DBS-C01 Workload-Specific Database Design Practice Question
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?
⚠ Common exam trap
Test-takers frequently 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.
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.
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.
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.
- ✗
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.
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