The answer is that the Glue job’s security group lacks an outbound rule allowing traffic to the RDS security group on port 3306. This is because AWS Glue ETL jobs run inside a VPC, and for any TCP connection to be established, the initiating side—Glue—must have an outbound rule permitting the traffic, even if the RDS security group has a corresponding inbound rule. Without that outbound rule, the TCP handshake fails, producing the “communications link failure” error. On the AWS Certified Data Engineer Associate DEA-C01 exam, this scenario tests your understanding of stateful security group behavior and the common trap of only checking inbound rules. A frequent mistake is assuming that allowing inbound traffic on the RDS side is sufficient, but Glue must also be allowed to send traffic out. Remember the mnemonic: “Outbound to open, inbound to welcome”—Glue must push out before RDS can pull in.
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 engineer is running an AWS Glue ETL job that reads from an Amazon RDS MySQL database and writes to Amazon S3. The job fails with a 'Communications link failure' error. The security group for the RDS instance allows inbound traffic from the Glue job's security group. What is the most likely cause of the failure?
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's security group does not allow outbound traffic to the RDS security group on port 3306.
Option C is correct because AWS Glue ETL jobs run in a VPC that requires outbound security group rules to initiate connections to RDS. Even if the RDS security group allows inbound traffic from the Glue security group, the Glue security group must also have an outbound rule allowing traffic to the RDS security group on port 3306 (MySQL default port). Without this outbound rule, the TCP handshake from Glue to RDS fails, causing a 'Communications link failure'.
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 JDBC connection string in the Glue job does not include the database name.
Why it's wrong here
Missing database name would cause a different error.
✗
The Glue job is using the wrong JDBC driver.
Why it's wrong here
Glue uses its own driver; this is unlikely.
✓
The Glue job's security group does not allow outbound traffic to the RDS security group on port 3306.
Why this is correct
Without outbound rule, the connection fails.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The IAM role used by the Glue job does not have rds:Connect permission.
Why it's wrong here
RDS does not use IAM for MySQL connections by default.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume only inbound rules matter for security groups, but outbound rules are equally critical for initiating connections from the client (Glue) to the server (RDS).
Detailed technical explanation
How to think about this question
The 'Communications link failure' error in JDBC often masks a lower-level TCP connectivity issue, such as a missing outbound security group rule or a network ACL blocking ephemeral ports. In AWS Glue, the job's elastic network interface (ENI) is attached to a security group that must permit outbound traffic to the RDS instance's security group on the database port (3306 for MySQL). A common real-world scenario is when the Glue job is in a different VPC or subnet without proper VPC peering or a transit gateway, causing the connection to time out.
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.
Data Operations and Support — This question tests Data Operations and Support — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The Glue job's security group does not allow outbound traffic to the RDS security group on port 3306. — Option C is correct because AWS Glue ETL jobs run in a VPC that requires outbound security group rules to initiate connections to RDS. Even if the RDS security group allows inbound traffic from the Glue security group, the Glue security group must also have an outbound rule allowing traffic to the RDS security group on port 3306 (MySQL default port). Without this outbound rule, the TCP handshake from Glue to RDS fails, causing a 'Communications link failure'.
What should I do if I get this DEA-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". 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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Question Discussion
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