFilter(),
+		)))
 
 	if err != nil {
 		log.Fatalf("failed to read recent rows: %v", err)
 	}
+	// update the cache with the latest rows
+	warmCache[cachePrefix] = mostRecentByKeySegment
 	return mostRecentByKeySegment
 }
 
@@ -44,10 +87,10 @@ func fetchMostRecentRows(tbl *bigtable.Table, ctx context.Context, prefix string
 	// key/value pairs are the start/stop rowKeys for range queries
 	rangePairs := map[string]string{}
 
-	for _, highestSequenceKey := range latest {
-		rowKeyParts := strings.Split(highestSequenceKey, ":")
+	for prefixGroup, highestSequence := range latest {
+		rowKeyParts := strings.Split(prefixGroup, ":")
 		// convert the sequence part of the rowkey from a string to an int, so it can be used for math
-		highSequence, _ := strconv.Atoi(rowKeyParts[2])
+		highSequence, _ := strconv.Atoi(highestSequence)
 		lowSequence := highSequence - numRowsToFetch
 		// create a rowKey to use as the start of the range query
 		rangeQueryStart := fmt.Sprintf("%v:%v:%016d", rowKeyParts[0], rowKeyParts[1], lowSequence)