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Question 467 of 1,730

Workload-Specific Database Design →mediumMultiple ChoiceObjective-mapped

DBS-C01 Workload-Specific Database Design Practice Question

This DBS-C01 practice question tests your understanding of workload-specific database design. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.

A company runs a MongoDB-compatible workload on Amazon DocumentDB. They notice that many read requests are returning stale data even though reads are directed to the primary instance. What is the MOST likely cause?

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.

A
The application's session is pinned to a secondary replica despite requesting the primary.
Why wrong: Session pinning does not override read preference; if primary is requested, the driver connects to primary.
B
The application is using a read preference that allows secondary reads.
If the read preference is set to 'secondaryPreferred' or similar, reads may go to secondary replicas which are eventually consistent.
C
The primary instance is experiencing high CPU utilization, causing delayed writes.
Why wrong: High CPU on primary does not cause stale reads on primary; writes are acknowledged before responding.
D
The storage volume is using the default eventually consistent configuration for primary reads.
Why wrong: DocumentDB primary reads are strongly consistent by default.

Full breakdown with real-world context →

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

The application is using a read preference that allows secondary reads.

Option B is correct because the most likely cause of stale reads from the primary instance is that the application is using a read preference that allows secondary reads. In Amazon DocumentDB, even if the connection string specifies the primary endpoint, the MongoDB driver's read preference setting (e.g., `secondaryPreferred` or `nearest`) can cause reads to be served from replica instances, which may have replication lag and thus return stale data. The default read preference is `primary`, but if the application explicitly sets a different preference, reads can be directed to secondaries without the developer realizing it.

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's session is pinned to a secondary replica despite requesting the primary.
Why it's wrong here
Session pinning does not override read preference; if primary is requested, the driver connects to primary.
✓
The application is using a read preference that allows secondary reads.
Why this is correct
If the read preference is set to 'secondaryPreferred' or similar, reads may go to secondary replicas which are eventually consistent.
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.
✗
The primary instance is experiencing high CPU utilization, causing delayed writes.
Why it's wrong here
High CPU on primary does not cause stale reads on primary; writes are acknowledged before responding.
✗
The storage volume is using the default eventually consistent configuration for primary reads.
Why it's wrong here
DocumentDB primary reads are strongly consistent by default.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates assume connecting to the primary endpoint always guarantees primary reads, but the MongoDB driver's read preference setting can silently redirect reads to secondaries, causing stale data even when the endpoint is correct.

Detailed technical explanation

How to think about this question

Amazon DocumentDB implements MongoDB's wire protocol, including read preference tags and the `$readPreference` parameter in queries. When an application sets `readPreference=secondaryPreferred`, the driver sends reads to secondaries unless no secondary is available, which can return stale data if replication lag exists. This is a common misconfiguration because developers often assume the primary endpoint in the connection string guarantees primary reads, but the driver-level read preference overrides this. In production, monitoring `replicationLag` in CloudWatch and auditing driver configuration can prevent this issue.

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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.

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.

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FAQ

Questions learners often ask

What does this DBS-C01 question test?

Workload-Specific Database Design — This question tests Workload-Specific Database Design — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The application is using a read preference that allows secondary reads. — Option B is correct because the most likely cause of stale reads from the primary instance is that the application is using a read preference that allows secondary reads. In Amazon DocumentDB, even if the connection string specifies the primary endpoint, the MongoDB driver's read preference setting (e.g., `secondaryPreferred` or `nearest`) can cause reads to be served from replica instances, which may have replication lag and thus return stale data. The default read preference is `primary`, but if the application explicitly sets a different preference, reads can be directed to secondaries without the developer realizing it.

What should I do if I get this DBS-C01 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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