Troubleshooting Scenario Questions

Check the Cloud Logging logs for the subscription

Why wrong: Cloud Logging records discrete Pub/Sub events such as publish operations, ack/nack calls, and subscription configuration changes, but it does not expose continuous gauges like the current backlong size or the age of the oldest unacknowledged message. Logs are an audit trail of what happened over time, whereas backlog is a real-time state measured by a metric. Thus, checking logs may show whether messages were acknowledged, but it cannot directly tell you the current subscription backlog or consumer lag.

Use gcloud pubsub subscriptions describe and check the ackDeadlineSeconds

Why wrong: The `gcloud pubsub subscriptions describe` command returns the subscription's configuration, including `ackDeadlineSeconds`, push endpoint, message retention period, and topic binding, but it does not return any current runtime metrics. `ackDeadlineSeconds` defines the time window a subscriber has to acknowledge a message before it is redelivered; it is a setting, not a measurement of how many messages are awaiting acknowledgment or the age of the oldest one. Therefore, this command cannot reveal whether a subscription is currently backlogged, only how the acknowledgement timeout is configured.

Check the Cloud Console Pub/Sub dashboard for the topic publish rate

Why wrong: The Cloud Console Pub/Sub topic dashboard's publish rate shows the rate at which messages are published to the topic, which is an ingress metric and not a measure of unacknowledged work on a particular subscription. A topic can have a high publish rate while all subscriptions are keeping up, or a low publish rate while a stuck subscriber allows the oldest unacked message to age indefinitely. Backlog is a subscription-side phenomenon, so you need a subscription-level metric rather than a topic-level publish rate.

The instance's firewall rules block egress traffic to port 80.

Why wrong: The stem explicitly states that the VPC's firewall rules allow all egress traffic, so no firewall rule is blocking port 80 or any other outbound port. Cloud NAT is compatible with permissive egress rules; it does not require a specific allow rule beyond what permits the traffic itself. Thus, this option contradicts the given facts and cannot be the cause of the connectivity failure.

The instance's service account does not have the compute.instances.update permission.

Why wrong: The instance's service account is used for authentication and authorization to Google Cloud APIs, not to control network connectivity or NAT behavior. Cloud NAT does not inspect the instance's service account when forwarding traffic. The relevant identity would be the Cloud NAT's own service account, which the deployment likely already has, so a missing compute.instances.update permission on the instance is irrelevant.

The VPC does not have a route for the instance's subnet to the internet gateway.

Why wrong: A route to the internet gateway (the default 0.0.0.0/0 route) is indeed required for Cloud NAT to forward traffic, and the scenario confirms this route exists in the VPC. Since the route is present, this option is factually incorrect as an explanation. Even when the route exists, a NAT gateway in the wrong region will still prevent the instance from reaching the internet, so the root cause remains the regional mismatch.

The instances do not have external IP addresses.

Why wrong: Instances lacking external IP addresses are exactly the use case Cloud NAT is designed to solve. Cloud NAT allows outbound connections from private instances by translating their private source IPs to a NAT IP address, while still preventing inbound direct connections. Therefore, the absence of external IPs is not a misconfiguration; it is the expected prerequisite for using Cloud NAT.

The firewall rules block egress traffic.

Why wrong: Cloud NAT operates at the network layer (IP translation) and does not interfere with firewall rules; VPC firewall rules independently control egress traffic based on source/destination IPs, ports, and protocols. The implicit egress rule allows all outbound traffic, so unless a custom deny rule with a lower priority exists, egress is permitted. A firewall block would cause connection timeouts, not the specific NAT failure pattern described.

The subnet does not have Private Google Access enabled.

Why wrong: Private Google Access is a separate VPC feature that enables private instances to reach Google APIs and services (like Cloud Storage) through the internal IP ranges 199.36.153.4/30 and 199.36.153.8/30, and it does not provide general internet access. Cloud NAT handles general internet egress, and an instance can have Cloud NAT without Private Google Access. Thus, disabling Private Google Access does not prevent internet-bound traffic from using NAT.

kubectl ssh api-pod-7d4f9 -n production

Why wrong: kubectl ssh is not a real kubectl subcommand; the Kubernetes CLI has no built-in SSH functionality because Pods are not VMs and typically lack sshd. To get a shell in a container you must use `kubectl exec -it <pod> -n <namespace> -- <command>`, which uses the container runtime's exec API rather than a network SSH daemon. Even if `kubectl ssh` existed, it would not know how to negotiate container-level access, so this command would fail with an unknown command error.

kubectl run debug --image=busybox --attach=api-pod-7d4f9

Why wrong: `kubectl run` is used to create a new workload resource (a Pod, Deployment, or Job) from an image — it does not attach to an existing Pod. The `--attach` flag on `kubectl run` only controls whether the CLI attaches to the newly created container's output, and it requires a command to run; it cannot target another Pod like `api-pod-7d4f9`. Therefore this command would create a separate busybox Pod (or Deployment) rather than giving you a shell inside the existing API Pod, which is not what you want for debugging that specific Pod.

gcloud container exec api-pod-7d4f9 --namespace=production -- bash

Why wrong: `gcloud container exec` is not a valid command in the Google Cloud CLI; `gcloud container` manages GKE clusters (e.g., `gcloud container clusters list`, `gcloud container clusters get-credentials`), but it does not provide a direct way to execute commands inside a Pod. Pod-level access is always performed through `kubectl`, which talks to the Kubernetes API server after you have authenticated to the cluster. To achieve the same result, you would first run `gcloud container clusters get-credentials <cluster>` and then use `kubectl exec -it api-pod-7d4f9 -n production -- bash`.

Cloud Monitoring

Why wrong: Cloud Monitoring specializes in time-series metrics, uptime checks, dashboards, and alerting policies — not raw log viewing. While it can display logs-based metrics derived from Cloud Run log entries, the actual log messages themselves live in Cloud Logging. To inspect the logs generated by a Cloud Run service, you must use Cloud Logging's Logs Explorer, not Cloud Monitoring.

Error Reporting

Why wrong: Error Reporting ingests and aggregates only exception and error signals, using machine learning to cluster them into meaningful groups with stack traces, so it is not a general log viewer. It consumes error-like entries from Cloud Logging but is neither the storage location nor the interface for browsing the full set of Cloud Run logs. A typical Cloud Run service also emits info and debug logs that Error Reporting ignores entirely.

Cloud Trace

Why wrong: Cloud Trace measures latency and traces end-to-end request paths across services, recording spans with timing metadata rather than log content. It can show how long each call took in a Cloud Run request, but it will not show you the textual or structured logs your service emitted. For Cloud Run log viewing, Cloud Trace is a complementary debugging tool, not the destination for logs.

VPC firewall rules are blocking egress traffic.

Why wrong: VPC firewall rules blocking egress traffic would prevent all outbound packets from leaving the instance, but this instance has no external IP address assigned (no accessConfigs in the output). Even with permissive egress firewall rules, the instance cannot reach the internet because its packets lack a public source IP for return traffic. The output does not indicate any firewall deny logs; it simply shows an absent external connectivity path.

The instance needs a Cloud NAT gateway for outbound connectivity.

Why wrong: Cloud NAT is a managed service that enables outbound internet from instances without external IPs, but it is a solution, not the root cause of the connectivity failure. The command output shows the instance has no external IP, which is the immediate problem; Cloud NAT could be configured as a workaround, but the lack of a public IP is what the output reveals. Additionally, Cloud NAT would require proper routing and firewall rules, but the underlying missing component is the external IP assignment.

The subnetwork is misconfigured.

Why wrong: The subnetwork is misconfigured would affect settings like the IP address range, gateway, or route propagation, but the output shows a valid subnet with a proper internal IP assigned to the instance. A subnet misconfiguration typically impacts all instances in that subnet or causes IP address conflicts, not the absence of an external IP. The instance's interface has a private IP from a healthy subnet, so the issue lies solely in the missing public IP, not the subnet itself.