The answer is a network ACL blocking outbound traffic from the Glue job’s subnet. When an AWS Glue job runs inside a VPC and connects to a JDBC source like PostgreSQL, the connection timeout error typically indicates a network-layer failure rather than an authentication or schema issue. While the security group may allow inbound traffic to the database, the network ACL (NACL) is a stateless firewall that controls both inbound and outbound traffic at the subnet level; if its outbound rules deny ephemeral ports or the database port, the Glue job’s response packets are silently dropped, causing the timeout. On the AWS Certified Data Engineer Associate DEA-C01 exam, this scenario tests your understanding of VPC networking components—specifically the difference between stateful security groups and stateless NACLs. A common trap is assuming that a correct security group alone guarantees connectivity, but you must verify both inbound and outbound NACL rules. Memory tip: “NACL blocks outbound, SG blocks inbound—check both for a clean connection.”
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 data engineer runs an AWS Glue job that reads from a JDBC connection to a PostgreSQL database. The job fails with a 'Connection timed out' error. The Glue job runs in a VPC with the appropriate security group. What is the most likely cause?
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 network ACL associated with the Glue job's subnet is blocking outbound traffic.
Option C is correct because a network ACL can block outbound traffic from the VPC to the database. Option A is wrong because the error is network, not authentication. Option B is wrong because the security group allows inbound, but need outbound. Option D is wrong because it's a network issue, not schema.
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.
✓
The network ACL associated with the Glue job's subnet is blocking outbound traffic.
Why this is correct
Network ACLs can block traffic.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The Glue job does not have permission to access the database.
Why it's wrong here
Permission issue would give access denied, not timeout.
✗
The security group does not allow inbound traffic from the Glue job.
Why it's wrong here
Inbound rule on database side needed, but Glue's security group needs outbound.
✗
The database credentials are incorrect.
Why it's wrong here
Would get authentication error, not timeout.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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: The network ACL associated with the Glue job's subnet is blocking outbound traffic. — Option C is correct because a network ACL can block outbound traffic from the VPC to the database. Option A is wrong because the error is network, not authentication. Option B is wrong because the security group allows inbound, but need outbound. Option D is wrong because it's a network issue, not schema.
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: "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?
CIDR notation defines the prefix length.
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