The correct answer is that the Glue job lacks a network path to the RDS instance because it is not attached to the same VPC subnet. When an RDS database resides in a private subnet, an AWS Glue ETL job must have a direct network connection to that subnet, typically achieved by attaching the Glue job to the same VPC and configuring an elastic network interface (ENI) in that subnet. A VPC endpoint for S3 only provides private connectivity to S3, not to RDS, so the job cannot reach the database, resulting in the communications link failure. On the AWS Certified Data Engineer Associate DEA-C01 exam, this scenario tests your understanding of Glue networking fundamentals versus service-specific endpoints—a common trap is confusing VPC endpoints for S3 with general VPC connectivity. Remember the memory tip: “S3 needs an endpoint, RDS needs a subnet.”
DEA-C01 Data Operations and Support Practice Question
This DEA-C01 practice question tests your understanding of data operations and support. 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.
A data pipeline uses AWS Glue ETL jobs to process data from Amazon RDS for MySQL to Amazon S3. Recently, the jobs have been failing with the error 'Communications link failure' during the connection phase. The RDS instance is in a private subnet, and the Glue job uses a VPC endpoint for S3. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The Glue job does not have a network path to the RDS instance because it is not attached to the same VPC subnet.
Option B is correct because the Glue job needs a VPC endpoint for S3 to access S3, but for RDS, a VPC endpoint is not sufficient; the job must have network connectivity to the RDS subnet, typically via an ENI in the same VPC. Option A is wrong because KMS permissions would cause access denied, not connection failure. Option C is wrong because connection pooling is not relevant. Option D is wrong because the error is 'Communications link failure', not authentication.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 RDS database has reached the maximum number of connections.
Why it's wrong here
That error would be 'too many connections', not 'communications link failure'.
✗
The Glue job does not have IAM permissions to decrypt the RDS database using AWS KMS.
Why it's wrong here
KMS issues cause access denied, not connection failure.
✗
The JDBC driver used by Glue is incompatible with the MySQL version.
Why it's wrong here
An incompatible driver would give a different error.
✓
The Glue job does not have a network path to the RDS instance because it is not attached to the same VPC subnet.
Why this is correct
Glue jobs need an ENI in the same VPC to connect to RDS.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related DEA-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Data Operations and Support — This question tests Data Operations and Support — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The Glue job does not have a network path to the RDS instance because it is not attached to the same VPC subnet. — Option B is correct because the Glue job needs a VPC endpoint for S3 to access S3, but for RDS, a VPC endpoint is not sufficient; the job must have network connectivity to the RDS subnet, typically via an ENI in the same VPC. Option A is wrong because KMS permissions would cause access denied, not connection failure. Option C is wrong because connection pooling is not relevant. Option D is wrong because the error is 'Communications link failure', not authentication.
What should I do if I get this DEA-C01 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related DEA-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". 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?
CIDR notation defines the prefix length.
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