The answer is that the instance is a Read Replica of another RDS instance, as confirmed by the presence of the `ReplicaLag` field in the CLI output. This field, with a value of `0` in this case, is a dedicated metric that only appears when an RDS instance is configured as a Read Replica, indicating the asynchronous replication lag from its source DB instance. On the AWS Certified Database Specialty DBS-C01 exam, this is a classic trap: candidates often focus on the lag value itself, but the key test is recognizing that `ReplicaLag` is a conditional field—it simply does not exist in the output for a standalone or Multi-AZ instance. The exam expects you to know that a Read Replica’s CLI output will always include this attribute, regardless of whether the lag is zero or non-zero. For a quick memory tip, remember the phrase “Lag equals Replica”—if you see `ReplicaLag` in the output, the instance is a replica, no matter what the number says.
DBS-C01 Workload-Specific Database Design Practice Question
This DBS-C01 practice question tests your understanding of workload-specific database design. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
Exhibit
Refer to the exhibit.
Consider the following AWS CLI command output for an Amazon RDS instance:
{
"DBInstances": [
{
"DBInstanceIdentifier": "mydb",
"DBInstanceClass": "db.r5.large",
"Engine": "mysql",
"DBInstanceStatus": "available",
"MasterUsername": "admin",
"Endpoint": {
"Address": "mydb.abcdef123456.us-east-1.rds.amazonaws.com",
"Port": 3306
},
"MultiAZ": false,
"StorageType": "gp2",
"AllocatedStorage": 100,
"ReadReplicaSourceDBInstanceIdentifier": "mydb-primary"
}
]
}
Based on the CLI output, what is true about this RDS instance?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The instance is a Read Replica of another RDS instance
The CLI output shows `ReplicaLag` with a value of `0`, which is a field that only appears when the RDS instance is configured as a Read Replica. A Read Replica maintains asynchronous replication from a source DB instance, and the lagmetric indicates how far behind the replica is. Since the output includes this field, the instance must be a Read Replica.
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 instance runs Amazon Aurora PostgreSQL
Why it's wrong here
Engine is mysql.
✗
The instance is a Multi-AZ deployment
Why it's wrong here
MultiAZ is false.
✓
The instance is a Read Replica of another RDS instance
Why this is correct
ReadReplicaSourceDBInstanceIdentifier is set.
Related concept
Read the scenario before looking for a memorised answer.
✗
The instance uses Provisioned IOPS (io1) storage
Why it's wrong here
StorageType is gp2.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates see `ReplicaLag: 0` and assume it means no replication is happening or that it indicates a Multi-AZ setup, but in reality, a lag of 0 simply means the replica is fully caught up, and the presence of the field itself confirms it is a Read Replica, not a Multi-AZ standby.
Detailed technical explanation
How to think about this question
When you create an RDS Read Replica, AWS automatically adds a `ReplicaLag` metric to the instance metadata, which is visible in the CLI output via `describe-db-instances`. This lag is measured in seconds and represents the delay between the source and replica. In contrast, Multi-AZ replicas are synchronous and do not expose this metric because they are not intended for read scaling; they exist solely for failover. The `ReplicaLag` field is a definitive indicator of a Read Replica configuration.
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.
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 instance is a Read Replica of another RDS instance — The CLI output shows `ReplicaLag` with a value of `0`, which is a field that only appears when the RDS instance is configured as a Read Replica. A Read Replica maintains asynchronous replication from a source DB instance, and the lag metric indicates how far behind the replica is. Since the output includes this field, the instance must be a Read Replica.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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