PCD Practice Test 17 — 15 Questions

Question 1

A company is migrating a Cassandra-based time-series application to Google Cloud. They need to maintain single-digit millisecond write latency at petabyte scale. Which Google Cloud database is the BEST replacement?

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

Accepted Answer

Cloud Bigtable. Bigtable is the closest equivalent to Cassandra for time-series data, offering similar performance and scalability. Cloud Spanner is relational, and Firestore is for document data.

Answer

Cloud Spanner

Answer

Firestore

Answer

BigQuery

Question 2

You are designing a Cloud Bigtable schema for an AdTech platform. Queries often filter by campaign ID and date range. Which three row key design practices should you consider to avoid hotspots and optimize performance? (Choose three.)

Accepted Answer

Promote the campaign ID to the beginning of the row key. Field promotion (moving high-cardinality fields to the key), reversed domain, and salted keys are common patterns to avoid hotspots. Single table is not a key design practice. Column families affect storage, not key design.

Answer

Use a single table for all data

Answer

Separate frequently accessed columns into a different column family

Question 3

A mobile app needs to synchronize user data across devices with offline support. The data is structured as documents with hierarchical relationships. Which Google Cloud database is best suited for this use case?

Accepted Answer

Firestore. Firestore provides offline persistence, real-time sync, and document data model, making it ideal for mobile apps. Cloud SQL does not offer offline sync, Bigtable is not document-oriented, and Memorystore is a cache.

Answer

Cloud Bigtable

Answer

Cloud SQL

Answer

Memorystore

Question 4

A company is using Cloud Bigtable for IoT data ingestion. They are experiencing high latency and write throttling. The current row key format is 'device_id#timestamp'. Which THREE actions can help distribute the write load and reduce hotspots? (Choose 3.)

Accepted Answer

Reverse the order of row key components: timestamp#device_id. Salted keys, field promotion, and reversed domain are common patterns to distribute writes. Using a single node is not a solution; it worsens performance.

Answer

Increase the number of Bigtable nodes

Answer

Use a single column family to reduce data size

Question 5

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 designed for exactly this use case — petabyte-scale, low-latency (single-digit ms), high-throughput NoSQL storage for time-series, IoT, and financial data. It scales horizontally by adding nodes. BigQuery is optimised for analytics (seconds-to-minutes latency), Cloud SQL is for OLTP (limited to tens of thousands of QPS), and Firestore is for document data with hierarchical structure.

Answer

BigQuery

Answer

Cloud Spanner

Answer

Firestore

Question 6

A company is designing a Bigtable schema for time-series data from millions of devices. Each device sends a reading every minute. The row key is currently 'device_id#timestamp'. The engineering team notices hot spots on a few popular devices. Which row key design change would BEST distribute writes across the cluster?

Accepted Answer

Append a random salt prefix to the row key. Salted keys (prepending a hash prefix) distribute writes across tablet servers. Reversing the timestamp or field promotion alone may not solve hotspots from popular device IDs.

Answer

Promote device ID to column family

Answer

Use reversed domain for device IDs

Answer

Use only timestamp as row key

Question 7

A company is migrating an Oracle database to AlloyDB for PostgreSQL using Database Migration Service. They have completed a full dump and are now in the CDC phase. They want to test the target database with minimal risk. What should they do?

Accepted Answer

Promote the continuous migration replica to a standalone instance. Database Migration Service allows creating a connection profile and a migration job. During the CDC phase, you can promote the replica to make the target database writable for testing. After testing, you can resume migration by re-creating the job.

Answer

Stop the migration job and promote the replica

Answer

Create a snapshot of the target instance

Answer

Use Cloud SQL Auth Proxy to connect

Question 8

A retail company is migrating its on-premises PostgreSQL OLTP database to Google Cloud. They require high availability with automatic failover and zero RPO. The application is latency-sensitive and must remain in a single region. Which TWO configurations should they choose? (Choose TWO.)

Accepted Answer

Deploy Cloud SQL for PostgreSQL with HA configuration. Cloud SQL HA uses synchronous replication to a standby in a different zone within the same region, providing automatic failover and zero RPO. AlloyDB is PostgreSQL-compatible and offers HA with synchronous replication. Cloud SQL cross-region replication is asynchronous, not meeting zero RPO. Spanner multi-region is overkill and introduces cross-region latency. Memorystore is not a relational database.

Answer

Deploy Cloud SQL for PostgreSQL with cross-region replication

Answer

Deploy Cloud Spanner multi-region configuration

Answer

Use Memorystore for Redis as a primary database

Question 9

An engineer is configuring Cloud SQL for PostgreSQL with HA. They notice that after a failover, the original primary instance does not automatically resume as a standby. What is the likely cause?

Accepted Answer

The original primary instance's disk is now in read-only mode. In Cloud SQL HA, after a failover, the original primary becomes a new standby using the same underlying disk. It does not create a new instance automatically. The disk is preserved, and the standby is recreated.

Answer

The standby instance was not configured in a different zone

Answer

The HA instance is using local SSD which is not durable

Answer

The original primary instance is deleted after failover

Question 10

A company is migrating a PostgreSQL database to AlloyDB using Database Migration Service. They need to ensure the migration has minimal downtime and can be tested before cutover. Which THREE steps should they include in their migration plan?

Accepted Answer

Create a migration job with 'Full dump + CDC' to enable continuous replication.

Answer

Test the migration by promoting a replica in a non-production environment before final cutover.

Answer

Use pg_dump to take a manual backup and restore it to AlloyDB before setting up DMS.

Question 11

A team is using Cloud Spanner and needs to query a secondary index with a STORING clause. What is the benefit of using STORING?

Accepted Answer

It avoids an index join (back to the base table). STORING allows storing additional columns in the index, enabling index-only scans and avoiding a join back to the base table, improving query performance.

Answer

It creates a global index

Answer

It reduces storage costs

Answer

It allows the index to be used for ordering

Question 12

You are designing a Bigtable row key for an IoT telemetry application that writes one row per device per minute. Devices are identified by a 12-character device ID. To avoid write hotspots, which row key design is most appropriate?

Accepted Answer

Row key: reversed deviceID + timestamp (e.g., '321cba#2024-03-15T10:30:00'). To distribute writes evenly across Bigtable tablets, the row key should have a non-monotonic prefix. Reversing the device ID (or salting) helps. Field promotion is not needed here.

Answer

Row key: timestamp + deviceID (e.g., '2024-03-15T10:30:00#abc123')

Answer

Row key: deviceID + timestamp (e.g., 'abc123#2024-03-15T10:30:00')

Answer

Row key: hash(deviceID) + deviceID + timestamp

Question 13

A corporation needs to back up its Cloud Spanner database for disaster recovery. They require the ability to restore to a specific point in time within the last 7 days. What backup strategy should they implement?

Accepted Answer

Use Spanner's built-in point-in-time recovery (PITR) which retains versions for 7 days.. Cloud Spanner supports full database backups exported to Avro in Cloud Storage. Incremental backups are not supported natively; point-in-time recovery is achieved via versioning (up to 7 days) but backups are full. For granular recovery, you can use the built-in versioning or schedule frequent exports.

Answer

Use Cloud Scheduler to run incremental backups daily.

Answer

Enable cross-region replicas and restore from the replica.

Answer

Export the database to Cloud Storage every hour using Dataflow.

Question 14

A company is using Cloud Bigtable for ad tech data. They have a single table with a column family containing frequently accessed columns and another with rarely accessed columns. To optimize performance, what column family design change should they implement?

Accepted Answer

Move frequently accessed columns to a separate column family. Separating frequently accessed columns into their own column family reduces the amount of data read per request, improving performance.

Answer

Move rarely accessed columns to a different table

Answer

Combine all columns into one column family

Answer

Use a single column family and set garbage collection to delete rarely accessed columns

Question 15

A company needs a globally distributed relational database that provides strong ACID transactions across regions and a 99.999% availability SLA. Which Google Cloud database service meets these requirements?

Accepted Answer

Cloud Spanner. Cloud Spanner is the only Google Cloud database that offers globally distributed ACID transactions and a 99.999% SLA. Cloud SQL and AlloyDB are regional, and Bigtable is not relational.

Answer

Cloud SQL

Answer

Cloud Bigtable

Answer

AlloyDB