Introduction

GCP Pub/Sub message delivery delayed when subscription backlog or flow control misconfigured. This guide provides step-by-step diagnosis and resolution.

Symptoms

Typical error output:

bash
Error: Subscription backlog exceeded threshold
Backlog size: 500000 messages
Ack deadline: 10s (too short for processing time)

Common Causes

  1. 1.Subscriber processing slower than publishing rate
  2. 2.Ack deadline shorter than processing time
  3. 3.Flow control settings too restrictive
  4. 4.Dead letter policy misconfigured

Step-by-Step Fix

Step 1: Check Current State

bash
gcloud pubsub subscriptions describe my-sub --project=my-project
gcloud pubsub topics describe my-topic
gcloud logging read "resource.type=pubsub_subscription"

Step 2: Identify Root Cause

bash
gcloud logging read --project=<project> --filter="severity>=ERROR"

Step 3: Apply Primary Fix

```bash # Update subscription ack deadline gcloud pubsub subscriptions update my-sub --ack-deadline=60 --max-delivery-attempts=5 --dead-letter-topic=projects/my-project/topics/my-dlq

# Enable exactly-once delivery gcloud pubsub subscriptions update my-sub --enable-exactly-once-delivery ```

Step 4: Apply Alternative Fix

```bash # Alternative fix: Check configuration gcloud resource describe <resource> --project=<project> --format=yaml

# Update specific properties gcloud resource update <resource> --project=<project> --<flag>=<value>

# Verify the fix gcloud resource describe <resource> --project=<project> --format=json ```

Step 5: Verify the Fix

bash
gcloud pubsub subscriptions describe my-sub --format="value(ackDeadlineSeconds)"
gcloud monitoring metrics list --resource-type=pubsub_subscription

Common Pitfalls

  • Forgetting to check regional quotas before provisioning
  • Not waiting for async operations to complete before next step
  • Missing IAM permissions for GCP resource operations
  • Confusing zone-level and region-level resources

Best Practices

  • Always check quotas before provisioning new resources
  • Use GCP Cloud Monitoring for observability
  • Implement proper error handling in gcloud scripts
  • Enable logging for all critical GCP resources
  • GCP Quota Exceeded
  • GCP Resource Deployment Failed
  • GCP Network Connectivity Issues
  • GCP IAM Permission Denied