The correct first step is to check the security group and network ACL rules for the RDS instance and the Glue connection. This is the right move because a JDBC communications link failure almost always points to a network connectivity problem—Glue cannot reach the RDS database, often because the private subnet’s inbound rules or the Glue connection’s associated security group are misconfigured. On the AWS Certified Data Engineer Associate DEA-C01 exam, this scenario tests your understanding of how AWS Glue connects to resources in private subnets, typically via VPC endpoints or a Glue connection with proper networking. A common trap is to jump to authentication or IAM roles, but the error message itself—"communications link failure"—is a TCP-level timeout, not a credential issue. Remember the mnemonic: "Link fails, check the rails"—meaning verify the network path (security groups, ACLs, and subnet routing) before touching permissions or code.
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 run ETL jobs that process data from Amazon RDS for MySQL and load it into Amazon S3. The job runs daily and processes incremental changes using the JDBC connection. Recently, the job has been failing with a 'Communications link failure' error. The RDS instance is in a private subnet. Which step should the engineer take first to diagnose the issue?
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
✓
Check the security group and network ACL rules for the RDS instance and the Glue connection.
Option B is correct because network connectivity between Glue and RDS is likely the issue, and checking security groups and subnets is the first step. Option A is wrong because the error is not about authentication. Option C is wrong because S3 permissions are not related to the link failure. Option D is wrong because the job type (Spark vs Python) does not affect connectivity.
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.
✗
Verify that the IAM role used by Glue has the correct permissions to access RDS.
Why it's wrong here
Permissions are not indicated by 'Communications link failure'.
✗
Change the Glue job type from Spark to Python shell.
Why it's wrong here
Job type does not resolve network errors.
✓
Check the security group and network ACL rules for the RDS instance and the Glue connection.
Why this is correct
Network misconfiguration is the most common cause of link failure.
Clue confirmation
The clue word "first" in the question point toward this answer.
Check that the JDBC driver is compatible with the Glue version.
Why it's wrong here
Driver compatibility does not cause intermittent link failures.
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: Check the security group and network ACL rules for the RDS instance and the Glue connection. — Option B is correct because network connectivity between Glue and RDS is likely the issue, and checking security groups and subnets is the first step. Option A is wrong because the error is not about authentication. Option C is wrong because S3 permissions are not related to the link failure. Option D is wrong because the job type (Spark vs Python) does not affect connectivity.
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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