DEA-C01 Data Operations and Support Practice Question
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?
⚠ Common exam trap
A common mix-up: 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).
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 Glue job's security group does not allow outbound traffic to the RDS security group on port 3306.
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'.
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
Omitting the database name from the JDBC URL yields a syntax or unknown-database error, not a communications link failure, which signals a TCP-level connection problem. The database name is genuinely required in the JDBC URL, so this option is tempting when troubleshooting connection strings, but it cannot produce this specific error.
- ✗
The Glue job is using the wrong JDBC driver.
Why it's wrong here
A wrong JDBC driver typically produces class-loading or connection-string errors, not a communications link failure, which indicates the TCP connection to MySQL was refused or dropped. It is tempting because driver mismatches do cause connection errors, and that would be the likely cause if Glue could not load the MySQL connector at all.
- ✓
The Glue job's security group does not allow outbound traffic to the RDS security group on port 3306.
Why this is correct
Security groups are stateful, but the Glue job's own outbound rules must still permit traffic to the RDS security group on MySQL port 3306. Inbound access alone is insufficient, so the missing egress rule explains the communications link failure.
- ✗
The IAM role used by the Glue job does not have rds:Connect permission.
Why it's wrong here
RDS MySQL authentication is enforced by database users and passwords, not IAM policies; rds:Connect is not an IAM action, so a missing permission cannot cause this error. IAM roles govern AWS API access, making this tempting when Glue jobs fail, but the failure here is at the MySQL protocol layer.
Visual reference
Quick reference
AWS S3 Storage Class Comparison
| Storage Class | Min Duration | Retrieval | Use Case |
|---|---|---|---|
| S3 Standard | None | Immediate | Frequently accessed data |
| S3 Standard-IA | 30 days | Immediate | Infrequent access, rapid retrieval |
| S3 One Zone-IA | 30 days | Immediate | Non-critical infrequent data |
| S3 Intelligent-Tiering | None | Immediate–hours | Unknown or changing access patterns |
| S3 Glacier Instant | 90 days | Milliseconds | Archive with instant retrieval |
| S3 Glacier Flexible | 90 days | Minutes–hours | Archive, flexible retrieval |
| S3 Glacier Deep Archive | 180 days | Hours | Long-term compliance archive |
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This DEA-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the DEA-C01 exam.