backend: Add instance_stats background job

Following #656 where we add the `instance_stats` table, we now implement a background task that populates this table every hour, with the purpose of committing a snapshot of live instance counts to the new database table. This task, using a Go ticker, is started at service startup and runs for the whole lifetime of the Nebraska server until it is stopped.

backend/pkg/api/instances.go

@@ -49,7 +49,7 @@ const (
 
 const (
 	validityInterval postgresDuration = "1 days"
-	defaultInterval  time.Duration    = 2 * time.Hour
+	defaultInterval  time.Duration    = time.Hour
 )
 
 // Instance represents an instance running one or more applications for which
@@ -642,6 +642,11 @@ func (api *API) instanceStatusHistoryQuery(instanceID, appID, groupID string, li
 		Limit(uint(limit))
 }
 
+// GetDefaultInterval returns the default interval used for instance stats queries.
+func (api *API) GetDefaultInterval() time.Duration {
+	return defaultInterval
+}
+
 // instanceStatsQuery returns a SelectDataset prepared to return all instances
 // that have been checked in during a given duration from a given time.
 func (api *API) instanceStatsQuery(t *time.Time, duration *time.Duration) *goqu.SelectDataset {
@@ -777,9 +782,9 @@ func (api *API) GetInstanceStatsByTimestamp(t time.Time) ([]InstanceStats, error
 	return instances, nil
 }
 
-// updateInstanceStats updates the instance_stats table with instances checked
+// UpdateInstanceStats updates the instance_stats table with instances checked
 // in during a given duration from a given time.
-func (api *API) updateInstanceStats(t *time.Time, duration *time.Duration) error {
+func (api *API) UpdateInstanceStats(t *time.Time, duration *time.Duration) error {
 	insertQuery, _, err := goqu.Insert(goqu.T("instance_stats")).
 		Cols("timestamp", "channel_name", "arch", "version", "instances").
 		FromQuery(api.instanceStatsQuery(t, duration)).

backend/pkg/api/instances_test.go

@@ -325,7 +325,7 @@ func TestUpdateInstanceStats(t *testing.T) {
 	ts := time.Now().UTC()
 	elapsed := ts.Sub(start)
 
-	err = a.updateInstanceStats(&ts, &elapsed)
+	err = a.UpdateInstanceStats(&ts, &elapsed)
 	assert.NoError(t, err)
 
 	instances, err = a.GetInstanceStats()
@@ -354,7 +354,7 @@ func TestUpdateInstanceStats(t *testing.T) {
 	ts3 := time.Now().UTC()
 	elapsed = ts3.Sub(ts2)
 
-	err = a.updateInstanceStats(&ts3, &elapsed)
+	err = a.UpdateInstanceStats(&ts3, &elapsed)
 	assert.NoError(t, err)
 
 	instances, err = a.GetInstanceStats()
@@ -387,7 +387,7 @@ func TestUpdateInstanceStatsNoArch(t *testing.T) {
 	// Use large duration to have some test coverage for durationToInterval
 	elapsed := 3*time.Hour + 45*time.Minute + 30*time.Second + 1000*time.Microsecond
 
-	err := a.updateInstanceStats(&ts, &elapsed)
+	err := a.UpdateInstanceStats(&ts, &elapsed)
 	assert.NoError(t, err)
 
 	instanceStats, err := a.GetInstanceStatsByTimestamp(ts)

backend/pkg/server/server.go

@@ -7,6 +7,7 @@ import (
 	"net/url"
 	"path"
 	"strings"
+	"time"
 
 	oapimiddleware "github.com/deepmap/oapi-codegen/pkg/middleware"
 	"github.com/getkin/kin-openapi/openapi3filter"
@@ -47,6 +48,7 @@ var (
 )
 
 // New takes the config and db connection to create the server and returns it.
+// It also starts a background job to update instance stats periodically.
 func New(conf *config.Config, db *db.API) (*echo.Echo, error) {
 	// Setup Echo Server
 	e := echo.New()
@@ -132,6 +134,24 @@ func New(conf *config.Config, db *db.API) (*echo.Echo, error) {
 		e.DefaultHTTPErrorHandler(err, c)
 	}
 	codegen.RegisterHandlers(e, handlers)
+
+	// setup background job for updating instance stats
+	go func() {
+		err := db.UpdateInstanceStats(nil, nil)
+		if err != nil {
+			logger.Err(err).Msg("Error updating instance stats")
+		}
+		ticker := time.NewTicker(db.GetDefaultInterval())
+		defer ticker.Stop()
+
+		for range ticker.C {
+			err := db.UpdateInstanceStats(nil, nil)
+			if err != nil {
+				logger.Err(err).Msg("Error updating instance stats")
+			}
+		}
+	}()
+
 	return e, nil
 }