The correct first step is to verify that the Glue job's VPC subnet and security group allow outbound traffic to the RDS instance. This is because a 'Communications link failure' error when using an AWS Glue JDBC connection to an RDS database in a private subnet almost always indicates a network connectivity issue—Glue must be able to reach the database over the network, and if the security group lacks an outbound rule or the subnet lacks a route to the RDS endpoint, the connection will time out. On the AWS Certified Data Engineer Associate DEA-C01 exam, this scenario tests your understanding of how AWS Glue jobs interact with VPC networking, a common trap being that candidates mistakenly blame database configuration or data format changes first. Remember, connection pooling errors or resource limits produce different error messages, so always isolate the network layer before tuning the database. Memory tip: "Glue to RDS? Check the VPC address."
DEA-C01 Data Ingestion and Transformation Practice Question
This DEA-C01 practice question tests your understanding of data ingestion and transformation. 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 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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
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.
Option B is correct because the Glue job's network connectivity to RDS is the likely issue; checking security groups and route tables is the first step. Option A is wrong because connection pooling is a database configuration, not a network issue. Option C is wrong because data format changes cause parsing errors, not connection failures. Option D is wrong because Glue resource limits would cause different errors (e.g., 'Resource exceeded').
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.
✗
Check the Glue job's DPU allocation; increase if too low.
Why it's wrong here
DPU limits cause performance issues, not connection errors.
✗
Review the Glue job script for data type mismatches.
Why it's wrong here
Data type mismatches cause parsing errors, not connection failures.
✓
Verify that the Glue job's VPC subnet and security group allow outbound traffic to RDS.
Why this is correct
Network connectivity is the first thing to check for link failures.
Clue confirmation
The clue word "first" in the question point toward this answer.
Increase the RDS instance's max_connections parameter.
Why it's wrong here
Connection limit would cause 'too many connections' error, not link failure.
Common exam traps
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 Ingestion and Transformation — This question tests Data Ingestion and Transformation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Verify that the Glue job's VPC subnet and security group allow outbound traffic to RDS. — Option B is correct because the Glue job's network connectivity to RDS is the likely issue; checking security groups and route tables is the first step. Option A is wrong because connection pooling is a database configuration, not a network issue. Option C is wrong because data format changes cause parsing errors, not connection failures. Option D is wrong because Glue resource limits would cause different errors (e.g., 'Resource exceeded').
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: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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