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Question 467 of 1,730

Workload-Specific Database Design →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is a read preference that allows secondary reads. This is the most likely cause because Amazon DocumentDB, while offering strongly consistent reads from the primary instance by default, uses a distributed storage volume with six copies across three Availability Zones where secondary replicas are eventually consistent. If the application’s read preference is set to 'secondaryPreferred' or any value that permits reads from a replica—even when the primary is available—those reads can return stale data. On the AWS Certified Database Specialty DBS-C01 exam, this question tests your understanding of how DocumentDB’s consistency model interacts with MongoDB-compatible read preferences, a common trap where candidates assume all reads are strongly consistent simply because they target the primary. A useful memory tip is: “Primary equals strong, secondary equals eventual—check your preference to prevent stale.”

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.

Question 1mediummultiple choice

Read the full NAT/PAT explanation →

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.

DocumentDB uses a distributed storage volume with six copies across three Availability Zones. Reads from the primary are strongly consistent by default. However, if the application uses read preferences that allow secondary reads (e.g., 'secondaryPreferred'), then reads may go to replicas which are eventually consistent. The question implies reads are directed to primary, but if the application uses 'secondaryPreferred' and the primary is unavailable, reads may go to secondary. Alternatively, if the application uses a read preference that does not require primary, stale reads can occur. Option D addresses the most common cause: read preference set to allow secondary reads.

Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.

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
Static NAT maps one inside address to one outside address.
✗
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: NAT rules depend on direction and matching traffic

NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.

Detailed technical explanation

How to think about this question

NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.

KKey Concepts to Remember

Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.

TExam Day Tips

Identify inside and outside interfaces first.
Check whether the scenario needs static NAT, dynamic NAT or PAT.
Do not confuse NAT matching ACLs with normal packet-filtering intent.

Key takeaway

NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.

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.

Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related DBS-C01 NAT questions on configuration and troubleshooting.

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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 — Static NAT maps one inside address to one outside address..

What is the correct answer to this question?

The correct answer is: The application is using a read preference that allows secondary reads. — DocumentDB uses a distributed storage volume with six copies across three Availability Zones. Reads from the primary are strongly consistent by default. However, if the application uses read preferences that allow secondary reads (e.g., 'secondaryPreferred'), then reads may go to replicas which are eventually consistent. The question implies reads are directed to primary, but if the application uses 'secondaryPreferred' and the primary is unavailable, reads may go to secondary. Alternatively, if the application uses a read preference that does not require primary, stale reads can occur. Option D addresses the most common cause: read preference set to allow secondary reads.

What should I do if I get this DBS-C01 question wrong?

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?

Static NAT maps one inside address to one outside address.

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