PCD Designing highly scalable, available, and reliable cloud-native applications — Timed 40 Questions

Question 1

A company running a high-traffic e-commerce platform on Google Cloud experiences occasional data loss in their Cloud SQL database during failover events. The database is configured with a failover replica in a different zone. What is the most likely cause of the data loss?

40 minute time limit — choose an answer to begin.

Accepted Answer

The database is using asynchronous replication to the failover replica.. Cloud SQL uses synchronous replication for failover replicas by default, ensuring that transactions are committed on both the primary and the replica before acknowledging the write. If asynchronous replication is configured, the replica may lag behind the primary, and during a failover, any transactions not yet replicated are lost. This is the most likely cause of data loss during failover events.

Answer

Automated backups are not enabled.

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The failover replica is configured as a read replica instead of a failover replica.

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The database is not using regional persistent disks.

Question 2

An organization wants to design a serverless data processing pipeline that is highly available and can automatically scale based on the number of incoming requests. The pipeline processes JSON messages from a Cloud Pub/Sub topic and writes results to BigQuery. Which service should be used as the compute component?

Accepted Answer

Cloud Run. Cloud Run is the correct compute component because it is a fully managed serverless platform that automatically scales from zero based on incoming HTTP requests, supports event-driven processing via Pub/Sub push subscriptions, and integrates natively with BigQuery. It provides high availability by default across zones and can handle burst traffic without provisioning overhead, making it ideal for a serverless pipeline that processes JSON messages and writes results to BigQuery.

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Cloud Dataflow

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Cloud Functions

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Compute Engine with managed instance groups

Question 3

A developer runs the command shown in the exhibit. They need to ensure that the application running on instance-3 can be restored quickly if it fails. What should they do?

Accepted Answer

Create a managed instance group using an instance template based on instance-3's boot disk.. A managed instance group (MIG) with an instance template based on instance-3's boot disk provides automated health checking, auto-healing, and managed instance replacement. If instance-3 fails, the MIG automatically recreates it from the template, ensuring rapid restoration without manual intervention. This aligns with the goal of designing highly available and reliable applications on Google Cloud.

Answer

Add instance-3 to an unmanaged instance group.

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Configure Cloud DNS with a health check pointing to instance-3.

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Take a snapshot of instance-3's disk and use it to create a new instance manually.

Question 4

A developer finds the JSON key shown in the exhibit in a Cloud Storage bucket that is publicly accessible. Which security best practice was violated?

Accepted Answer

The key was stored in a publicly accessible Cloud Storage bucket.. Storing a JSON key (a service account private key) in a publicly accessible Cloud Storage bucket directly violates the principle of least privilege and exposes sensitive credentials to unauthorized users. Any entity with read access to the bucket can retrieve the key and impersonate the service account, potentially gaining unauthorized access to Google Cloud resources.

Answer

The key is not rotated regularly.

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The key was created as a user-managed key instead of a Google-managed key.

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The key was not encrypted using Cloud KMS.

Question 5

A team is migrating a monolithic application to microservices on Google Kubernetes Engine (GKE). They want to ensure that if one microservice fails, it does not cascade to other services. Which design pattern should they implement?

Accepted Answer

Circuit Breaker pattern. The Circuit Breaker pattern is correct because it prevents cascading failures by monitoring for failures in a downstream microservice and, once a threshold is exceeded, immediately failing requests to that service without attempting the call. In GKE, this can be implemented using tools like Istio or Envoy sidecar proxies, which can be configured with circuit breaker settings to stop traffic to unhealthy pods, allowing the system to recover gracefully.

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Event-driven architecture

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Retry with exponential backoff

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Bulkhead pattern

Question 6

A company runs a stateful application on Compute Engine instances with local SSDs. They need to perform maintenance that requires stopping the instances. What is the best approach to ensure data durability and minimal downtime?

Accepted Answer

Migrate data to persistent disks and configure the application to use persistent disks. Local SSDs provide ephemeral storage that is tied to the lifecycle of the Compute Engine instance. When an instance is stopped or terminated, data on local SSDs is permanently lost. To ensure data durability during maintenance that requires stopping the instance, the application must use persistent disks, which are durable network-attached storage that persists independently of the instance. Option D is correct because migrating the application to persistent disks ensures data survives the stop and allows the instance to be restarted with the same data, minimizing downtime.

Answer

Create a snapshot of the local SSD before stopping the instance

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Use instance groups with autohealing to automatically recreate instances

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Enable live migration on the instance

Question 7

An administrator runs the above command to create a Compute Engine instance. However, the nginx service does not start. What is the most likely cause?

Accepted Answer

The instance has no external IP address and cannot reach the internet to download packages.. The command likely creates a Compute Engine instance without specifying an external IP address (e.g., using `--no-address` or omitting `--address`). Without an external IP, the instance cannot reach the internet to download the nginx package from repositories, causing the startup script that installs and starts nginx to fail. This is the most direct cause of the nginx service not starting.

Answer

The metadata key is misspelled; it should be 'startup-script-url'.

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The instance does not have the compute.instance.update permission.

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The startup script runs before the boot disk is fully mounted.

Question 8

A company is designing a highly available application on Google Cloud using multiple regions. Which TWO strategies should they implement to achieve this?

Accepted Answer

Use a global load balancer to distribute traffic across regions.. A global load balancer (e.g., Google Cloud External HTTPS Load Balancer) can distribute traffic across multiple regions, providing cross-region failover and low-latency routing. This is a fundamental pattern for multi-region high availability, as it allows traffic to be directed to healthy backends in any region, even if an entire region fails.

Answer

Use zonal persistent disks for stateful data.

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Deploy a single instance group in one region for simplicity.

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Store all data in a single Cloud Storage bucket.

Question 9

A company is designing a globally distributed application using Cloud Spanner. The application requires strong consistency and the ability to handle high read/write throughput. The team is concerned about inter-continental latency. Which two design choices would optimize performance while maintaining strong consistency? (Choose two.)

Accepted Answer

Enable leader-optimized routing to direct reads to the nearest leader region.. A is correct because leader-optimized routing directs read requests to the nearest region that contains the leader replica for the requested data, reducing inter-continental latency while still reading from the leader to ensure strong consistency. C is correct because placing a multi-region Spanner instance in geographic locations close to users minimizes network round-trip time, and Spanner's synchronous replication across regions maintains strong consistency even with high read/write throughput.

Answer

Use read-only replicas in each continent to serve reads locally.

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Implement client-side caching with a short TTL for frequently accessed data.

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Increase the number of nodes in the Spanner instance to improve throughput.

Question 10

A company runs a stateful application on Compute Engine instances with persistent disks. The application must be highly available and be able to recover from a zonal failure with minimal data loss. The current architecture uses a single instance in one zone. Which design should the team implement?

Accepted Answer

Use a regional persistent disk attached to a managed instance group across two zones.. A regional persistent disk synchronously replicates data across two zones, and when attached to a managed instance group (MIG) spanning those zones, it provides automatic failover with minimal data loss. This design ensures that if one zone fails, the MIG can detach the disk from the failed instance and attach it to a healthy instance in the surviving zone, preserving state with near-zero RPO.

Answer

Use a standard persistent disk and configure a global load balancer to failover.

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Create a snapshot schedule and restore the snapshot to a new instance in another zone on failure.

Answer

Migrate to Cloud Filestore for shared file storage across zones.

Question 11

A company runs a stateful application on Compute Engine with regional persistent disks. They want to achieve high availability with automatic failover in case of a zone failure. Which architecture meets these requirements?

Accepted Answer

Use regional persistent disks with a managed instance group spanning two zones. Regional persistent disks synchronously replicate data across two zones, and when combined with a managed instance group (MIG) spanning those same two zones, the application can automatically fail over to the healthy zone if one zone fails. The MIG's autohealing and health-check mechanisms detect the failure and recreate instances in the surviving zone, while the regional PD remains accessible from either zone, ensuring high availability without manual intervention.

Answer

Use Cloud Storage FUSE to mount a multi-regional bucket as a filesystem

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Use standard persistent disks with scheduled snapshots to a multi-region bucket

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Use zonal persistent disks with a managed instance group in a single zone

Question 12

A company is migrating a monolithic application to a microservices architecture on Google Cloud. They want to decouple services and ensure that a failure in one service does not impact others. Which pattern should they implement?

Accepted Answer

Implement circuit breaker pattern using a service mesh like Istio. The circuit breaker pattern, implemented via a service mesh like Istio, is the correct approach because it prevents cascading failures by monitoring service health and stopping requests to a failing service until it recovers. Istio's Envoy sidecar proxies enforce circuit breaking at the network layer, allowing the system to degrade gracefully without impacting other services.

Answer

Implement caching with Memorystore

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Increase the number of instances of each service

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Use synchronous HTTP calls with retries

Question 13

Match each Google Cloud service to its primary purpose.

Accepted Answer

Cloud Run: A fully managed compute platform for containerized applications.. Cloud Run deploys containers, Cloud Functions runs event-driven functions, and Cloud Storage stores unstructured objects. Common confusions include swapping the definitions of Cloud Run and Cloud Functions, or mistaking Cloud Storage for a database service like Cloud SQL.

Answer

Cloud Run: A serverless execution environment for building and connecting cloud services.

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Cloud Storage: A fully managed relational database for MySQL, PostgreSQL, and SQL Server.

Question 14

Match each Cloud SQL database engine to its description.

Accepted Answer

Cloud SQL for MySQL: Fully managed relational database based on MySQL community edition.. Cloud SQL offers managed relational databases; AlloyDB and Spanner are for higher scale.

Answer

Cloud SQL for MySQL: A globally distributed relational database with strong consistency.

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Cloud SQL for PostgreSQL: A fully managed relational database compatible with PostgreSQL and designed for high-scale analytical workloads.

Question 15

An online gaming platform uses Cloud Spanner as its globally distributed database. They notice that write latency increases significantly during peak hours. The application performs many single-row writes with high consistency requirements. Which design change would most effectively reduce write latency?

Accepted Answer

Use interleaved tables to colocate related rows.. Interleaved tables in Cloud Spanner physically colocate parent and child rows, reducing the number of splits and cross-node round trips for related single-row writes. This minimizes distributed transaction overhead and write latency, especially under high consistency requirements, without requiring additional nodes or sacrificing consistency.

Answer

Increase the number of nodes in the Spanner instance.

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Switch to eventual consistency mode for writes.

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Split the table into multiple smaller tables.

Question 16

A development team is using Cloud Build to deploy containerized applications to GKE. They want to ensure that only containers that have passed security scans and unit tests are deployed to production. Which approach should they use?

Accepted Answer

Use Cloud Build with a multi-step pipeline that includes test and security scan steps, and only promote to production after successful completion.. Cloud Build is a CI/CD service that executes a series of build steps defined in a cloudbuild.yaml file. By defining a multi-step pipeline that includes unit test execution and container security scanning (e.g., using Container Analysis or a scanner like Trivy) as prerequisite steps, the build will fail if any step returns a non-zero exit code. Only after all steps succeed does the pipeline proceed to deploy to GKE, ensuring that untested or vulnerable containers never reach production. This approach directly enforces the quality gate within the build process itself.

Answer

Deploy to a staging cluster first, then manually promote to production using kubectl.

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Use Cloud Deploy to automate delivery with approval gates.

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Configure Cloud Build triggers to deploy directly to production on every push.

Question 17

A startup is building a REST API on Cloud Run. They expect unpredictable traffic spikes and want to ensure the service can scale from 0 to many instances automatically. What scaling configuration should they use?

Accepted Answer

Set min instances to 0 and max instances to 1000.. Cloud Run's autoscaling allows min instances to be set to 0, enabling the service to scale down to zero when idle (cost-efficient), and max instances to 1000 to handle unpredictable traffic spikes by scaling out horizontally. This configuration ensures the service can start from zero and automatically add instances up to the maximum limit as demand increases, which is ideal for unpredictable workloads.

Answer

Set max instances to 1 to control costs.

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Use manual scaling with a fixed number of instances.

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Set min instances to 5 and max to 100.

Question 18

A company runs a critical application on Compute Engine with a stateful database. They need to achieve 99.99% availability for the database tier. Which architecture should they implement?

Accepted Answer

A Compute Engine instance group with managed instance groups and a regional persistent disk configured for synchronous replication.. A managed instance group with a regional persistent disk configured for synchronous replication provides the necessary 99.99% availability by ensuring the database runs across two zones with synchronous writes to both replicas. This architecture allows automatic failover within seconds if one zone fails, meeting the high-availability requirement without data loss.

Answer

Use Cloud SQL with automatic failover and read replicas.

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Two Compute Engine instances in different zones with a shared Zonal persistent disk.

Answer

Single Compute Engine instance with a persistent disk snapshot scheduled every hour.

Question 19

A team created the instance template above and used it in a managed instance group. However, instances fail to serve web traffic. What is the most likely cause?

Accepted Answer

The startup script does not configure a firewall rule to allow HTTP traffic.. The instance template likely includes a startup script that installs and starts Nginx, but does not configure a firewall rule (e.g., via `gcloud compute firewall-rules create` or `iptables`) to allow inbound HTTP traffic on port 80. By default, GCP VPC firewall rules deny all ingress traffic unless explicitly allowed, so even if Nginx is running, external requests will be blocked. This is the most common reason why a managed instance group fails to serve web traffic despite the application being installed.

Answer

The image family debian-11 does not have the necessary packages.

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The machine type e2-medium is too small for Nginx.

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The instance template is missing a service account.

Question 20

A security engineer applied the IAM policy above to a Cloud Storage bucket. The service account "my-sa" is used by an application that needs to read and write files to the bucket. The application reports that it cannot write files. What is the issue?

Accepted Answer

The "roles/storage.objectCreator" role only allows creating new objects, but not overwriting existing ones.. The 'roles/storage.objectCreator' role grants permission to create new objects in a Cloud Storage bucket, but it does not allow overwriting existing objects. To overwrite objects, the 'roles/storage.objectAdmin' or 'roles/storage.legacyObjectOwner' role is required, which includes the storage.objects.update permission. Since the application needs to both read and write (including overwrite) files, the objectCreator role is insufficient.

Answer

The policy is missing the "roles/storage.objectAdmin" role.

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The service account lacks permission to list bucket contents.

Answer

The policy has duplicate bindings that cause a conflict.