The answer is a monotonically increasing primary key creating a hotspot. This occurs because Cloud Spanner distributes data across splits based on the primary key range, so an auto-increment integer funnels all new writes into the highest split, overwhelming that single node and causing high write latency. On the Google Professional Cloud Database Engineer exam, this scenario tests your understanding of distributed database design, specifically how to avoid write contention by using a non-sequential key, such as a UUID or a composite key with a shard prefix. A common trap is assuming auto-increment works well in Spanner as it does in traditional SQL databases, but the exam expects you to recognize that monotonically increasing keys defeat Spanner’s parallel write capability. Memory tip: think “hot key, cold performance”—a sequential key creates a single hot spot, while a random key spreads the load.
PCDE Plan and manage database infrastructure Practice Question
This PCDE practice question tests your understanding of plan and manage database infrastructure. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
An e-commerce application uses Cloud Spanner for its global inventory database. The application experiences high write latency during peak hours. After reviewing the schema, the database engineer notices that the primary key is an auto-incrementing integer. What is the most likely cause of the high write latency?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Clue: "primary"
Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The monotonically increasing primary key creates a hotspot.
Cloud Spanner distributes data across splits based on the primary key range. A monotonically increasing integer primary key, such as an auto-incrementing ID, causes all new writes to target the same split (the highest key range), creating a hotspot. This single split becomes a bottleneck, leading to high write latency during peak hours, as Spanner cannot parallelize the writes across multiple nodes.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The application is using read-only transactions instead of read-write transactions.
Why it's wrong here
Read-only transactions cannot cause high write latency; they are for reads only.
✗
The database is configured with a backup retention period of 2 seconds.
Why it's wrong here
Backup retention does not affect write latency during normal operations.
✗
The application is using read replicas that are out of date.
Why it's wrong here
Read replicas do not impact write latency; they only affect read performance.
✓
The monotonically increasing primary key creates a hotspot.
Why this is correct
Spanner distributes data by key range; sequential keys cause all new rows to be written to a single node, creating a hotspot.
Clue confirmation
The clue words "most likely", "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that auto-incrementing keys are always optimal for performance, but in distributed databases like Spanner, they cause hotspots; candidates may incorrectly attribute latency to backup settings or read replicas instead of the key design flaw.
Detailed technical explanation
How to think about this question
Cloud Spanner uses a distributed storage layer where data is partitioned into splits, each served by a set of tablets. A monotonically increasing primary key leads to a 'hotspot' where all new rows are inserted into the same split, overwhelming that split's resources. To avoid this, use a key that distributes writes evenly, such as a UUID or a hash-prefixed key, which allows Spanner to spread load across multiple splits and nodes.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Plan and manage database infrastructure — This question tests Plan and manage database infrastructure — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The monotonically increasing primary key creates a hotspot. — Cloud Spanner distributes data across splits based on the primary key range. A monotonically increasing integer primary key, such as an auto-incrementing ID, causes all new writes to target the same split (the highest key range), creating a hotspot. This single split becomes a bottleneck, leading to high write latency during peak hours, as Spanner cannot parallelize the writes across multiple nodes.
What should I do if I get this PCDE question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely", "primary". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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