[ws-rollout] Add prometheus init check

When given a non-usable `--prometheus-url`, We start the rollout without verifying if the prometheus is reachable or not. This is a problem as we will be unable to get the metrics from prometheus and hence the rollout will be reverted later causing unnecessary time waste. This can be prevented by performing a simple check to see if the prometheus is reachable or not. `up` query is used instead of key metrics as we can't be sure of their existence. Signed-off-by: Tarun Pothulapati <tarun@gitpod.io>

Author: Pothulapati

components/workspace-rollout-job/cmd/root.go

@@ -96,6 +96,13 @@ var rootCmd = &cobra.Command{
 return err
 }
 
+// Check if prometheus is reachable
+err = analysis.CheckPrometheusReachable(ctx, conf.prometheusURL)
+if err != nil {
+log.WithError(err).Fatal("init: prometheus is not reachable")
+return err
+}
+
 prometheusAnalyzer, err := analysis.NewWorkspaceKeyMetricsAnalyzer(ctx, config, conf.prometheusURL, conf.targetPositivePercentage, 30305)
 if err != nil {
 log.WithError(err).Fatal("failed to create a prometheus client")

components/workspace-rollout-job/pkg/analysis/analyzer.go

@@ -6,6 +6,10 @@ package analysis
 
 import (
 "context"
+"time"
+
+"github.com/prometheus/client_golang/api"
+v1 "github.com/prometheus/client_golang/api/prometheus/v1"
 )
 
 type Decision int
@@ -21,3 +25,24 @@ type Analyzer interface {
 // repeatedly to determine whether to move forward on the rollout or not.
 MoveForward(ctx context.Context, clusterName string) (Decision, error)
 }
+
+func CheckPrometheusReachable(ctx context.Context, prometheusURL string) error {
+if prometheusURL == "" {
+return nil
+}
+
+client, err := api.NewClient(api.Config{
+Address: prometheusURL,
+})
+if err != nil {
+return err
+}
+
+v1Client := v1.NewAPI(client)
+_, _, err = v1Client.Query(ctx, "up", time.Now())
+if err != nil {
+return err
+}
+
+return nil
+}