Improve probe setup

Liveness probe is now a TCP probe. This has the effect that if the
server is overloaded and does not respond to HTTP requests in time that
it will not be killed as long as the server still listens on the TCP
port (i.e. the server is actually alive). Therefore it can finish the
requests (this shouldn't take forever due to gunicorn request timeouts).

The readiness probe now is an HTTP probe, so if the server is
overloaded, it won't receive any new requests.

Instead of initialDelaySeconds, a startup HTTP probe is used which
queries the service every second, so that it's marked as available as
soon as it's actually available. This is vital for one of our use cases
where we scale the service to zero and only activate it on the first
request, i.e. users wait for the service to actually become available.

helm-chart/eoapi/templates/services/deployment.yaml

@@ -40,6 +40,13 @@ spec:
           - "--root-path=/{{ $serviceName }}{{ $.Release.Name }}"
           {{- end }}{{/* needed for proxies and path rewrites on NLB */}}
         livenessProbe:
+          tcpSocket:
+            port: {{ $.Values.service.port }}
+          failureThreshold: 3
+          periodSeconds: 15
+          successThreshold: 1
+          timeoutSeconds: 1
+        readinessProbe:
           httpGet:
             {{- if (eq $serviceName "stac") }}
             path: /_mgmt/ping
@@ -48,9 +55,20 @@ spec:
             {{- end }}
             port: {{ $.Values.service.port }}
           failureThreshold: 3
-          initialDelaySeconds: 30
           periodSeconds: 15
           successThreshold: 1
+        startupProbe:
+          httpGet:
+            {{- if (eq $serviceName "stac") }}
+            path: /_mgmt/ping
+            {{- else }}
+            path: /healthz
+            {{- end }}
+            port: {{ $.Values.service.port }}
+          # check every sec for 1 minute
+          periodSeconds: 1
+          failureThreshold: 60
+          successThreshold: 1
         ports:
           - containerPort: {{ $.Values.service.port }}
         resources: