PCD Practice Test 38 — 15 Questions

Question 1

You are designing a Cloud Spanner schema for a global user database. Users are identified by a UUID. You notice hotspotting on a specific node during writes. What is the most effective way to resolve this?

Choose an answer to begin — your selection is scored in the full session.

Accepted Answer

Use a composite primary key with a hash prefix of the UUID. Hotspotting in Cloud Spanner occurs when writes are concentrated on a single node due to a monotonically increasing primary key (like a UUID without a hash prefix). By using a composite primary key with a hash prefix of the UUID, you distribute writes evenly across all nodes because the hash prefix ensures that successive writes land on different splits. This prevents a single node from becoming a bottleneck, thereby resolving the hotspotting issue.

Answer

Enable interleaved tables to store child rows with parent

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Change the primary key to a monotonically increasing timestamp

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Add a secondary index on the UUID column

Question 2

Your mobile app uses Firestore to store user profiles. You need to restrict access so that users can only read/write their own data. Which Firestore feature should you use?

Accepted Answer

Security Rules. Firestore Security Rules are the correct choice because they allow you to define granular access controls based on user identity. By using the `request.auth.uid` variable in your rules, you can restrict read and write operations to documents where the user's UID matches a field in the document (e.g., `resource.data.user_id == request.auth.uid`), ensuring users can only access their own data.

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Data Bundles

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Composite indexes

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IAM roles

Question 3

A Cloud Spanner table uses a UUID as the primary key. The application is experiencing high write latency and hotspotting. Which design change would BEST resolve the issue?

Accepted Answer

Use a composite primary key with a shard prefix (e.g., hash of UUID). Adding a shard prefix (e.g., a hash of the UUID) as the first part of a composite primary key distributes writes evenly across Cloud Spanner's splits, eliminating hotspotting. UUIDs alone cause hotspotting because they are randomly distributed but still lead to a single split being overwhelmed if the table is small or the UUIDs are inserted in order; a hash prefix ensures that consecutive writes target different splits, reducing write latency.

Answer

Switch to a monotonically increasing integer key

Answer

Add a secondary index on the UUID column

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Enable interleaved tables

Question 4

The alert is not firing even though error_count metric occasionally spikes above 10. What is the most likely reason?

Accepted Answer

The duration of 300s means the condition must remain >10 for 5 minutes, so brief spikes do not trigger.. The alert condition is configured with a duration of 300 seconds (5 minutes), meaning the error_count metric must remain above 10 for the entire 5-minute window before the alert fires. Brief, transient spikes that exceed 10 but do not persist for the full duration will not trigger the alert, which is the most likely reason the alert is not firing despite occasional spikes.

Answer

The aggregations are incorrect; should use REDUCE_MAX.

Answer

The filter specifies gke_container but the metric might be from other resources.

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The comparison should be COMPARISON_GT_OR_NAN.

Question 5

You need to design a database for a global inventory management system that requires strong consistency across regions and can handle millions of updates per day. The system must maintain ACID transactions across multiple warehouses. Which database service and configuration would you choose?

Accepted Answer

Cloud Spanner multi-region instance. Cloud Spanner is the only Google Cloud database service that provides ACID transactions across regions with strong external consistency, making it ideal for a global inventory system requiring millions of updates per day. Its multi-region instance configuration uses synchronous replication and the TrueTime API to ensure linearizable transactions, meeting the need for strong consistency across warehouses.

Answer

Bigtable with cluster replication

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Firestore in multi-region mode

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Cloud SQL with cross-region replicas

Question 6

A company needs to store petabytes of time-series IoT sensor data and query it with single-digit millisecond latency at millions of reads per second. The data has a simple key-value structure with timestamps. Which Google Cloud database is MOST appropriate?

Accepted Answer

Cloud Bigtable. Cloud Bigtable is a fully managed, scalable NoSQL database designed for large analytical and operational workloads, making it ideal for petabyte-scale time-series IoT sensor data. It supports single-digit millisecond latency for reads and writes at millions of operations per second through its underlying storage engine (based on Google's Chubby and SSTable technology) and its native integration with the HBase API, which provides efficient key-value access with timestamp-based versioning.

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BigQuery

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Firestore

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Cloud Spanner

Question 7

A financial services company is designing a globally distributed ledger system using Cloud Spanner. They need to support high write throughput, avoid hotspots, and ensure fast queries on a customer_id column. The primary key is currently a UUID. Which THREE design changes should they implement? (Choose THREE.)

Accepted Answer

Use a composite primary key with customer_id as the first column. Placing `customer_id` as the first column in a composite primary key enables Spanner to distribute writes across splits based on the leading key column, avoiding hotspots that occur when using a monotonically increasing UUID as the sole primary key. This design also allows fast point-lookups and range scans on `customer_id` without requiring a secondary index, as Spanner can directly locate the row via the primary key.

Answer

Enable point-in-time recovery to reduce storage costs

Answer

Create a secondary index on customer_id

Question 8

A company wants to achieve 0 RPO for a Cloud SQL for MySQL instance. Which configuration should they use?

Accepted Answer

Configure a Cloud SQL HA instance. Cloud SQL HA (high availability) instances use synchronous replication to a standby VM in a different zone within the same region, providing automatic failover with zero data loss (0 RPO). This configuration ensures that committed transactions are immediately replicated to the standby before acknowledging the commit, guaranteeing that no committed data is lost even if the primary zone fails.

Answer

Enable cross-region replication to a replica in another region

Answer

Enable automatic backups with point-in-time recovery

Answer

Use a read replica in the same zone

Question 9

A financial services company is implementing a distributed transaction across multiple Cloud Spanner instances in different regions. They require strict ACID compliance. What is the best approach?

Accepted Answer

Use Cloud Spanner with multi-region configuration. Cloud Spanner with a multi-region configuration is the best approach because it provides native support for distributed ACID transactions across regions using synchronous replication and the TrueTime API, ensuring strong consistency, atomicity, and isolation without requiring application-level coordination. This eliminates the complexity and latency of managing distributed transactions manually while meeting strict ACID compliance requirements.

Answer

Use Cloud Pub/Sub to orchestrate eventual consistency

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Use Cloud SQL with cross-region replication and two-phase commit

Answer

Implement a saga pattern with compensating transactions

Question 10

You are designing a Bigtable schema for an ad-tech platform that tracks ad impressions. Each impression has a unique ID, timestamp, user ID, and campaign ID. Queries frequently filter by user ID and time range. Which row key design is MOST appropriate to avoid hotspots and support efficient range scans?

Accepted Answer

Use a composite key: hashed user ID prefix + timestamp. Using a hashed user ID prefix distributes writes across Bigtable tablets, preventing hotspots from sequential user IDs, while appending the timestamp enables efficient range scans within a user's data. Bigtable's lexicographic ordering on row keys means that the hashed prefix ensures load balancing, and the timestamp suffix allows scanning a specific time range for a given user without scanning irrelevant rows.

Answer

Use the user ID as the row key

Answer

Use a composite key: campaign ID + timestamp

Answer

Use the impression ID as the row key

Question 11

You are a developer at a company that runs a critical pricing engine on Compute Engine instances in a managed instance group (MIG) behind an internal TCP load balancer. The pricing engine is a stateful application that stores state in memory and also writes to a Cloud Bigtable instance for persistence. The application uses a custom port 8080. You need to migrate this application to Cloud Run for better scalability and reduced operational overhead. The application must maintain session affinity so that requests from the same client are routed to the same instance (since the in-memory state is not yet fully externalized). The application currently uses a health check on /healthz that returns 200 OK. You have containerized the application. When you deploy to Cloud Run, you notice that traffic is not sticky; every request might go to a different revision. You also need to ensure that Bigtable writes are performed asynchronously to avoid slowing down the pricing calculations. What should you do?

Accepted Answer

Use Cloud Run with an HTTP(S) External Load Balancer and enable session affinity on the backend service.. Cloud Run does not natively support session affinity, but you can achieve it by placing Cloud Run behind an external HTTP(S) load balancer and enabling session affinity on the backend service. This ensures that requests from the same client are routed to the same Cloud Run revision, maintaining session stickiness for the in-memory state. Option A is incorrect because Cloud Run does not support custom TCP health checks; it only supports HTTP health checks, and Cloud Run automatically handles health checks via the /healthz endpoint. Option B is incorrect because setting concurrency to 1 does not provide session affinity; it only limits the number of concurrent requests per container, but subsequent requests from the same client may still go to different containers without a load balancer with session affinity. Option D is incorrect because Cloud Run cannot be placed behind an internal TCP/UDP load balancer; it only supports HTTP(S) traffic and must use an external load balancer for session affinity.

Answer

Implement a custom health check on TCP port 8080 in Cloud Run to ensure only healthy instances receive traffic.

Answer

Increase the container concurrency setting to 1 to force each container to handle one request at a time.

Answer

Deploy the application on Cloud Run and configure an Internal TCP/UDP Load Balancer in front of it with session affinity.

Question 12

A financial services company is migrating a globally distributed trading application to Cloud Spanner. They need strong consistency and low-latency reads across regions. Which THREE configurations should they choose? (Select THREE.)

Accepted Answer

Use interleaved tables for related data. Interleaved tables physically co-locate parent and child rows in Cloud Spanner, enabling efficient joins and low-latency reads for related data. This is critical for a globally distributed trading application that requires strong consistency and fast access to hierarchical data, such as orders and their line items, without cross-node coordination.

Answer

Use stale reads with bounded staleness

Answer

Regional instance configuration to reduce latency

Question 13

A company is migrating an Oracle database to Cloud SQL for PostgreSQL using Database Migration Service (DMS). During the full dump phase, the migration fails with an error stating that the source database user does not have sufficient privileges. Which THREE privileges are required for the DMS user on the Oracle source? (Choose three.)

Accepted Answer

EXECUTE on DBMS_FLASHBACK. Database Migration Service (DMS) for Oracle to Cloud SQL for PostgreSQL uses Oracle Flashback technology to capture consistent change data during the migration. The DMS user must have the EXECUTE privilege on the DBMS_FLASHBACK package to enable Flashback Query operations, which are essential for reading the source database in a transactionally consistent state without blocking writes.

Answer

CREATE SESSION

Answer

CREATE TABLE

Question 14

A mobile app team wants to store user session state with low latency and support for pub/sub messaging. They need a managed service that automatically scales. Which database is BEST suited for this?

Accepted Answer

Memorystore for Redis. Memorystore for Redis is the best choice because it provides an in-memory data store with sub-millisecond latency, ideal for session state caching, and natively supports pub/sub messaging via Redis Pub/Sub. As a managed service, it automatically handles scaling and failover, meeting the team's requirements without operational overhead.

Answer

Bigtable

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Cloud SQL

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Firestore

Question 15

A developer wants to containerize a Node.js application and deploy it to Cloud Run. They need to ensure the container is as small as possible. What should they do?

Accepted Answer

Use a multi-stage Dockerfile with a distroless base image.. A multi-stage Dockerfile allows you to separate the build environment from the runtime environment. By using a distroless base image (e.g., gcr.io/distroless/nodejs), you include only the application and its runtime dependencies, omitting package managers, shells, and other OS utilities. This results in a significantly smaller container image, which reduces attack surface and improves deployment speed on Cloud Run.

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Use a full Ubuntu base image with all dependencies.

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Use a node:latest image and remove unnecessary files.

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Use a simple FROM scratch image.