DEA-C01 Data Ingestion and Transformation Practice Question

A company uses AWS Glue to process data from Amazon RDS MySQL into Amazon S3. The Glue job uses a JDBC connection and runs on a schedule. Recently, the job has been failing with a 'Communications link failure' error. The RDS instance is in a private subnet. Which troubleshooting step should the data engineer take FIRST?

⚠ Common exam trap

Test-takers frequently assume 'Communications link failure' is a database-side issue (like connection limits or timeouts) and jump to tuning RDS parameters, when in fact it is most commonly a network connectivity problem in a VPC environment.

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

✓

Verify that the Glue job's VPC subnet and security group allow outbound traffic to RDS.

The 'Communications link failure' error typically indicates a network connectivity issue between the Glue job and the RDS instance. Since the RDS instance is in a private subnet, the Glue job must be configured with a VPC subnet and security group that allows outbound traffic to the RDS instance's security group on port 3306 (MySQL). Without this network path, the JDBC connection cannot be established, making verifying the VPC and security group configuration the first logical troubleshooting step.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
Check the Glue job's DPU allocation; increase if too low.
Why it's wrong here
DPU allocation governs compute and memory for transformation, not TCP reachability to a private-subnet RDS endpoint; a 'Communications link failure' indicates the JDBC socket cannot be established. Raising DPUs is the right lever when jobs fail with out-of-memory or executor timeouts, not network-level connection errors.
✗
Review the Glue job script for data type mismatches.
Why it's wrong here
Data type mismatches surface as conversion or schema errors during read/write, not as a 'Communications link failure', which is a JDBC socket-level fault occurring before any row is parsed. Script review is the correct first step when jobs fail with casting exceptions or malformed records.
✓
Verify that the Glue job's VPC subnet and security group allow outbound traffic to RDS.
Why this is correct
Glue jobs in a VPC need a subnet route and security group egress permitting TCP 3306 to the RDS instance; without it, the JDBC handshake fails with 'Communications link failure'. Verifying outbound connectivity from the Glue ENI directly addresses the private-subnet constraint before deeper checks.
✗
Increase the RDS instance's max_connections parameter.
Why it's wrong here
max_connections governs concurrent session limits, and exceeding it returns 'Too many connections', not a 'Communications link failure'. Raising it is the right step when the RDS instance rejects new sessions under load, not when the Glue job cannot reach the private-subnet endpoint at all.

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

Go deeper

DEA-C01 Data Ingestion and Transformation Practice Question

Why each option matters

Option-by-option breakdown

AWS S3 Storage Class Comparison

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